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Exogenous Application of Signaling Compounds Enhances Rice Allelopathic Potential in Rhizosphere Soil.

Byline: Khalid Mahmood, Muhammad Bismillah Khan, Muhammad Ijaz, Habib Ullah, Athar Mahmood6, Yuan Yuan Song, Shi Ming Luo and Ren Sen Zeng

Abstract

In this study, allelopathic induction in rhizosphere after exogenous application of signaling compounds methyl jasmonate and methyl salicylate was evaluated. Post applications of signaling compounds to rice seedlings imposed allelopathic affect on tester plant species barnyard grass (Echinocloa cress-galli L.) and lettuce (Lactuca sativa L.). Two rice cultivars with contrasting allelopathic abilities were evaluated through rhizosphere soil method, aqueous leaf and root leachates bioassays. Rhizosphere soil of high allelopathic rice cultivar (BR-41) reduced the shoot length by 35-38% and root length by 52-53% of lettuce seedlings after methyl jasmonate and methyl salicylate application, respectively, while lower allelopathic rice cultivar (Huanjingxian) wane down the lettuce shoot length (25 and 27%) and root length 31 and 36%, respectively.

The aqueous leaf and root leachates of both rice cultivars also imposed inhibition on germination and seedling growth of barnyard grass after signaling compounds treatment. Transcriptome analyses of genes responsible for momilactone biosynthesis (OsCyC1) were also up regulated. The relative expression of OsCyC1 was higher over basal levels present in control plant leaves. OsPAL is another key gene involved in phenolic compound biosynthesis and found up regulated after signaling compounds treatments. The maximum up regulation of OsPAL after methyl jasmonate and methyl salicylate application was 10 and 12 times higher at 12 h, respectively as compared to control plants. Exogenous application of signaling compounds can induce allelopathic potential for effective weed control in crop plants. (c) 2013

Friends Science Publishers

Keywords: Allelopathy; Root exudates; Methyl jasmonate; Methyl salicylate; Rhizosphere soil

Introduction

Methyl jasmonate and methyl salicylate are important signaling compounds and have pronounced affect on plant growth and development (Smith et al., 2009; Gimenez- Ibanez and Solano, 2013). These signaling molecules mediate plant defense responses through induction of secondary metabolites in plants (Reymond and Farmer, 1998; Van Wees et al., 2000; Smith et al., 2009). The jasmonates and salicylates function as key signal molecules in plant defense (Kessler and Baldwin, 2002). These signal molecules act as elicitors for production of plant secondary metabolites (Chen et al., 2006). The jasmonate and salicylate increase expression pattern of defense related genes (Thomma et al., 1998) and regulate plant secondary metabolism (Zhao et al., 2005). Jasmonate can stimulate momilactone: a major phytoalexin and possible phytotoxic agent in rice (Nojiri et al., 1996), which is involved in resistance in plant species against insect herbivores (Baldwin, 1998).

Jasmonate, salicylate and their methyl esters are the naturally occurring regulators of higher plants and can induce endogenous level of secondary metabolites after exogenous application (Bi et al., 2007).

Weeds are important constraints in agricultural production systems (Peltzer et al., 2009; Bennett et al., 2011). In recent years, agricultural yield has been increased but at the cost of our environment (Tilman et al., 2002; Baumgartner et al., 2005). Many reports emphasized on herbicide resistance in plants through activation of specialized enzymes (Macias et al., 2004; Arias et al., 2005; Owen and Zelaya, 2005; Funke et al., 2006; Gressel, 2009; Busi et al., 2011). Allelopathy has increasingly received attention by scientists and has played important roles in plant biodiversity and sustainable agriculture (Chung et al., 2006; Zeng et al., 2008). Allelopathy provides an alternative for weed management (Macias et al., 2004; Macias et al., 2007).

Allelopathy has been considered an environmental friendly approach for weed control in rice (Oryza sativa L.) production (Olofsdotter et al., 2002). Plants can release allelopathic compounds into the environment through root exudation, leaching by dews and rains, volatilization, or decaying plant tissue (Rice, (1984). The rice allelopathy and weed control has been the focus of research (Dilday et al., 1994; Olofsdotter et al., 1999; Ebana et al., 2001; Khanh et al., 2007; Jensen et al., 2008). A number of secondary metabolites were isolated and identified as allelochemicals in rice (Chou et al., 1991; Inderjit 1996; Mattice et al., 1998; Seal et al., 2004a, b; Kato-Noguchi et al., 2007). The momilactones has shown to be constitutively secreted from the roots of rice seedlings (Kato-Noguchi and Ino, 2003) as an allelopathic agent. The genetic markers associated with allelopathic activity in rice has been identified (Ebana et al.,2001; Jensen et al., 2008).

There are several ways to induce rice allelopathic potential such as incorporating allelopathic character into desired cultivar and through induction of physiological processes that can impose allelopathic effect. Enhancing these traits in crop plants may also increase their resistance to insects and soil microorganisms. Improving the allelopathic ability of rice through genetic engineering provides an alternative for weed management but have had limited success and no rice cultivar released yet for commercial farming. The exogenous application of signaling molecules elevate allelopathic potential and researchers are working on these lines (Bi et al., 2007; Kato- Noguchi, 2011; Mahmood, 2013).

Plants attain a range of mechanisms to combat invasion of other plants. These mechanisms include pre- existing physical and chemical barriers as well as inducible defense responses that become activated upon exogenous application of signaling compounds. Allelochemicals are secreted to the rhizosphere and suppress the growth of neighboring plants (Bais et al., 2004) but still there is no evidence that methyl jasmonate and methyl salicylate can induce allelopathic potential strong enough to impose inhibition in soil condition. This study was conducted to establish whether allelopathic potential of rice can be induce in the rhizosphere. For this purpose, rhizosphere soil method and expression analysis of genes responsible for biosynthesis of allelochemicals after defense compounds treatment were used to evaluate allelopathic induction.

Materials and Methods

Plant Material

Two rice (Oryza sativa L.) cultivars Huajingxian and BR-41 with different known allelopathic potential were used. Rice cultivar BR-41 with known allelopathic potential (Kim et al., 2005) was obtained from International Rice Research Institute, Los Banos, Philippines and Huajingxian, China, a low allelopathic cultivar (Bi et al., 2007) was provided by Prof. Zhi-Qiang Chen at the South China Agricultural University. Barnyard grass (Echnicloa cruss-galli L.) seeds were collected from the rice field of South China Agriculture University, Guangzhou, China, and lettuce seeds purchased from local vegetable market of Guangzhou. Methyl jasmonate and Methyl salicylate were purchased from Sigma (St. Louis, MO, USA). TRIzol reagent, AMV reverses transcriptase, Taq polymerase, and deoxynucleotide triphosphates were purchased from TaKaRa (Shuzo Co. Ltd., Shiga, Japan). MOPS and DEPC were bought from AMRESCO (Solon, OH, USA).

Growth Conditions

Rice seeds were surface-sterilized with 1% NaClO for 30 min, washed with distilled water several times and pre- germinated in petri-dishes for 3 days on moistened filter paper. All these and further manipulations were carried out under sterilized conditions. After three days, 20 germinated seeds were transplanted in each plastic pot (20x25 cm) filled with soil. These were raised in a growth chamber for forty days at 24-26degC with 150 mMd m-2 sec-1 light and a photoperiod of 12-h light/12-h darkness.

The pots were watered and fertilized with Hoagland nutrient solution every alternate day. A 5 mL each of methyl jasmonate (0.05 mM) and methyl salicylate (5 mM) were applied by brushing on rice leaves and were grown further for ten days in the growth chamber for bioassay studies.

Bioassays

Rhizosphere soil: After methyl jasmonate and methyl salicylate application on rice leaves; plants were carefully removed from the pots without disturbing the soil. Plant roots were shaken softly to separate/collect the adhering soil from the surroundings of the root, called rhizosphere soil. Collected soils from different treatments were sieved in a 1 mm mesh to remove root hair and other organic debris as much as possible. This rhizosphere soil was put into agar (5%) and cooled at 42oC. Ten mL of prepared agar along with rhizosphere soil added into multi-dish. The five lettuce seeds were placed on the agar culture media and grown in dark condition to avoid light effect on germination (Mahmood et al., 2013). The lettuce seeds were also sown in non-rice rhizosphere soil as control. Three replicates were maintained for each treatment and experiment was repeated four times. Root and shoot lengths of the lettuce seedlings were measured 3rd day after their growth.

Aqueous leachates: Aqueous extracts were prepared by extracting 10 g of treated rice leaves with 100 mL of distilled water for 24 h Extracts were filtered through filter paper and stored at 4degC until used. Barnyard grass (Echinochloa crus-galli) seeds were grown on the aqueous extract of rice leaves. Root and shoot lengths of the E. crus- galli seedlings were determined after 7 d. Three replicates were maintained for each treatment. Experiment was repeated three times.

RNA Isolation and Quantitative Polymerase Chain Reaction

Total RNA was isolated from rice leaves after signaling molecules treatments using Trizol reagent (Invitrogen, Carlsbad, CA, USA). For real time-PCR analysis, first- strand cDNAs were synthesized from DNaseI-treated total RNA using SuperscripII reverse transcriptase (Invitrogen) according to the manufacturer's instructions. Real time PCR was performed in an optical 96-well plate with an ABI PRISM 7500 Real-time PCR System (Applied Biosystems, Foster City, CA, USA). Each reaction contained 10 uL 2x SYBR Green Master Mix Reagent (Applied Biosystems), 1.0 uL cDNA samples and 200 nM of gene-specific primer in a final volume of 20 uL. The thermal cycle was used as follows: 95oC for 3 min; 39 cycles of 94oC for 45 sec, 58oC for 30 sec, 72oC for 15 sec and 82oC for 10 min. The degenerate primer was used for amplification of putative genes. Actin gene was used as internal control with primers. The sequences of genes specific primer were presented below (Table 1). The relative expression levels were determined.

Statistical Analysis

SAS 8.0 (SAS Institute, Cary, North Carolina) package for windows was used for statistical analysis. The data were analyzed with a one-way analysis of variance with treatment differences among means tested using Tukey test at P0.05.

Results

Rhizosphere Soil

Exogenous applications of methyl jasmonate and methyl salicylate to rice shoot increased inhibitory effect on neighboring plants. Rhizosphere soil of rice plants of Huajingxian and BR-41 cultivars after exogenous application of signaling compounds inhibited the lettuce shoot length as compared to untreated rice plants. In case of BR-41, 35 and 38% inhibition of lettuce shoot was recorded, while even Huajingxian imposed 25 and 27% inhibitory effect on lettuce shoots length after treatment of signaling compounds (Table 2). The inhibitory effect on lettuce root length after signaling molecules application was very obvious. BR-41 caused 52 and 53% inhibition in lettuce root length as compared to untreated rice root adhering soil, while inhibitory effect of Huajingxian was 31 and 36%, respectively as compared to non-rhizosphere soil. After application of methyl jasmonate and methyl salicylate, rhizosphere soil (Table 2). of BR-41 reduced the germination% of lettuce plants.

From these results, it's inferred that signaling compounds can increase rice root exudates secretion into rhizosphere soil that impose inhibition on lettuce seedling growth.

Leaf Leachates

Aqueous leaf leachates of both the rice cultivars (high allelopathic and low allelopathic) inhibited barnyard seedling (Echincloa cruss-galli L.) growth after treatment of methyl jasmonate and methyl salicylate. Aqueous leaf leachates of BR-41 wane down the shoot (31% each), root lengths (49 and 56%), germination% (13 and 17), fresh weight (36 and 39%) and dry weight (36 and 40%) of barnyard grass as compared to control. Low allelopathic rice variety (Huajingxian) also caused inhibition to barnyard grass growth after exogenous application of signaling compounds. Huajingxian reduced the shoot (28 and 30%), root lengths (46 and 47%), germination (10% each), and (35% each) and dry weight (39% each) of barnyard grass as compared to control plants (Table 3).

Root Leachates

Aqueous root leachates of both rice cultivars also imposed inhibition on barnyard grass growth but inhibitory effects are less compared to aqueous leaf leachates. Aqueous root leachates of BR-41 reduced the shoot length (23% each), root length (40% each), germination (10%), fresh weight (36 and 38%) and dry weight (36 and 40%) of barnyard grass after methyl jasmonate and methyl salicylate treatment, respectively. Low allelopathic rice cultivar also caused reduction in barnyard grass growth after signaling compound exposure. Huajingxian root leachates wane down the shoot length (17 and 18%), root length (25 and 26%), germination (3 and 7), fresh (17 and 23%) and dry weight (25 and 32%), respectively.

Expression Analysis of OsCYC1 and OsPAL

Signaling compounds can activate the genes responsible for momilactone biosynthesis and phenolic compound accumulation in rice leaves (Table 4). We found that expression pattern of OsCYC1 and OsPAL was dramatically up regulated when analyzed through quantitative or real time polymerase chain reaction (QRT-PCR) after methyl jasmonate and methyl salicylate treatment. Exogenous application of methyl salicylate induce the relative expression pattern of OsCyC1 as 4, 12, 3.5 and 2 fold higher after 6, 12, 24 and 48 h, respectively as compared to control plants (Fig. 1A). Methyl jasmonate application on rice leaves also activates OsCYC1 in the same manner. The up regulation of OsCYC1 was 2, 14, 10 and 6 times higher after 6, 12, 24 and 48 h, respectively over the basal levels present in control leaves (Fig. 1B). The OsPAL is a key gene involved in phenolic compounds biosynthesis. Exogenous application of Methyl salicylate

Table 1: Sequence of the genes specific primer used for Real-Time QRT-PCR

Gene###Accession Number###Specific Primer

OsCYC1###AB066270###F: 5'- gaggagatagaccagcaagtgga-3'

###R: 5'-tgagcagtaggcgacatagca -3'

OsPAL###X16099###F: 5-cacaagctgaagcaccaccc-3

###R: 5-gagttcacgtcctggttgtg-3

Actin###X15865###F: 5'-ctgacggagcgtggttac-3'

###R: 5'-ggaaggcgggaagaggac-3'

Table 2: Effects of rhizosphere soils of two rice cultivars on seed germination and seedling growth of lettuce after methyl jasmonate (MeJA) and methyl salicylate (MeSA) application

Observations###Huanjingxian###BR-41###Non rhizosphere soil

###Control###MeJA###MeSA###Control###MeJA###MeSA

Shoot length (cm)###3.3+-0.06ab###2.3+-0.09c###2.3+-0.09c###3.4+-0.03a###2.0+-0.04d###1.9+-0.10d###3.1+-0.07b

Root length (cm)###1.4+-0.02ab###0.9+-0.05c###0.8+-0.05c###1.4+-0.06a###0.6+-0.02d###0.6+-0.01d###1.3+-0.04b

Germination (%)###100+-0.0a###100+-0.0a###100+-0.0a###100+-0.0a###85+-5.77b###95+-3.33ab###100+-0.0a

Data are expressed as means +-SE values (n=3) followed by the same letter are not significantly different according to Tukey test (P less than 0.05)

Table 3: Effects of aqueous leaf leachates of two rice cultivars (Huanjingxian and BR-41) exposed to methyl jasmonate (MeJA) and methyl salicylate (MeSA) on seed germination and seedling growth of barnyard grass (Echinocloa cruss-galli L.)

Observations###Huanjingxian###BR-41###Control (water)

###Control###MeJA###MeSA###Control###MeJA###MeSA

Shoot length (cm)###6.2+-0.14ab###4.8+-0.23c###4.7+-0.14c###5.8+-0.18b###4.6+-0.21c###4.6+-0.20c###6.6+-0.05a

Root length (cm)###3.8+-0.09b###2.6+-0.15c###2.6+-0.14c###3.3+-0.13b###2.5+-0.25c###2.3+-0.12c###4.9+-0.32a

Germination %###97+-3.33 ab###90+-5.77abc###90+-3.33abc###93+-3.33abc###87+-3.33bc###83+-3.33c###100+-0.0a

Fresh weight (g)###0.39+-0.01b###0.30+-0.01c###0.30+-0.01c###0.35+-0.02b###0.30+-0.01c###0.29+-0.01c###0.47+-0.02a

Dry weight (g)###0.04+-0.004ab###0.03+-0.001cd###0.03+-0.001cd###0.04+-0.003bc###0.03+-0.005cd###0.03+-0.002d###0.05+-0.002a

Data are expressed as means +-SE values (n=3) followed by the same letter are not significantly different according to Tukey test (P less than 0.05)

Table 4: Effects of aqueous root leachates of two rice cultivars (Huanjingxian and BR-41) exposed to methyl jasmonate (MeJA) and methyl salicylate (MeSA) on seed germination and seedling growth of barnyard grass (Echinocloa cruss-galli L.)

Observations###Huanjingxian###BR-41###Control (water)

###Control###MeJA###MeSA###Control###MeJA###MeSA

Shoot length (cm)###6.1+-0.10b###5.5+-0.28c###5.5+-0.18c###5.9+-0.038bc###5.2+-0.15d###5.1+-0.17d###6.7+-0.05a

Root length (cm)###4.0+-0.03b###3.6+-0.12c###3.5+-0.18c###3.4+-0.13c###2.9+-0.11d###2.8+-0.18d###4.8+-0.22a

Germination (%)###100+-0.00a###97+-3.33a###93+-3.33a###100+-0.00a###90+-5.77a###90+-5.77a###100+-0.00a

Fresh weight (g)###0.42+-0.01b###0.38+-0.01c###0.36+-0.02c###0.35+-0.01c###0.30+-0.02d###0.29+-0.01d###0.46+-0.01a

Dry weight (g)###0.05+-0.002a###0.04+-0.001b###0.03+-0.001c###0.04+-0.003b###0.03+-0.003c###0.03+-0.002c###0.05+-0.001a

Data are expressed as means +-SE values (n=3) followed by the same letter are not significantly different according to Tukey test (P less than 0.05)

activate the expression of OsPAL as 4, 10, 5 and 3 fold higher after 6, 12, 24 and 48 h, respectively over the basal levels present in control leaves (Fig. 2A). After Methyl jasmonate application corresponding gene activation was 7, 12, 5 and 4 times higher at 6, 12, 24 and 48 h, respectively as compared to control rice plant leaves (Fig. 2B).

Discussion

Plants are subjected to various biotic and abiotic stresses and adapt changing climate through inducing internal mechanisms (Atkinson and Urwin, 2012). Plant chemical defense against herbivores and pathogens are inducible and regulated by both the jasmonate and salicylate signaling pathways (Thaler et al., 2002). This study found that rice phytotoxic effect on barnyard grass and lettuce seedling could be induced through methyl jasmonate and methyl salicylate. The phytotoxicity of the root exudates of rice at higher plant densities increased after treatment with methyl jasmonate and methyl salicylate at natural soil conditions evaluated through rhizosphere soil method. Aqueous extracts of both leaves and roots of methyl jasmonate and methyl salicylate treated Huajingxian and BR-41 rice cultivars showed enhanced inhibitory effects on barnyard grass root growth. Exogenous addition of jasmonate increases the resistance of wild plants to insects in the field (Baldwin, 1998).

Many studies have shown that signal compounds can induce plant defense mechanism against insect herbivores and microbial pathogens but lack of information available to demonstrate that signaling pathways are involved in allelopathy. A paper has been

Fig. 1: Relative expression of gene transcriptome of OsPAL (A) and OsCYC1 (B) of rice cultivar BR-41 at 4 leaf stage after exogenous application of methyl jasmonate. Data are expressed as means +-SE values (n=3) followed by the same letter are not significantly different according to Tukey test (P less than 0.05) published by Bi et al. (2007) demonstrated that signaling compounds can induce rice allelopathic potential but still no evidence that emphasize on rice allelopathy in soil conditions.

This study suggests that signaling compounds up- regulate gene transcription of OsPAL and resulting in increased biosynthetic activities and accumulation of phenolic acids, which likely lead to enhanced phytotoxicity. We found that methyl jasmonate and methyl salicylate treated rice plants showed up-regulation of OsCyC1, which is a key gene for momilactone biosynthesis. The momilactone is key compounds for allelopathy of rice (Kato-Noguchi et al., 2007). Exogenous application of signaling compounds up-regulate gene transcription of OsCyC1, resulting in increases biosynthetic activities and production of momilactone, which likely lead to enhanced allelopathic potential. The findings are ecologically significant in a sense that jasmonate and salicylate signaling pathways may control the production and release of phytotoxins (Dixon et al., 2002).

Thus, plants may respond to competition or stress by activating physiological mechanisms to release chemicals that interfere with a neighbor's growth (Martin et al., 2003).

In crux, signaling compounds can enhance synthesis and accumulation of secondary metabolites in rice rhizosphere that resulted in rice phytotoxicity and its allelopathic potential (Metlen et al., 2009). We are reporting for the first time, exposure to signaling compounds to one Fig. 2: Relative expression of gene transcriptome of OsPAL (A) and OsCYC1 (B) after exogenous application of methyl salicylate to BR-41 rice cultivar at 4-leaf stage. Data are expressed as means +-SE values (n=3) followed by the same letter are not significantly different according to Tukey test (P less than 0.05)

plant species that causes inhibition on the growth of neighboring plants in natural soil conditions, although at higher plant density. Plant roots recognize and respond to the identities of their neighbors and changes in root ecology on exogenous application of methyl jasmonate and methyl salicylate may affect root neighbors.

Acknowledgements

This research was financially supported by the National 973 project of China (2011CB100400), National Natural Science Foundation of China (31070388, 31028018, 30870390), Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2010), Guangdong Science and Technology Plan Project (2008A030101008, 2008B021500001) and Ph.D. Programs Foundation of the Ministry of Education of China (20104404110004) and Ministry of Education Pakistan.

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State Key Laboratory of Conservation and Utilization of Subtropical Agricultural Bio-resources, South China Agricultural University, Wushan, Guangzhou 510642, P.R. China

Institute of Tropical and Subtropical Ecology, South China Agricultural University, Wushan, Guangzhou 510642, P.R. China

Department of Agronomy, Bahauddin Zakariya University, Multan, Pakistan

Department of Agronomy, The Islamia University of Bahawalpur, Pakistan

Department of Horticulture, Bahauddin Zakariya University, Multan, Pakistan

University College of Agriculture, University of Sargodha, Pakistan

Faculty of Life Science, University of Copenhagen, Denmark

For correspondence: khalid@life.ku.dk; ijazhi@yahoo.com
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Author:Mahmood, Khalid; Khan, Muhammad Bismillah; Ijaz, Muhammad; Ullah, Habib; Mahmood, Athar; Yuan Yuan S
Publication:International Journal of Agriculture and Biology
Article Type:Report
Geographic Code:9PAKI
Date:Dec 31, 2013
Words:4611
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