Isolation of ABA-response mutants in allohexaploid bread wheat (Triticum aestivum L.): Drawing connections to grain dormancy, preharvest sprouting, and drought tolerance

This dissertation describes the characterization of seed dormancy and drought tolerance phenotypes of abscisic acid (ABA) response mutants in allohexaploid wheat (Triticum aestivum L.). The plant hormone ABA is required for the induction of seed dormancy, stomatal closure in drying soils, and drought tolerance. Preharvest sprouting (PHS), the germination of seed on the mother plant, occurs when cool moist conditions persist before harvest. PHS resistance is correlated with seed dormancy resulting from red grain color, higher ABA accumulation or sensitivity. ABA only inhibits the germination of imbibing dormant wheat grain, not of after-ripened grain. ABA hypersensitive lines were isolated based on the phenotype of maintaining higher ABA sensitivity when partially after-ripened. ABA hypersensitive phenotypes were stronger and persisted longer in the soft white spring 'Zak' compared to the hard red springs Chinese Spring and 'Scarlet'. ZakERA (Zak Enhanced Response to ABA) 0 and 19A mutants showed increased dormancy associated with a reproducible increase in ABA sensitivity in germination. These mutants required more time to after-ripen than Zak wild type (WT), and still showed ABA sensitivity after 3 years after-ripening. Warm1 (Wheat ABA- responsive mutant) and Warm4 showed the strongest ABA-hypersensitive germination phenotype among Chinese Spring mutants, and were accompanied by decreased germination in the absence of ABA. Although these mutants required more time to after-ripen than WT, they lost ABA sensitivity within 3-12 months. ScERA ( Scarlet Enhanced Response to ABA) 10A, 68A, 10C, and 4A showed the strongest ABA-hypersensitive germination phenotype. ScERA 4A is unique in having enhanced ABA sensitivity without any apparent increase in seed dormancy. ABA insensitive mutants in Scarlet, ScABI (Scarlet ABA- Insensitive) 2A, 2C, 1B, and 1C, were insensitive to ABA inhibition of germination with little or no after-ripening. ABA mutants are also expected to show altered water relations. All of the ScERA or ScABI mutants tested so far appear to have only seed germination phenotypes. Consistent with the role of ABA in down-regulating transpiration in drying soils, Warm3, Warm4, and ZakERA 19A are associated with decreased leaf transpiration. In Warm4 and ZakERA 19A this decrease in transpiration is associated with higher canopy temperatures under field conditions.