Genetic and Physiological Bases of Heat-Induced Floret Sterility in Wheat

Abstract

The global temperature is increasing at an alarming rate, which is a major concern for wheat growers due to the adverse effects of these temperatures on crop productivity. The current study focussed on the impact of brief episodes of high temperatures during the booting stage on floret fertility in wheat, using a combined approach of plant physiology and quantitative genetics. Wheat plants were exposed to a brief heat stress (3 days, 37/27 °C day/night) when the main tiller reached a particular auricle interval length (AIL) stage. Plants were then moved back into the greenhouse where they were evaluated at maturity for floret fertility and several other morphological and physiological traits. A total of 136 durum wheats genotypes from different part of the world (including Australia) and 26 hexaploid wheat genotypes were tested (Chapter 3) to identify heat-induced floret sterility tolerance. Both heat stressed hexaploid wheat and durum genotypes showed variability for several studied traits, including floret fertility components. The result from this experiment indicating the possibility for further genetic improvement of the wheat crop through selection and cross breeding. A total of 144 F1-derived double haploid (DH) lines developed from crosses between hexaploid wheat cvs. Drysdale and Waagan were used to identify QTLs for tolerance to heat-induced floret sterility. A total of 29 QTL were identified. Six of the 21 wheat chromosomes, namely 1B, 2B, 3B, 4B, 4D, and 7A, showed QTL for heat-induced floret sterility tolerance with individual QTL explaining between 6 and 49 % of the phenotypic variance (Chapter 4). Both parents contributed favourable alleles for heat tolerance. A region on chromosome 2B strongly affected heat responses of floret fertility and co-located with a locus controlling resistance to the yellow rust disease, with heat tolerance being coupled with rust resistance. Tolerance QTLs for two yield components (grain size and grain number) were independent, suggesting that breeders needed to apply selection for both of these traits when breeding for hot environments. The QTL detected on 2B may provide an appropriate target for fine mapping and marker assisted selection for improving heat tolerance in wheat. As the QTL for tolerance to heat induced floret sterility on chromosome 2B was so strong, the genotype for the tolerance locus could be confidently inferred in many of the Drysdale × Waagan DHs, allowing the locus to be placed as a single point on the genetic map (Chapter 5). Additional gene based markers in the region were designed using wheat genomics information assembled within the Diversity Among Wheat Genome (DAWN) tool, including the NRGene wheat cv. Chinese Spring genomic sequence. These markers were then scored on the DH lines. The QTL was thereby mapped to 9.1 cM interval on the genetic map, corresponding to a region of 31.5 Mb in the genome containing 203 predicted genes. Pairs of near isogenic lines (NILs; 32 lines total) were developed from eight heterogeneous inbred families for further field, physiological and molecular studies of the locus. The 2B tolerance QTL was located 21.2 cM from the centromere in a region of high recombination, which favours prospects for positionally cloning the gene. Experiments to identify the pollen developmental stage most sensitive to heat were conducted in Chapter 6. The intolerant cv. Drysdale was most sensitive at auricle interval lengths (AILs) of 7 cm to before 13 cm. These stages corresponded to early meiosis to late uni-nucleate microspore stage of pollen development. In Chapter 7, mechanisms of heat-induced floret sterility tolerance conferred by the chromosome 2B QTL, and its mode of expression, were investigated. Chromatin and pollen starch staining in the cvs. Drysdale and Waagan, and RIL families derived from these cvs., showed that intolerance from Drysdale was associated with failure of starch grains to accumulate starch, but mitotic divisions in the microspores were unaffected by heat treatment. Tolerance controlled by the chromosome 2B locus was expressed in a mainly dominant manner, and was sporophytic. However no major effect of the tolerance locus on female reproduction was detected. Implications of this information for breeding strategies and the identification of the underlying tolerance gene were discussed.