Wheat is one of the most important crops in the world. However, its production is being constantly hampered by abiotic environmental stresses such as heat stress. Three experiments were conducted in the present study to investigate the existence of genetic variation and to dissect the genetic control of heat tolerance at early plant growth stage of wheat.
A novel screening method was developed to assess the existing genetic variation for early stage heat tolerance and to identify heat tolerant genotypes. A total of 499 wheat genotypes were evaluated for heat tolerance by subjecting them to heat stress (35 ºC) and non-stress (25 ºC) conditions at the early seedling growth stage. Seedling length (SL) was measured to assess response of genotypes to heat stress. It was found that the SL at 25 ºC ranged from 7.8 cm to 41.9 cm, while SL at 35 ºC ranged from 0.4 cm to 29.9 cm, with mean SL reduced by an average of 48.5% at 35 ºC compared to that at 25 ºC. A damage index (DI) for each genotype was calculated to rank the genotypes, and the DI ranged from 0.98 to -0.28. Three hexaploid genotypes, Pakistan W 20B, Perenjori, and SST16, were ranked as extremely heat tolerant (EHT). Hexaploid genotypes Synthetic and Stiletto, along with two tetraploid genotypes, (T. turgidum ssp dicoccoides) G3211 and G3100 were found to be extremely heat susceptible (EHS). All other genotypes fell between these extremes. Genotypes from seven different geographic regions responded differently to heat stress. Asian genotypes displayed the highest tolerance to heat stress followed by genotypes from Africa, Europe, South America, the Middle East, North America and Australia.
Five genotypes including two heat tolerant genotypes (Tevere and W156), one moderately tolerant genotype (Chara) and two susceptible genotypes (Blanco Sin Barbillas and Cascades) were crossed in a half 5 × 5 diallel analysis. Analysis of variance of heat tolerance index (HTI) for the parents and their half diallel F2 progenies revealed the involvement of additive (to a higher degree) and non-additive genetic effects governing the inheritance of HTI. Susceptibility was dominant over tolerance for heat stress. The recessive alleles were concentrated in the two heat tolerant parents Tevere and W156. The analysis also revealed that both major and minor genes were involved in controlling the trait. High broad-sense (86.5%) and moderate narrow-sense (41.0%) heritabilities were observed for heat tolerance.
Quantitative Trait Locus (QTL) analysis was also conducted in a recombinant inbred line (RIL) population derived from a cross between two hexaploid wheat genotypes, Synthetic and Opata. A total of 78 RILs from the cross was assessed based on HTI at early seedling stage under environment controlled condition. The bread wheat Opata was more tolerant to heat stress (35 ºC) than Synthetic. Four major QTLs were detected for HTI, one (HTI3A) on chromosomes 3A, two (HTI3B.1, HTI3B.2) on 3B and the other one (HTI4D) on 4D. These QTL explained up to 88.4% of phenotypic variation.
An association analysis population (92 tolerant and 92 susceptible genotypes) was genotyped using four markers Xbarc217, Xgwm285, Xbarc84 and Xbarc197 that were the most closely linked simple sequence repeat (SSR) markers to the heat tolerance QTLs identified. A total of 16 different allelic combinations were found with mean diversity index (DI) values ranging from 0.82 to 0.14. The lowest DI (0.14) was recorded with genotypes that showed combinations of 102 bp (Xbarc217), 223 bp (Xgwm285), 123 bp (Xbarc84) alleles and an absence of the 136 bp (Xbarc197) allele.
This group included six heat tolerant hexaploid wheat genotypes, namely, AGT Scythes, Aus 17513, Machete, Opata, Perenjori and Rageni. The highest DI (0.82) was recorded in genotypes that presented only allele 136 bp and lacked alleles 102, 223 and 123 bp. This group included four heat susceptible hexaploid wheat genotypes (Chihuahuena, India 85, Red Bobs and Synthetic), two tetraploid wheat genotypes, (T. turgidum ssp dicoccoides G3211 and T. timopheevii ssp araraticum KU 8567), and three diploid genotypes of T. monococcum G 2847, G 2511 and Thoudar-2. The presence of the 223, 123 and 102 bp alleles across the 184 genotypes increased HTI by an average of 23.2, 20.0 and 17.0%, respectively, while the presences of the 136 bp allele reduced HTI by an average of 14.5%. This validation of the closely linked markers to the major QTLs across this diverse population is an indication for those markers to be useful for marker assisted selection (MAS) in wheat heat-tolerance breeding.
|Qualification||Doctor of Philosophy|
|Publication status||Unpublished - Feb 2016|