Characterization and inheritance of adult plant stem rust resistance in durum wheat.
The adult plant resistance gene Sr2 has remained effective since its transfer from the tetraploid Yaroslav Emmer to the bread wheat cultivar Hope in 1926 (McFadden, 1930). This gene remains the foundation of stem rust resistance durability in numerous bread wheat cultivars grown on millions of hectares across many nations each year (Rajaram, 1988). Therefore, great reliance is placed on the continued effectiveness of this gene in combination with other resistance genes (Rajaram, 1988).
Despite the fact that valuable genes have been derived from tetraploid wheat genotypes, limited research has been conducted on the stem rust resistances existing in wheats of this genome. Virtually all reports have considered seedling resistance, while none, in recent times at least, have examined adult plant resistance. Adult plant resistance in this paper is that resistance only evident during mature stages of plant growth from 39 on the Zadok's scale, as modified by Tattam and Makepeace (1979). This study investigates the character and inheritance of adult plant resistance to stem rust in two durum cultivars, Glossy Huguenot and Marouani.
MATERIALS AND METHODS
Four durum cultivars were selected for disease progress and genetic analysis following a screening of durum cultivars for seedling infection types (IT) and field adult plant reactions to a group of P. graminis tritici pathotypes. Glossy Huguenot (W304) and Marouani (W308) displayed adult plant resistance while Bansi Strain 168 (W990) and Russian 1364-3 (W831) were susceptible. All four cultivars give high ITs as seedlings (Table 1). Numbers preceded by "W" refer to the University of Sydney Wheat Accession Register.
Pathotypes used in seedling tests were selected on two criteria: (i) differential virulence-avirulence combinations to resistance genes known to occur in tetraploid wheat, and (ii) avirulence to all these genes. The six pathotypes were 21-0 (University of Sydney culture number 54129); 21-9 (71178); 116-2,3,7 (61352); 126-6,7,11 (334); 222-1,2,3,4,5,6,7, (70-L5); and culture 56-E1. The pathotype designation system used in Australia was described by Watson and Luig (1963) and updated in Luig and Watson (1977). Their respective virulence on the tetraploid derived genes are listed in Table 1. Cultivars which gave high ITs were assumed to carry no effective resistance to any members of the Australian P. graminis tritici flora.
Seedlings at the first leaf stage were sprayed with uredin-iospores suspended in a light mineral oil, maintained overnight in a moist chamber, then placed on glasshouse benches. Infection types were scored 12 to 14 d after inoculation and were based on a zero-to-four IT scale described by Stakman et al. (1962).
Urediniospores of pathotypes, injected into stems of susceptible spreader plants at the booting stage for the field evaluation in 1972 and 1973 were 21-2,3,4,5,7 (68016); 34-2,4,5,7,11 (64231); 34-1,2,3,6,7 (66-L2), and 222-2,3,7,8 (71107). These pathotypes, among the most virulent variants then available, were placed in the field primarily for the evaluation of breeding materials in adjoining field trials, at the Plant Breeding Institute, University of Sydney, Castle Hill. Test plots were naturally infected with inoculum derived from the susceptible buffers. Cultures 66-L2 and 222-1,2,3,4,5,6,7 (70-L5) were developed by mutagenesis in the laboratory at the Plant Breeding Institute.
Characterization of Adult-Plant Reaction
In 1972, Bansi Strain 168, Marouani and Russian 1364-3 were grown in three replications of small plots (3 rows 15 cm apart by 3 m long). To hinder inter-plot effects, a stem rust and leaf rust (Puccinia recondita Rob. ex. Desm. f. sp. tritici) resistant breeding selection, Tr308, was sown solidly between plots (1 m around plots). The experimental area was surrounded by inoculated susceptible buffers.
The 1973 experiment consisted of Glossy Huguenot with the above cultivars sown in four replications of small isolated plots (1 row by 3 m with 1-m free ground surrounding each plot). The resistant buffer was considered unnecessary. The [TABULAR DATA FOR TABLE 1 OMITTED] experimental area was enclosed by inoculated susceptible breeding materials.
Leaf rust was selectively controlled in both years by the systemic fungicide, RH124 (Rohm and Haas Co., Philadelphia, formulation number), which contained the active ingredient 4-n-butyl-1,2,4,-triazole. This chemical was applied at the rate of 2 kg per hectare by aqueous spray to plants and soil at the four-leaf stage before leaf rust was present. Soil moisture was maintained near field capacity by overhead irrigation to allow for the continued renewal of a protective dose into the plant. Protection of fully susceptible cultivars was nearly complete (maximum of 2% disease cover at flag leaf senescence). The active ingredient showed no activity towards stem rust.
Following the first appearance of stem rust each year, percentage uredinia covering the penultimate internode on each of ten random tillers in each plot was estimated weekly. A disease assessment scale developed by James (1971) was used. On this scale, the percentage of infection represents the actual area of the stem covered by the stem rust uredinia. Previous experience had suggested that the penultimate stem and sheath sections remained viable and infective over the longest period of the epiphytotic, thus offering the greatest opportunity for disease development and consequent discrimination between cultivars.
The completion date of anthesis was recorded for each cultivar. A disease index for each assessment date was calculated by summing the individual estimates for the penultimate internode within a plot. The mean disease intensity index and its standard deviation was then calculated from the three replications.
Glossy Huguenot and Marouani, both adult plant resistant, were intercrossed, and each was crossed to the adult plant susceptible cultivars Bansi Strain 168 and Russian 1364-3. Plants from the [F.sub.1] generation, [F.sub.3] families (offspring of individual [F.sub.2] plants), and [F.sub.3] plants from segregating [F.sub.2] families were exposed to a field rust epiphytotic at the Tamworth Centre for Crop Improvement in 1978 and 1979. The [F.sub.3] families were grown as randomized replicated plots (20-30 plants per 1-m single row, two replications) in each year. All experimental plots were naturally infected from susceptible border rows of 'Morocco' (W2145). An aqueous urediniospore suspension of pathotype 343-1,2,3,5,6 (common virulent field pathotype) was hypodermically injected into elongating culms within the borders. The optimal time period for scoring the adult-plant reactions extended from the milk to early dough stages of grain development (Hare and Mcintosh, 1979).
Adult Plant Reactions of Parental Cultivars
Epiphytotic progress curves were constructed by plotting the mean disease intensity indices against time (days), starting from the first appearance of stem rust on Bansi Strain 168 and Russian 1364-3 [ILLUSTRATION FOR FIGURE 1 OMITTED]. The vertical axes in Fig. 1 were scaled differently for susceptible (disease intensity of 0-250) and resistant cultivars (0-10) to amplify the low level of disease development on the adult-plant resistant cultivars. Disease development was dependent on maturity. The late-maturing cultivars were exposed to greater inoculum loads at higher temperatures and longer day lengths than the earlier maturing cultivar, Bansi Strain 168; hence, the epiphytotic progress curve for Bansi Strain 168 was cautiously compared with those of the other three cultivars.
In addition, Bansi Strain 168 was very susceptible to powdery mildew (Erysiphe graminis DC. f. sp. tritici E. Marchal) (approximately 25% leaf area covered; James, 1971) while the other cultivars were only slightly infected (approximately 1-2% leaf area covered) in both years. The stem and sheath areas occupied by powdery mildew lesions may have suppressed the level of stem rust infection on Bansi Strain 168 but only to a minimal extent on the other cultivars.
Cultivars grown over both years behaved similarly with respect to the development of stem rust. Despite maturing 25 d before the other cultivars, Bansi Strain 168 was extremely susceptible to stem rust (maximum or terminal rust intensity: 1972, 215; 1973, 87). Russian 1364-3 was susceptible (1972, 55; 1973, 200) while Glossy Huguenot (1973, 10) and Marouani (1972, 1; 1973, 1) were adult plant resistant.
Genetic Analysis of Adult Plant Resistance
Crosses between the resistant parents Glossy Huguenot and Marouani and the susceptible parents Bansi Strain 168 and Russian 1364-3, produced resistant [F.sub.1] progeny, indicating that resistance was dominant. The results from [F.sub.3] family responses are summarized in Table 2. In each of the four crosses, [F.sub.3] family segregation conformed satisfactorily with ratios expected on the assumption that dominant alleles at a single locus conditioned resistance. The results of families tested in two years generally showed good agreement. Combined results from the two years were used in the chi-square analysis. Of 727 families tested in 1978, the classifications for 12 did not agree in 1979. All were classified as homozygous resistant in one year and segregating in the other. Since resistance was dominant, it is possible that plot samples were not of sufficient size to include a susceptible plant in such plots. Therefore, each of these 12 families was included in the segregating class for analysis. Examples of parental and [F.sub.3] plant reactions are illustrated in Fig. 2.
Between 15 and 20 [F.sub.3] plants from segregating [F.sub.2]-derived families in Marouani/Russian 1364-3 and Glossy Huguenot/Russian 1364-3 were individually classified for adult plant reaction in 1979 (Table 3). All results from crosses of resistant by susceptible parents indicated that resistance in each case can be explained on the basis of dominant alleles at a single locus.
The cross, Glossy Huguenot/Marouani, produced resistant [F.sub.1] plants while 50 [F.sub.3] families were all uniformly resistant. If rust resistance was determined by different loci in each parent, and assuming that the population was of insufficient size to include a segregating or homozygous susceptible family through sampling chance, then [TABULAR DATA FOR TABLE 2 OMITTED] at P = 0.05, the maximum recombination value (r) for these loci is given by the expression (Hanson 1959):
2r - 3[r.sup.2]/2 = 1 - [(0.05).sup.1/50],
therefore, r = 3% (0.03).
The adult plant resistance displayed by Glossy Huguenot and Marouani significantly reduced the number of stem rust pustules per unit area and delayed the onset of disease when compared with the susceptible controls, Bansi Strain 168 and Russian 1364-3. These pustules were similar in morphology to those on the susceptible cultivars, and occurred at any location on the exposed penultimate stem sheath. This type of disease development is indicative of "slow rusting," or "late rusting," depending on the criteria used in scoring it relative to susceptible controls. The adult plant resistance could not be detected at the seedling stage, despite the use of pathotypes specifically selected for avirulence. Bolat and Roelfs (1991) were also unable to show seedling resistance in Glossy Huguenot to a range of North American stem rust pathotypes. These authors chose pathotypes with differential avirulence to known tetraploid resistance genes (Sr9d, Sr9g, Srdp-2). Bansi Strain 168 and Russian 1364-3 exhibited no detectable resistance at any growth stage; consequently, these cultivars were useful susceptible parents in genetic studies.
Genetic analyses of crosses involving resistant by susceptible parents indicated that adult plant resistance was determined by dominant alleles at a single locus in each parent. All progeny derived from a cross between the resistant parents Glossy Huguenot and Marouani displayed adult plant resistance. Failure to detect a single susceptible plant from 50 intercross [F.sub.2] progeny families suggested that the gene in question is probably common to each parent; however, a two gene hypothesis could not be excluded. A maximum likelihood analysis gave a maximum recombination value of 3% for two resistance genes. A lack of observable variation across progenies in the expression of adult plant resistance and infection type reaction (i.e., occasional compatible pustules) supported the common gene hypothesis.
The resistant parents may share a common ancestral Algerian parent, the putative source of the resistance gene. Glossy Huguenot is believed to be a non-glaucous [TABULAR DATA FOR TABLE 3 OMITTED] variant derived from the Australian durum cultivar Huguenot, because it resembles Huguenot in many other morphological features. Huguenot is an awnless selection from the awned cultivar Medeah, an introduction from Algeria (Sutton, 1910). Marouani is an Algerian durum cultivar (Zeven and Zeven-Hissink, 1976). Both Medeah and Huguenot, reported to be "rust resistant" (Valder, 1900), were cultivated widely from the 1890s to 1922 in Western Australia, South Australia and New South Wales. "Rust resistance" in this context most probably refers to stem rust resistance, a prevalent wheat disease in Australia during this period. The adult plant resistance in Huguenot has remained effective even though it could have been challenged by the stem rust pathogen on a number of occasions. Huguenot and susceptible breadwheat cultivars were propagated frequently and concurrently over small individual areas along the warm humid coastal plains of northern New South Wales; hence, viable inoculum should have been present to infect the durum (Valder, 1900). Huguenot is still grown on a small scale in the Barossa Valley, South Australia, where it remains resistant to stem rust. While the adult plant resistance of Glossy Huguenot and Marouani has not been extensively exposed to Australian stem rust pathotypes within a commercial crop situation, it appears to have remained effective for the past 90 yr. This performance suggests the possibility of the presence of a durable resistance.
Even though the Sr2 gene was derived from the tetraploid wheat Yaroslav Emmer, it is unlikely to be the same gene as that from Glossy Huguenot and Marouani. Two lines of evidence suggest genetic and physiological difference. Pustules on plants carrying Sr2 tend to be distributed above the stem nodes and are often surrounded by necrotic tissue, while pustules on Glossy Huguenot and Marouani derivatives are randomly spread and without necrosis (Hare and Mcintosh, 1979). Also, Sr2 resistance, although at the hexaploid level, is conditioned by recessive alleles, in contrast to the dominant Glossy Huguenot and Marouani resistance (Hare and Mcintosh, 1979). The Glossy Huguenot and Marouani gene when transferred to the hexaploid level should provide a valuable and potentially durable adult plant resistance and a supplement to Sr2.
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