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Super Hybrid Rice in China and India: Current Status and Future Prospects.

Byline: S.K. Singh, P.K. Bhati, Amita Sharma and Vikas Sahu

Abstract

Rice is a staple food for over half of the world's population and has the second largest cereal production after maize. Rice yield has experienced many fold jumps since the 1950s. This happened primarily as the result of genetic improvement and increasing harvest index by reducing plant height using the semi-dwarf genes and utilization of heterosis by producing hybrids. Hybrid rice technology is one of the strongest tools to break the yield barrier. To make hybrid rice technology practically feasible it needs strong system of hybrid seed production at commercial rate. Unlike maize, rice is a self-pollinated crop and needs special techniques, like utilization of male sterility system for hybrid seed production. The first hybrid rice variety for commercial cultivation was released by China in 1976. Seeing the China's success many countries started the hybrid rice breeding programme. India also started this programme in 1989. In about two decades of efforts India released 65 hybrid rice varieties till 2013.

Though, China is cultivating about 50% of its rice area under hybrid rice varieties, India is still struggling to enhance its acreage under hybrid rice from 4% to more. Because of complicated seed production system, higher seed cost and less preferred qualities of hybrid varieties it could not cover more area under cultivation in India. In 1998, China started work for developing super hybrid rice by combining an ideotype approach with the use of inter-sub specific heterosis. Till 2011, the Ministry of Agriculture, China approved 56 hybrid cultivars with great yield potential as super hybrid rice. Hence, India and China still have to work hard for developing high yielding super hybrids coupled with good cooking qualities and resistance/tolerance to major biotic and abiotic stresses. Suitable technology should also be developed to make hybrid seed production easier and cheaper. Copyright 2015 Friends Science Publishers

Keywords: Oryza; Hybrid rice; Male sterility; CGMS; TGMS; PGMS

Introduction

Rice (Oryza sativa L.) belongs to the genus Oryza of family Poaceae. The genus Oryza is known to consist of two cultivated species i.e. Asian rice (O. sativa, 2n=24=AA) and African rice (O. glaberrima, 2n=24=AA) and 22 wild species (2n=24, 48). About half of the world's population depends on rice for their survival. Rice is being cultivated in around 113 countries of the world. It contributes 19.62%, 24.63% and 40.52% of total food grain production of world, China and India respectively. In India, 56.68% of total rice area is grown on irrigated land, 27.21% on rainfed lowland and 5% on rainfed uplands (Rice Knowledge Management Portal, Hyderabad, 2012). India has the largest area under rice among the rice growing countries in the world and ranks second in production after China. India is one of the major rice export countries (22.7%) including Thailand (28%), Vietnam (24.5%), Pakistan (14%) and United States (10.8%).

The major rice import countries are Nigeria (26%), Iran (22.5%), Philippines (17.3%), China (17.3%) and Indonesia (16.8%) (USDA 2012). The present world rice area, production and productivity is 158.93 mha, 465.03 mt and 4.36 t ha-1, respectively. In India, it is being grown in 42.86 mha area with production of 104.32 mt and productivity of 3.59 t ha-1 and contributes 25% to agricultural GDP (Foreign Agriculture Services/USDA, Office of Global analysis, April 2012). To feed the ever growing population, the targeted rice production of the world, for the year 2030 is envisaged as 771.02 million tonnes (Alexandratos and Bruinsma, 2012). To get success in achieving the target, the increase in rice productivity is the only option left, since the other alternatives like cultivable land, water and other natural resources are either stagnant or declining (Yashitola et al., 2002).

Hence, there is an urgent need to boost the rice production through its enhanced productivity. Hybrid rice technology is one of the most important and practically feasible technologies to enhance the rice productivity. Like other crops, rice is also showing enough heterosis (15-25%) (Siddiq, 1997; Singh and Haque, 1999; Singh et al., 2013). Unlike maize, the hybrid seed production in rice is not possible without availability of strong male sterility system.

A wild rice plant with aborted pollen in China's Hainan Island and the available fertility restorer genes in indica rice led the beginning of hybrid rice technology in 1970. The hybrid rice variety for commercial cultivation was released by China in 1976. Seeing the China's success many countries started hybrid rice breeding programme. India also started this programme in 1989. The study tours and fellowship trainings of a large number of Indian scientists in reputed institutes in China, IRRI, Philippines and in other countries helped in accelerating the hybrid rice research. In about two decades of extensive efforts, India has so far released 65 rice hybrids by both public and private sectors for commercial cultivation (Directorate of Rice Development Report, 2013, Patna). These hybrids have yield advantage of 1.0-1.5 t ha-1 over the highest yielding inbred varieties. During the year 2011, around 2 mha area was planted with hybrid rice in India.

Hybrid rice is being cultivated predominantly in the states of Utter Pradesh, Jharkhand, Bihar, Punjab, Haryana, Madhya Pradesh and Chhattisgarh.

Some of the popular hybrids grown in the country are Arize-6444, PHB-71, KRH-2, PRH-10, PA 6129, Saihhadri, Suruchi, JKRH-2000, PAC 837 and DRRH-2. Other rice growing countries have also started hybrid rice research and its cultivation. Because of complicated seed production system, higher seed cost and less preferred qualities of hybrid rice varieties the enhancement in area coverage under hybrid rice in India is rather slow. Hence, proper strategies have to be adopted for research and development to overcome the above limitations to enhance hybrid rice acreage and ultimately to boost the rice production. The attempts have been made in super rice breeding in China and other countries in recent years. However, there were three main problems in super rice breeding: 1. the super rice varieties were still rare; 2. most super rice varieties exhibited narrow adaptability; and 3. current breeding theories emphasized too much on the rice growth model, but they were impractical in guidance for rice breeding.

The super parent of hybrid rice should exhibit excellent performance in all agronomic traits, with the yield or sink capacity reached the level of the hybrid rice control in regional trials. The super hybrid rice combination should meet the following criteria: good rice quality, wide adaptation, lodging resistance, resistance to main insects and diseases, and the yield exceeded above 8% over the control varieties in the national and provincial regional trials. To achieve the goal, the technical strategies, such as selecting optimal combination of the parents, increasing selection pressure, paying more attention to harmony of ideal plant type, excellent physiological traits and all the agronomic traits, should be emphasized. The yield of seed production should reach 3.75 t ha-1 and 5.25 t ha-1 for the super hybrid rice combinations derived from early-season and middle-season types of male sterile lines, respectively (Chen et al., 2007).

Hybrid Rice Technology

Heterosis or hybrid vigour is regarded as superiority of F1 hybrids over their parents and or over check varieties/hybrids. Heterosis is exploited through development of hybrids, synthetics and composites. In case of self pollinated crops like rice, synthetics and composites may not be used. Hence, to exploit heterosis in rice, development of high yielding hybrids is the only option where we may use full magnitude of available heterosis. To exploit the heterosis at commercial level it is necessary to have:

a. High magnitude of heterosis. Means, F1 hybrid must show high economic heterosis.

. Economical method of hybrid seed production.

Heterosis in Rice

Exploitation of heterosis was initiated in cross pollinated crops like maize, where inbreds exhibit inferior phenotypic performance due to inbreeding depression. Rice is a self- pollinated crop where pure lines are naturally formed with high phenotypic performance and little or no inbreeding depression. Although Jones (1926) was first to report existence of hybrid vigour in rice, progress in hybrid rice development was stalled due to two principal reasons, superior performance of pure lines and unavailability of suitable pollen control mechanisms.

In India, heterosis was first reported by Ramiah (1935) and Kadam et al. (1937). Being a self-pollinated crop with numerous bisexual flowers per inflorescence, hand emasculation for hybrid development is impractical in rice. Since, it is a self pollinated crop and farmers have to replace their seed every year, hybrid seed production at commercial scale is not possible without having a stable male sterility system. Therefore, by any means we have to make one of the parents as female to exploit it at commercial level. Hence, developing a suitable female parent is the major task to make hybrid rice technology practically feasible.

History of Hybrid Rice Technology in China and India

In 1964, Yuan Long Ping first put forward the idea of utilizing the heterosis in rice and initiated the research on hybrid rice in China.

In1970, a pollen abortive wild rice plant (Wild Abortive; WA) was discovered among the plants of common wild rice at Nanhong Farm of Hainan Island of China and the available restorer genes in indica rice led the beginning of hybrid rice technology.

In 1972, the first group of CMS lines such as Zhenshan 97A, V20A were developed by using WA as the donor of male sterile genes by way of successive backcrossing method.

In 1976 - First commercial three-line rice hybrid released in China.

In 1973 - PTGMS in rice was discovered in China.

In 1994 - first commercial two-line rice hybrid released in China.

In India- ICAR launched a mission mode project on hybrid rice in December, 1989.

IRRI, Philippines collaborated with the project by providing the needed germplasm and technical support. This project was further strengthened with financial support from UNDP and technical support from FAO since September, 1991-1996.

Mahyco Research Foundation (MRF), now popularly known as Barwale Foundation provided financial assistance to the hybrid rice project during 1996-2002 to fill critical gaps, which is a good example wherein a private foundation supported a public sector research.

In 1994 first time hybrid rice (APHR1, APHR2, MGR1 and KRH1) were released in India.

The research network consists of 12 active research centers across the country each with a specific mandate. India released 65 hybrid rice varieties till 2013 (Table 1).

The History of Super-Hybrid Rice Breeding

In 1982, the Japanese government initiated super high yielding rice breeding program. China and some other countries have paid great attention to super high-yielding rice breeding. The target was to achieve the yield of the brown rice output to 7 500 - 9 800 kg ha-1 in the medium and low yield areas, and over 10, 000 kg ha-1 in the high yield areas, to increase rice yield by 50% than control varieties in 15 years, by breeding high-yielding rice varieties and by developing the corresponding cultivation technology. Rice varieties such as Akenohoshi and Akichikara, which achieved nearly 10 000 kg ha-1 in the high yield areas, were developed at several breeding stations in Japan in the later eight years. However, these varieties were not being extended in large areas due to the lack of cold resistance, poor grain quality and low seed setting rate.

To break the yield barrier, scientists at the International Rice Research Institute (IRRI) proposed the plan of breeding new plant type rice' (also called super rice') in 1989. The goal was to breed new rice varieties with the yield potential of 13 000 - 15 000 kg ha-1 and increase of 20-30% than the control varieties. However, due to low biomass production, poor grain filling, low seed setting rate and susceptibility to diseases and insects, the new varieties have not been released for rice production in farmers' fields.

The China's super rice project was started in 1996. By the cooperation of nationwide breeders and researchers of China more than 20 super rice varieties or hybrid rice combinations such as Liangyoupeijiu and Xieyou 9308 were bred. The yields of these varieties or hybrids were higher than 10.5 t ha-1 in larger planted areas, and increased by 15% comparing to the hybrid rice combinations widely planted at present in China. The new hybrid rice combinations were also called super hybrid rice', which stood for the success of super rice breeding in China (Chen et al. 2000; 2007).

The Strategies for Super Hybrid Rice Breeding

The main strategies for breeding super hybrid rice combinations with wide adaptability, high yield potential, good rice quality and multiple disease and insect resistances are to combine the ideal plant type with physiological vigor and to harmonize all the growth traits, by improving the selective pressure on the basis of crosses from the super parents. Firstly, optimal combination of the parents is one of the key strategies for hybrid rice breeding. Optimal combination means that the hybrid rice combination derived from the parents with a reasonable genetic difference, such as from (1) lowland rice and upland rice varieties, (2) different varieties with geographically remote distance, (3) different ecological types, (4) different dominant varieties, and (5) indica and japonica (Cheng et al., 2007).

However, what needs to be emphasized in particular is that we can exploit only part of the heterosis between the indica and japonica subspecies, but not the heterosis between the typical indica and japonica rice or the one including excessive indica and japonica ingredients. Generally, the hybrid rice crossed from typical indica and japonica rice would result in the poor adaptability and stability of seed setting rate. Secondly, maintaining high harmony in a great degree of plant type, leaf shape, yield components and physiological function is another key strategy of super hybrid rice breeding. Increasing selection pressure is the third important strategy for breeding super hybrid rice with wide adaptability. For some important traits, such as resistance to higher or lower temperatures and rice blast, and the sterility of T(P)GMS and CMS lines, we usually grow the candidate plants in the conditions where it is easy to identify these traits.

The super hybrid rice will be more vigorous and competitive only when bred under the conditions with added selection pressure. However, the breeding efficiency will be improved greatly after pyramiding multi-resistance and other favourable genes into the super hybrid rice parents by using the molecular breeding technology (Chen et al., 2007).

Problems and Prospects in Super Hybrid Rice Breeding

Since launching the project of super rice breeding' in China in 1996, great achievements in super rice breeding have been acquired in ten years. However, we should be well aware of the super rice varieties which are still rare, especially for growing in double-cropping rice regions in southern China. In addition, most of the currently super hybrid rice combinations have low seed setting rate, poor yield stability and weak adaptability, caused by their genetic disharmonies. These problems have been the factors inhibiting extension of these super rice varieties to the large area. On the other hand, with the increasing in yield of the new rice varieties and level of breeding strategy, the theories of super high-yielding breeding have to be further improved. Exploitation of indica/japonica heterosis can heighten the level of yield.

With the development of molecular marker technology in rice, the subspecies differentiation of parents can be determined and the proper contribution of indica and japonica genes in hybrids can be established for high yields in combination with harmonious plant types. Molecular marker-assisted selection has provided an approach to pyramid beneficial alleles of QTLs for improving yield and other important traits. Recently, some restorer lines carrying beneficial alleles of yield QTLs and major genes for disease resistance are under evaluation in the super hybrid rice breeding program. Incorporation of the characteristics of high photosynthetic rate from other species into rice plants is of importance for future super hybrid rice breeding (Chen et al. 2007).

Techniques of Hybrid Rice Seed Production

Since rice is a self pollinated crop, one of the parents to be used as female must be made male sterile through proper technique. Several types of techniques for making female may be tested and developed for hybrid seed production are:

Cytoplasmic Genetic Male Sterility (CGMS), also known as three line system.

Environmental sensitive Genetic Male Sterility (EGMS), also known as two line system.

Apomixis, also known as One Line System. Chemically Induced Male Sterility (CIMS).

CGMS System (3-Line System for Hybrid Seed Production)

In India, this system is being used for hybrid rice seed production at a commercial scale. No other system is being used commercially in this country. However, China first started using this system at a commercial scale but now is focusing towards EGMS systems for hybrid rice seed production apart from CGMS systems. Since in CGMS system, 3 lines viz., A' line (CMS line), B' line (maintainer line) and R' line (Restorer line) are needed every year therefore this system is designated as 3-line system of hybrid seed production. The cytoplasmic-genetic male sterility (CGMS) system is the result of interaction between specific sterility inducing cytoplasm and the nuclear gene(s). To get male sterility expression both sterile cytoplasm and recessive (rf) nuclear genes are required. Hence, it becomes necessary to identify several male sterile sources in rice.

As in China, the WA type of CMS source was developed, the other sources are also to be developed for diversification of genetic materials to exploit usable heterosis. Some of the promising CMS lines developed in India are listed in Table 2.

A' Line (CMS line): It is a female parent being used for the purpose of hybrid seed production. It has male sterile gene (S) in its cytoplasm. As rice flowers are bisexual hence, it is not possible to use normal parents as female by hand emasculation at commercial scale. Therefore, it is necessary to have a female parent as cytoplasmic male sterile (CMS line). Since, it is a CMS system, male sterility inheritance will be of maternal type. This results the F1 hybrid to be sterile if the male parent being used is not having the dominant restorer gene (Rf gene) in its nucleus. Hence, it is necessary to have R' line as a male parent for producing hybrid rice seed (Fig. 1).

R' Line (Restorer Line): This parent is being used as male parent for hybrid rice seed production. As it has restorer gene(s) (Rf gene) in its nucleus when it is hybridized with A' line then the hybrid seed produced on A' lines will have the fertility restorer gene in its nucleus in heterozygous condition (Rf1rf1). Hence, these hybrid seeds will be fertile which will be used by the farmers for hybrid rice crop cultivation (Fig. 2).

B' Line (Maintainer Line): It is the isogenic of A' line and being maintained every year only for maintaining the A' lines (Fig. 3).

Maintenance of Parental Lines (A, B and R lines)

Since, B' and R' lines are normal inbred lines hence their maintenance needs not require any special techniques. These are being maintained in a similar manner as other inbred rice varieties are being maintained in isolation. Hence, maintenance of only A' line requires special techniques.

Maintenance of A' line: It is generally observed that A' lines are late in flowering than to their respective B' lines by 3-5 days. Therefore, it is necessary to have staggered sowing of A' and B' lines to synchronize the flowering of both. Hence, in seed beds, A' lines are sown 3-5 days earlier than B' lines. Generally, 21-25 days old seedlings are transplanted in the field. The maintenance field of A'line should have an isolation distance of 400 m and 200 m for breeder and foundation seed production respectively from the other plots to avoid unwanted pollen contamination to A' line.

Transplanting of A' line and B' lines should be done in such a way that helps more and more pollen dispersal from B' line to A' line to enhance higher seed setting in A' line, which will be used for hybrid seed production in next year. The row ratio of B' line and A' line is generally kept at 2: 4 or 2: 6. To help pollen dispersal, generally stick' shaking or rope' shaking is done once or twice daily at the time of peak anthesis. Plant and row spacing should be adjusted as per the agro-climatic conditions being used in rice cultivation.

Special supervision should be taken for avoiding off type plants. Hence, regular roguing must be done to remove all the off type plants from A' as well as from B' lines.

Table 1: Hybrid Rice Released in India

S. No. Rice Hybrids###Year of###Duration###Yield###Developed by###Recommended for

###release###(Days)###(tha-1)

1.###APHR 1###1994###130-135###7.14###APRRI, Maruteru (ANGRAU), Hyderabad###Andhra Pradesh

2.###APHR 2###1994###120-125###7.52###APRRI, Maruteru (ANGRAU), Hyderabad###Andhra Pradesh

3.###MGR 1###1994###110-115###6.08###TNAU, Coimbatore###Tamil Nadu

4.###KRH 1###1994###120-125###6.02###VC Farm , Mandya, UAS, Bangalore###Karnataka

5.###CNRH 3###1995###125-130###7.49###RRS, Chinsurah (W.B.)###West Bengal

6.###DRRH 1###1996###125-130###7.30###DRR, Hyderabad###Andhra Pradesh

7.###KRH 2###1996###130-135###7.40###VC Farm , Mandya, UAS, Bangalore###Bihar, Karnataka, Tamil Nadu, Tripura,

###Maharashtra, Haryana, Uttarakhand, Orrisa,

###West Bengal, Pondicherry and Rajsthan

8.###Pant SankarDhan 1###1997###115-120###6.80###GBPUAT and T, Pantnagar###Uttar Pradesh

9.###PHB 71###1997###130-135###7.86###Pioneer Overseas Corporation, Hyderabad###Haryana, U.P., Tamil Nadu, Andhra Pradesh,

###Karnatka

10.###CORH 2###1999###120-125###6.25###TNAU, Coimbatore###Tamil Nadu

11.###ADTRH 1###1999###115-120###7.10###TNRRI, Aduthurai (TNAU)###Tamil Nadu

12.###Sahyadri###1998###125-130###6.64###RARS, Karjat (BSKKV)###Maharashtra

13.###NarendraSankarDhan 2###1998###125-130###6.15###NDUAT and T, Faizabad###Uttar Pradesh

14.###PA 6201###2000###125-130###6.20###Bayer Bio-Science, Hyderabad###Andhra Pradesh, Karnataka, Bihar, Orissa,

###Madhya Pradesh, Uttar Pradesh, West

###Bengal,Tamil Nadu and Tripura

15.###PA 6444###2001###135-140###6.11###Bayer Bio-Science, Hyderabad###U. P., Tripura, Odisha, Andhra Pradesh,

###Karnataka, Maharashtra, Uttarakhand

16.###Pusa RH 10###2001###120-125###4.35###IARI, New Delhi###Haryana, Delhi, Western U.P. and Uttarakhand

17.###PRH-122R (Ganga)###2001###130###5.64###Paras Extra Growth Seeds Ltd., Hyderabad###Bihar, Orissa, Punjab, U.P., Uttarakhand,

###Nagaland, Haryana

18.###RH 204###2003###120-126###6.89###Parry Monsanto Seeds Ltd., Bangalore###Andhra Pradesh, Karnataka, Tamil Nadu,

###Haryana, Uttarakhand and Rajsthan

19.###Suruchi 5401###2004###130-135###5.94###Mahyco Ltd., Aurangabad###Haryana, Andhra Pradesh, Karnataka, Gujarat,

###Odisha, Chattishgarh and Maharashtra

20.###Pant SankarDhan 3###2004###125-130###6.12###GBPUAT and T, Pantnagar###Uttarakhand

21.###Narendra###Usar###Sankar 2005###130-135###5.15###NDUAT and T, Faizabad###Saline and alkaline areas of U.P.

###Dhan 3

22.###DRRH 2###2005###112-116###5.35###DRR, Hyderabad###Haryana, Uttarakhand, W.B and Tamil Nadu

23.###Rajlakshmi (CRHR 5)###2005###130-135###5.84###CRRI, Cuttack###Boro areas of Aasam,Orissa

24.###Ajay (CRHR 7)###2005###130-135###6.07###CRRI, Cuttack###Irrigated areas of Orissa

25.###Sahyadri 2###2005###115-120###6.50###RARS, Karjat (BSKKV)###Maharashtra

26.###Sahyadri 3###2005###125-130###7.5###RARS, Karjat (BSKKV)###Maharashtra

27.###HKRH-1###2006###139###9.41###RARS,Karnal (CCSHAU)###Haryana

28.###CORH-3###2006###115###-###TNAU, Coimbatore###Tamil Nadu

29.###JKRH 401###2006###125###6.22###JK Agri. Genetics Ltd. Hyderabad###Bihar, Odisha, W.B.

30.###KJTRH 2###2006###N.A.###N.A.###RARS, Karjat (BSKKV)###Maharashtra

31.###Haryana Shankar Dhan-###2006###139###9.40###HAU, Haryana RARS,Kaul(CCS,HAU.)###Haryana

###1 (HKRH-1)

32.###HRI-152 (IET 18815)###2007###120 (Mid###NA###Bayer Bio-Science, Hyderabad###Punjab and Tamil Nadu

###early)

33.###JRH-4###2007###110-115###7.50###JNKVV, Jabalpur###Madhya Pradesh

34.###JRH-5###2007###105-108###7.50###JNKVV, Jabalpur###Madhya Pradesh

35.###Indira Sona###2007###120-125###7.0###IGKKV, Raipur###Chhattisgarh

36.###PA 6129###2007###115-120###6.58###Bayer Bio-Science, Hyderabad###Punjab, Tamil Nadu, Pondichery

37.###GK -5003###2008###128###6.04###Ganga Kaveri Seeds Pvt. Ltd., Hyderabad###Andhra Pradesh, Karnataka

38.###Sahyadri - 4###2008###115-120###6.80###RARS, Karjat (BSKKV)###Haryana, W. B., Maharashtra, U.P. and Punjab

39.###JRH- 8###2008###105-110###7.50###JNKVV, Jabalpur###Madhya Pradesh

40.###DRH - 775###2009###97###7.70###Methelix Life Sciences, Pvt. Ltd. Hyderabad.###Chhattisgarh, Jharkhand, West Bengal

41.###HRI -157 (IET 19511,###2009###130-135###6.50###Bayer Bio-Science, Hyderabad###Chhattisgarh, Gujarat, Bihar, Jharkhand,

###91H97226)###(Arize###Odisha, Maharashtra, A.P.,T.N. Karnataka,

###Prima)###Madhya Pradesh, Uttar Pradesh, Tripura

42.###PAC 835 (PAC 80035)###2009###130###5.60###Advanta India Ltd., Hyderabad###Odisha, Gujarat

###(IET 18178) Hybrid

43.###PAC 837 (PAC 80037)###2009###130###6.30###Advanta India Ltd., Hyderabad###Gujarat, Chhattisgarh, JandK, Andhra Pradesh,

###(IET 19746) Hybrid###Karnataka

44.###NK - 5251###2009###128###6.65###Syngenta India Ltd., Secundrabad###Andhra Pradesh, Gujarat, Karnataka,

###Maharashtra, Tamil Nadu

45.###DRRH- 3###2009###131###6.07###DRR, Hyderabad###Andhra Pradesh, Gujarat, Madhya Pradesh,

###Odisha, U.P.

46. US - 312###2010###125-130###5.76###Seed Works International, Hyderabad.###Andhra Pradesh , Bihar , Karnataka, Tamil

###Nadu, Uttar Pradesh, W. B.

47###CRHR-32###2010###125###5.43###CRRI, Cuttack, Odisha###Bihar, Gujarat

48###INDAM 200-017###2010###120-125###6.60###Indo-American seeds, Hyderabad###Odisha, Chattishgarh, Gujarat Maharashtra, A.P.

49###27P11###2010###115-120###5.67###PHI Seeds(P) Ltd.###Karnataka, Maharashtra

50###VNR 2245 (IET 20716)###2011###90-95###6.83###VNR Seeds Pvt. Ltd., Raipur-492099###Chhattisgarh, Tamil Nadu

###(VNR-204)

51###VNR 2245 (IET 20735)###2011###100-105###5.75###VNR Seeds Pvt. Ltd., Raipur-492099###Uttar Pradesh, Uttarakhand, West Bengal,

###(VNR-202)###Maharashtra, Tamil Nadu

52###Shyadri-5 (Hybrid)###2011###110-115###NA###RARS, Karjat (BSKKV)###Konkan Region of Maharashtra

53###CO (R) H-4###2011###130-135###7.34###TNAU, Coimbatore###Tamil Nadu

54###Hybrid CO 4###2012###130-145###7.34###NA###Tamil Nadu

55###US 382 (IET 20727)###2012###125-130###6.70###Seed Works International Pvt.###Ltd., Tripura, Madhya Pradesh, Karnataka

###Hyderabad-34.

56 27P31 (IET 21415)###2012###125-130###8.0-.9.0###PHI Seeds Pvt. Ltd. Hyderabad- 82.###Jharkhand, Maharashtra, Karnataka, Tamil

###Nadu

57 27P61 (IET 21447)###2012###132###6.70###PHI Seeds Pvt. Ltd. Hyderabad- 82.###Chhattisgarh, , Gujarat, Andhra Pradesh,

###Karnataka,Tamilnadu

58 25P25 (IET 21401)###2012###110###6.70###PHI Seeds Pvt. Ltd. Hyderabad- 82.###Uttarakhand, Jharkhand, Karnataka

59 ArizeTej (HRI 169) (IET 2012###125###70.0###Bayer Bio Science Pvt. Ltd, Hyderabad 81. Bihar, Chhattisgarh, Gujarat, Andhra Pradesh,

###21411)###Tamil Nadu

60 PNPH 24 (IET 21406)###2012###120-130###NA###Nuziveedu Seeds Limited,###Bihar, West Bengal, Odisha

###Medchal Mandal, Ranga Reddy- 501401 (A.P.)

61 PNPH 924-1 (IET 21255) 2012###125-135###NA###Nuziveedu Seeds Limited, Medchal Mandal, West Bengal, Assam

###Ranga Reddy- 501401 (A.P.)

62 NK 5251 (IET 19738)###2012###NA###NA###NA###Tamil Nadu, Karnataka, Andhra Pradesh,

###Maharashtra, Gujarat

63 JKRH 3333 (IET 20759)###2013###135-140###5.98###JK Agri Genetics Ltd, Hyderabad- 16.###West Bengal, Bihar, Chhattisgarh, Gujarat,

###Andhra Pradesh

64 RH- 1531(Frontline Gold) 2013###118-125###NA###Devgen Seeds and Crop Technology, Major Hybrid rice growing regions ( Madhya

###(IET 21404)###Hyderabad###Pradesh, Uttar Pradesh, Andhra Pradesh,

###Karnataka, Maharashtra)

65 CO 4 (IET 21449) (TNRH 2013 NA###NA###TNAU, Coimbatore###Tamil Nadu, Gujarat, Maharashtra

###174)

Table 2: Promising CMS lines developed in India

Centre Promising###CMS lines developed

DRR, Hyderabad###DRR 6A, 9A, 10A, 12A, 14A, 15A

IARI, New Delhi###Pusa 3A, 4A, 5A, 6A, 10A, 11A

PAU, Ludhiana###PMS 3A, 10A, 12A, 17A

RARS, Karjat###KJTCMS 1A, 2A, 3A, 4A

ARS, Ratnagiri###RTN 8A, 10A, 13A, 14A

APRRI, Maruteru###APMS 6A, 8A, 9A

CRRI, Cuttack###CRMS 32A, CRMS 31A

Table 3: Some Important Sources of Male Sterility Inducing Cytoplasm in Rice

Designation###Cytoplasmic source###First nuclear donor

CMS-WA###Wild rice with abortive pollen###Zhen shan 97 V20, V41

CMS-DA###Dwarf wild rice with abortive pollen###Xue Qin Zhao

CMS-IP###Indonasian paddy###II-32

CMS-DT###Dissi type###297

CMS-HL###Hong lian###Lian Tana Chao

CMS-KR###Oryzarufipogon###Taichung 65

CMS-BT###Chinsurahboro II###Taichung 65

CMS-TN###TN 1###Pankhari 203

CMS-GAM###Gambiaca###Chao yang 1

CMS-ARC###Assam rice collection IRRI Acc-13829###IR 10179-3-2-1

CMS- O. perennis###O. perennis, Acc. 104823###IR 64R

Particular attention should also be given for roguing of pollen shedder plants observed in A' lines. All recommended agronomic practices should be adopted for raising good crop. At maturity, B' lines should be harvested first and should not be used for seed production. This will avoid mixing of seeds from B' line to A' line. Later A' line is harvested, threshed, dried and stored properly to use it in next year for hybrid seed production (Fig. 4). Some important sources of male sterility inducing cytoplasm in rice are listed in Table 3.

Hybrid Seed Production

This is done at certified seed level. Till foundation seed level, only parental lines are multiplied. For hybrid seed production, A' line and R' line are used and seeds harvested from A' lines will be the F1 hybrid seed (Fig.5).

EGMS System (2 Line System of Hybrid Seed Production)

A new kind of genetic male sterility i.e. Environment Sensitive Genic Male Sterility has been deployed for developing commercial hybrids particularly in rice. In this system, male sterility condition is due to the interaction of nuclear genes with environmental factors such as photoperiod, temperature or both. Hybrid seed production through two line system is simpler if it becomes feasible. Magnitude of heterosis in two-line hybrids is usually 5 to 10% higher than in three line hybrids because of no cytoplasmic penalty. The EGMS comprises of the following three types.

PGMS (Photoperiod Sensitive Genic Male Sterility)

It was discovered as spontaneous mutation in japonica cultivar Nongken 58 in China. It includes genic male sterile lines which respond to the photoperiod or duration of day length for expression of pollen sterility and fertility behavior. Most of the PGMS lines remain male sterile under a long day (greater than 14 h) condition and revert back to fertility under short day (less than 13.5 h) condition.

TGMS (Thermo Sensitive Genic Male Sterile Lines)

It was discovered by Zhao et al. (1988) as spontaneous mutation in China. Male sterility/fertility alteration is conditioned by different temperature regimes. Most of the TGMS lines remain male sterile at a high temperature (Max.greater than 30C) and they revert back to partial fertility at a lower temperature (less than 30C). The critical sterility/fertility points vary from genotype to genotypes. The critical thermo sensitive stage for fertility alteration in TGMS line varied from 15-25 days before heading and 5-15 days after panicle initiation.

Li et al. (2006) identified it which is induced by short day length (less than 13 h) instead of long day length. This line is controlled by the interaction of photoperiod and temperature. PTGMS is just similar to the TGMS system in all respects except for the temperature regime in between the Critical sterility point (CSP) and Critical fertility point (CFP), where the photoperiod sensitivity is observed. Fertility was restored when day length was greater than 13.5 h (reverse of PGMS). This reversal is attained within temperature range of 23-28C and is known as short day male sterility (SDMS).

Hybrid Seed Production through EGMS System

This system could not become practically feasible in India as well as in other countries till date. This needs a specific environment for multiplication of EGMS lines and separate environment for production of hybrid seed where EGMS gene(s) are able to express. However, this system has become practically feasible in China where it is being used at commercial scale. Since EGMS system has many advantages over other systems, it is necessary to explore possibilities for its practical feasibilities in other countries also. India is working hard in this direction. Table 4 gives a list of important EGMS lines identified.

Apomixis

In case of apomixis, the F1 plants will produce seeds without fertilization which will be true to the type of parent. In such case, the heterozygosity will be fixed. Hence, the hybrid seed production will be as easier as of normal seed production of inbred varieties. But till date, this technique could not be developed in rice. Efforts in identifying apomixis in rice and its wild accessions did not provide promising results. Induction of apomixis has been reported in rice through use of physical and chemical mutagens (Chen, 1992).

Chemically Induced Male Sterility (CIMS)

In India, this system is not being used at commercial scale due to environmental concerns. This non-genetic method of inducing male sterility involves the use of chemicals called hybridizing agents (CHA's) or Gametocides. These chemicals kill the male gametes and make the plant male sterile. This method is very useful for plants with bisexual flowers in which it is difficult to obtain GMS or CGMS. Chemicals which have been evaluated in rice are, arsenics, GA3, ethrel, FW450, MH etc. (Table 5). Out of these, only zinc methyl arsenate and sodium methyl arsenate have been reported to be effective for producing commercial hybrids in China (Zhao et al., 1988). Hybrids produced by chemically induced male sterility are also called two-line hybrids in rice. Chemically induced male sterility is used sporadically because the effective and safe chemicals for inducing male sterility are not available.

Advantages and Disadvantages of Hybrid Rice Technology

Advantages

1. Average yield of rice hybrids is 15-35% higher when compared to the high yielding varieties of that group.

2. Increased vigour which makes them more competitive with weeds.

3. Quantity of seed used in hybrid rice is significantly less than the seed used in conventional high yielding varieties.

4. Average productivity and total return to total input cost is significantly higher in hybrid rice than in conventional high yielding varieties.

5. The nursery area required is lesser as compared to conventional high yielding varieties.

6. Hybrids are short duration in nature with resistance/tolerance to major biotic and abiotic stresses.

7. Hybrid seed production generates more employment.

Table 4: Some Important EGMS lines identified

Environment Sensitive Genic Male Sterility (EGMS) lines

Photoperiod sensitive GMS###Temperature sensitive GMS

Nongken 58 S (China)###Annong 810 S (China)

EGMS (USA)###Hennong S (China)

201 (USA)###5460 S (China)

CIS 28 10 S (China)###ATG-1 (India)

Zhenong S (China)###Norin PL 12 (Japan)

X 88 (Japan)###IR 32364 (IRRI)

Pei-Ai64S (China)###IR 68945 (IRRI)

7001 S (China)###IR 68949 (IRRI)

Table 5: List of chemicals, concentration and growth stages for application

Chemical###Concentration###Growth stage for application

Ethrel###800-1000 ppm###Prior to anthesis

Monosodium methyl 0.02 % or 2000 ppm Uni-nucleate pollen stage

or Arsenate (MGI)

Sodium methyl###0.02 % or 2000 ppm Five days before heading

arsenate

Table 6: CMS type, source and method

CMS type###Source###Method

Guang Nong 1###O. sativa###Irradiation

CMS-FA###O. rufipogon###Wide hybridization

Dong B 11A###O. rufipogon###Wide hybridization

D62wxA###O. sativa###Irradiation

FueA6###O. sativa###Irradiation

Vytilla-3###O. sativa###Spontaneous mutation

IR66707A###O. perennis###Wide hybridization

IR69700A###O. glumaepetata###Wide hybridization

Disadvantages

1. Farmers may not use their own seed from one year to next year hence they have to purchase fresh seed every year.

2. Seed cost is almost 2.5 times higher for hybrids than of conventional high yielding varieties.

3. Hybrid rice seed production technology is both labour and knowledge intensive.

4. Majority of the hybrids developed till date are lacking in quality traits.

5. Unpredictable environmental condition may affect sterility expression during seed production resulting to seed purity problems.

6. Hybrids require more doses of fertilizers for higher gain.

Future Strategies to Make Hybrid Rice Technology more Popular and Economical

Enhancing the hybrid rice seed productivity to reduce the seed cost:

a) Enhancing row ratio in three line system by proper technique of higher pollen dispersal. In India, a ratio of 2: 8 or 2: 10 (R: A) is followed. Seed production has been improved in China by increasing the ratio up to 2:16, which should be made possible in other countries too. The hybrid seed productivity should be enhanced atleast up to 2.5-3.0 tonnes ha-1 to make the availability of hybrid seed at cheaper rate.

) Adoption of two line system: It does not require maintainer line, which reduce the labour, expenditure and area requirement. Magnitude of heterosis in two line hybrid is also 5-10% higher than in three line hybrids as it does not have cytoplasmic penalty.

Enhancing heterosis:

a) We have to enhance the magnitude of heterosis in rice to make it more economical and income generating.

) Rao and Kulkarni (2004) found heterobeltiosis for grain yield in inter-subspecific (I/J) hybrids 25.2% and standard heterosis 56.8%, whereas in intra-subspecific I/I) hybrids 9.3% heterobeltiosis and 19.5% standard heterosis.

c) Vaithiyalingan and Nadarajan (2010) studied 42 inter and intra subspecific hybrids utilizing seven wide compatible varieties (WCVs) including two indica and five tropical japonica for nine biometrical characters including grain yield. For most of the characters, the mean heterosis per cent were in the order of indica/japonica F1 greater than Tropical japonica/indica F1 greater than indica/indica F1 greater than Tropical japonica/japonica F1. Breeding for Parental Diversification (A and R lines)

Single CMS line may create great threat in future; hence we should be ready to diversify the CMS sources. In India, all the hybrids developed till date is based on WA' CMS source. In Maize, Texas cytoplasm (CMS-T) became susceptible to Southern leaf blight epidemic caused by Drechslera maydis during 1970 in USA then other sources CMS-C and CMS-S began to be used for hybrid seed production. Rice hybrids also pose the risk of same vulnerability to disease and pests due to narrow genetic background of CMS sources. Considering this, search for new CMS sources and their integration in hybrid rice breeding is a major thrust in three line hybrid rice breeding strategy. Some new sources have been developed for diversification of male sterile cytoplasm in rice (Table 6). However, none of them have been found stable and effective as WA' cytoplasm. The D-type CMS has been used in China to develop a few popular hybrid rice varieties.

Breeding Super Hybrid Rice

At present super hybrid rice combinations developed in China have low seed setting rate, poor yield stability and weak adaptability, caused by their genetic disharmonies. Incorporation of the characteristics of high photosynthetic rate from other species into rice plants is of importance for future super hybrid rice breeding.

Table 7: List of identified genes and linked marker

S. Gene###Linked Marker###Recurrent Parent###Donor parent###End product###References

No.

Bacterial blight (BB)

1. Xa 21###pta248:Xa21###Minguhi63###IRBB21###BB resistance Improved hybrid###Chen et al.,2000

###(Restorer line)###developed

2.###xa13 and Xa 21###RG136:xa13###Pusa Basmati-1###IRBB55###Developed Improved Pusa Basmati-###Joseph et al.,2004 Gopala

###pta248:Xa21###1(Pusa 1460)###Krishnan et al., 2008

3.###Xa7 and Xa21###M1,M2,M3, M4###Minguhi63###Minguhi63 (Xa7) Improved version of Minguhi63 (Xa7)###Zhang et al., 2006

###and M5:Xa7###and###Minguhi63 and Minguhi63 (Xa21) developed

###pta248:Xa21###(Xa21)

4.###xa5,xa13 and Xa21###RG556:xa5###Sambha Mahsuri###SS1113###Developed BB resistance Improved###Sundaram et al., 2008

###RG136:xa13###Sambha Mahsuri

###pta248:Xa21

5.###xa13 and Xa 21###RG136:xa13###Pusa 6B and PRR78 Pusa 1460###Developed BB resistance Pusa1601 and Basavaraj et al., 2010

###pta248:Xa21###(Parental line of###Pusa1605

###PRH10)

6.###xa5, xa13 and Xa21 RG556:xa5###BPT5204###IRBB55###Developed BPT5204 with 3 gene Kottapalli et al., 2010

###RG136:xa13###IRBB13 IRBB21###combination

###pta248:Xa21###IRBB59

Bacterial blight + Restorer gene

7. Xa21,Rf3 and Rf4 pTA248:Xa21###KMR-3R###S1113###Developed BB resistance KRH2###Hari et al., 2011

###RM10313:Rf3

###RM25654:Rf4

Blast

8. Piz5 and Pi54###AP5930:Piz5###PRR78###C101A51###Developed Blast resistance PRR78###Singh et al., 2012

###RM206:Pi54###Tetep

Brown Plant Hopper (BPH)

9. Bph1 and Bph2###em24G:Bph1###Tsukushibare###PL4:Bph1###BPH Resistance pyramided ABL Sharma et al., 2004

###KPM2:Bph2###IR1154243:Bph2###developed

10. Bph25 Bph26###S00310:Bph25###Taichung 65###ADR52###Developed lines carrying Bph25 and Myint et al., 2012

###RM5479:Bph26###Bph26

Gall Midge

11. Gm-2###XRG476: Gm-2###Duokang###Duokang (Gm2)###Developed lines with both genes###Katiyar et al., 2001

###Gm-6(t)###Gm-6(t)###Phalguna###Phalguna (Gm6t)

Breeding for Transfer of Resistance for Biotic Stress in A and R Lines through Marker Assisted Selection

Marker assisted selection (MAS) refers to indirect selection for a desired plant phenotype based on the banding pattern of linked molecular (DNA) markers. Marker-assisted backcrossing is the simplest form of MAS, in which the goal is to incorporate a major gene from an agronomically inferior source (the donor parent) into an elite cultivar or breeding line (the recurrent parent). The desired outcome is a line containing only the major gene from the donor parent, with the recurrent parent genotype present everywhere else in the genome. Now this technique is being utilized to transfer the resistance genes for biotic stress into A and R lines to improve their overall genetic constitution which will ultimately improve the constitution of hybrid seeds produced thus making them more resistance towards stresses (Table 7).

Screening of Large Number of Restorer Lines through Marker Assisted Selection/Conventional Breeding

Molecular markers for R genes are now available to screen the full restorer lines hence, it is helping the breeders to screen all the genotypes in a lesser time as restorers and maintainers will ultimately help to develop new CMS lines as well as new genotypes. Restoration of WA-CMS in rice is controlled by two nuclear gene Rf3 and Rf4. The SSR marker RM1 is linked with Rf3 gene on the short arm of 1st chromosome (Ahmadikhah et al., 2007) and Microsatellite markers RM443 and RM315 were flanking Rf3 gene at a genetic distance of 4.4 and 20.7 cM on chromosome 1 respectively (Shah et al., 2012). The gene Rf4 was flanking by two SSR markers RM171 and RM6737 on the long arm of 10th chromosome at distance of 3.2 and 1.6cM, respectively (Shah et al., 2012). Sheeba et al. (2009) reported that Rf4, a major fertility restoration locus on Chromosome 10 was constructed and SSR marker RM6100 was observed to be very close to the gene at a distance of 1.2 cM.

The accuracy of the marker RM6100 in predicting fertility restoration was validated in 21 restorers and 18 maintainers. Revathi et al. (2013) reported that the SSR marker RM6100 linked to Rf4 gene on chromosome 10 and RM10313 linked to Rf3 gene on chromosome 1 showed eighty five and eighty one percentages, respectively. Though, conventional breeding takes more time and space to identify large number of genotypes even then it is still being used to identify the fertility restorer lines to be used for developing new hybrids in rice (Sharma et al., 2012).

Breeding for Different Agro-climatic Zones and Agro- ecosystem

The available hybrids are popular in the irrigated upland to medium lands. However, there is need to develop hybrids suited to rain fed lowlands as well for other situations also.

Hybrids for Longer Duration (140-150 Days) to Replace Longer Duration Mega Varieties like, MTU 7029 and BPT 5204 in India

Majority of the rice hybrids developed in India are generally of short duration which may not be compared for yield potential with the mega varieties of longer duration. Hence, there is an urgent need to develop longer duration hybrids in rice.

Hybrid for Quality Traits

Majority of the hybrids developed in India are lacking in good quality which is a major problem in large scale spread of hybrid rice in the country. Majority of the Indians prefer non-sticky cooked rice whereas most of the early hybrids are showing stickiness. Farmers get higher price of long slender aromatic varieties which is lacking in most of the hybrids developed in India. If hybrids with these quality traits are developed, it will certainly help the country in large scale adoption of hybrid rice technology. However, India has become the first country in releasing Basmati type of hybrid (PRH-10) in 2001 which became very popular in the country.

Conclusion

Rice is one of the most important staple foods. Hybrid rice exploits the phenomenon of heterosis which increases yield about 15-25% over HYVs. In case of rapidly increasing population and decreasing natural resources, hybrid rice is one of the most important and practically feasible technologies to achieve the targeted food grain production. Fast advances in molecular biology and biotechnology offer new hopes to design hybrid rice plants with higher yielding potential, better nutritional quality, resistance to biotic and abiotic stresses with higher nutrient and water use efficiency.Three types of male sterility (CGMS, EGMS, CHA's induced) have been used in hybrid rice seed production. India is using CGMS system (3 line system) for hybrid rice seed production. Improvement in hybrid rice seed production technology will further reduce the cost of hybrid rice seed. Hybrid rice seed production technology is both labour and knowledge intensive. This will also generate more and more employment.

The available hybrids are popular in the irrigated upland to medium lands. However, there is need to develop hybrids suited to rainfed lowlands as well as of longer duration to replace longer duration mega inbred varieties. Hybrids for meeting specific cooking qualities should also be developed to enhance the hybrid rice area in the country.

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