Genetic mapping and physiological analysis of heat tolerance in wheat

Abstract

High temperature induced loss of wheat production is a global phenomenon and posing a threat to food security. This study is focused on the genetic mapping of heat tolerance traits of wheat to understand the genetics and underlying mechanism of heat tolerance at reproductive stages. A total of six varieties of varying heat tolerance for floret fertility and grain filling were tested for the effects of heat applied at 5 cm auricle interval (AI) and 10 days after anthesis (DAA) tiller stages in the presence and absence of shallow standing water to investigate the potential effects of standing water on the responses to this heat treatment protocol (Chapter 2). Heat reduced grain number, chlorophyll content and single grain weight in heat susceptible varieties but not in the tolerant genotypes. No additional effect of standing water was observed therefore keeping plant pots in standing water could be considered as safe watering method for this heat tolerance screening protocol. In Chapter 3, 34 homozygous NILs from nine families contrasting for two grain filling and chlorophyll heat tolerance QTLs, QHsgw.aww-3B and QChlr13.aww-6B, were phenotyped to study effects of the QTLs on heat tolerance. Heat treatment reduced single grain weight in three lines containing the Drysdale (intolerance) allele at the 3B locus, relative to their corresponding tolerant sibling lines, while no significant effects of the QTL were found in the remaining lines. Shoot weight and culm length at maturity, and anthesis date, remained unaffected for both QTL alleles after heat treatment. Lines carrying the Drysdale allele at QHsgw.aww-3B locus showed a small amount of chlorophyll loss just after heat treatment but the loss increased by two weeks after heat treatment, and the loss was greater than in the lines carrying the tolerance allele from Waagan. Homozygous NIL pairs from the WW30674 family showed contrasting phenotypes for all the key traits and had also resulted in recombination in the 3B locus region, allowing the locus to be delimited further. In Chapter 4, further mapping was undertaken to further delimit the QHsgw.aww-3B locus on the tip of the short arm of chromosome 3B. New markers that were further distal, or targeting the large gap in the map between positions 3.2 and 34.6 cM, were designed using available genetic maps and genome sequence information. Twelve new markers were developed, of which two were positioned distal of the distal-most markers from the previous map, four were mapped 1.5 cM proximal of the previous most distal marker, and two of which were generated in the upper part of the gap region. In Chapter 5, the stem rust resistance gene Sr2, and genes NRT2.5 and GoGat involved in nitrogen utilization, were tested as candidates for the QHsgw.aww-3B heat tolerance effect. The csSr2 semi-diagnostic marker for Sr2, the pseudo black chaff pleotropic effect of Sr2, and the Lr27 leaf rust resistance locus tightly linked gene to Sr2 were scored in 144 Drysdale x Waagan DH lines. All of these loci were found to be tightly linked to the heat tolerance effects, hence the Sr2 stem rust resistance gene (or the gene encoding PBC, if different from that conditioning rust resistance), was considered to be a good candidate for the gene controlling the heat tolerance effect. Marker assays designed for NRT2.5 and GoGat failed to show polymorphism. A panel of 101 hexaploid wheat genotypes, for which there were grain filling and chlorophyll heat tolerance data available, were scored for csSr2, to further test the link between Sr2 and the 3B heat tolerance locus. On average, genotypes carrying the null csSr2 marker allele (associated with rust susceptibility at Sr2) appeared more tolerant to the effects of heat on final grain size than those carrying either the second marker allele associated with rust-susceptibility (Marquis allele), or the resistance-associated marker allele (CS (Hope 3B) allele). Therefore, selection of Sr2 stem rust resistance in breeding might come at a cost of enhanced heat susceptibility, and if not selecting for Sr2, the particular rust-susceptibility allele that is present may influence heat tolerance. If they are not the same gene then they could be separated through breeding. In Chapter 6, a population of 250 Young x Reeves DH lines, with parents previously shown to contrast for heat tolerance of grain filling and floret fertility, were used to identify heat tolerance QTL. Plants were heat treated at the 6 cm AI and 10 DAA stages to target the effects on grain number and grain size, respectively. No grain size heat tolerance QTL were detected. Two floret fertility heat tolerance QTL were detected, on chromosomes 2B and 6A. In Chapter 7, 21 Australian hexaploid wheat varieties were screened for heat tolerance applied at 6 cm AI and 10 DAA to identify tolerant varieties for the farmers and breeders. Baxter and EGA Gregory were classified as tolerant for both effects.