The WtmsDW Locus on Wheat Chromosome 2B Controls Major Natural Variation for Floret Sterility Responses to Heat Stress at Booting Stage

Abstract

Heat stress at booting stage causes significant losses to floret fertility (grain set) and hence yield in wheat (Triticum aestivum L.); however, there is a lack of well-characterized sources of tolerance to this type of stress. Here, we describe the genetic analysis of booting stage heat tolerance in a cross between the Australian cultivars Drysdale (intolerant) and Waagan (tolerant), leading to the definition of a major-effect tolerance locus on the short arm of chromosome 2B, Wheat thermosensitive male sterile Drysdale/Waagan (WtmsDW). WtmsDW offsets between 44 and 65% of the losses in grain set due to heat, suggesting that it offers significant value for marker-assisted tolerance breeding. In lines lacking the WtmsDW tolerance allele, peaks in sensitivity were defined with reference to auricle distance, for various floret positions along the spike. Other (relatively minor) floret fertility response effects, including at the Rht-D1 dwarfing locus, were considered likely escape artifacts, due to their association with height and flowering time effects that might interfere with correct staging of stems for heat treatment. Heat stress increased grain set at distal floret positions in spikelets located at the top of the spike and increased the size of spikelets at the base of the spike, but these effects were offset by greater reductions in grain set at other floret positions. Potentially orthologous loci on chromosomes 1A and 1B were identified for heat response of flowering time. The potential significance of these findings for tolerance breeding and further tolerance screening is discussed.

Keywords: QTL; auricle distance; floret sterility; heat tolerance; male sterility; wheat.

FIGURE 1

Average trait heat responses (%) of Drysdale × Waagan DH lines, relative to controls. All responses were significant at p < 0.01 unless indicated. nd, not determined; ns, not significant. (A) Fertility-related traits. (B) Growth and development traits. See Table 1 for trait key.

FIGURE 2

Floret fertility response curves (polynomial regression, order-3) in Drysdale × Waagan DH lines that had been heat treated at different stem developmental stages as defined by auricle distance, in the top, middle, and bottom third of the spike. Only lines carrying the Drysdale (intolerance) allele at WtmsDW are represented. (A) Floret positions 1 and 2 in the spikelets. (B) Floret positions > 2 in the spikelets. Average fertility in control plants C are represented by the lines to the right of the plots.

FIGURE 3

Drysdale × Waagan DH genetic map predominantly for the short arm of wheat chromosome 2B. Mbp locations of markers in the IWGSC v1.0, Chinese Spring reference genome sequence are shown to the left. The centromere position is from Alonge et al. (2020). Positions of temperature-responsive fertility QTL from four other studies are shown on the right. Temperature and/or photoperiod-responsive male-sterility loci Wtms1, TmsBS20T, and Wptms2 studied for hybrid breeding research are also shown (Xing et al., 2003; Guo et al., 2006a; Ru et al., 2015). Positions of the loci from other studies were located approximately, based on position of markers from the respective studies in the genomic sequence.

FIGURE 4

Floret fertility (floret positions 1 and 2; means ± SE) in WtmsDW near-isogenic lines, NIL-I (Drysdale allele, intolerant) and NIL-T (Waagan allele, tolerant). All means were significantly different at p < 0.001 except among the “No heat” means.