Evaluating Seed Dormancy, Hormone Response, and Pre-Harvest Sprouting Tolerance of An ABA Hypersensitive Mutant ZakERA8.

This study characterized the effect of the wheat ZakERA8 (ENHANCED RESPONSE TO ABA8) mutant on seed dormancy, after-ripening, and on preharvest sprouting tolerance in the soft white spring ‘Zak’.  The plant hormone abscisic acid (ABA) induces seed dormancy during embryo maturation and maintains dormancy in mature seed, whereas gibberellin (GA) stimulates seed germination and dormancy loss. Seed dormancy is lost during a period of dry storage called after-ripening.  Previous work showed that ZakERA8 has higher seed dormancy and is more sensitive to ABA inhibition of seed germination than wild-type Zak.  This study found that ZakERA8 lost ABA sensitivity more slowly than Zak over a detailed after-ripening time course.  Moreover, ZakERA8 seeds were initially more dormant than Zak as evidenced by the fact that they were less responsive to germination-rescue by GA.   GA sensitivity increased gradually over the after-ripening time course.  Since apparent ABA hypersensitive seed germination can result both from altered signaling and from increased endogenous ABA levels, the levels of ABA, GA, and several other hormones were examined in very dormant, dormant, after-ripened, and fully after-ripened seeds.  ZakERA8 had initially higher ABA levels and dormancy than wild type, but continued to fail to germinate as ABA levels dropped with after-ripening.  Based on this result, we suspect that ZakERA8 results from a change in ABA signaling.  ZakERA8 also resulted in reduced induction of GA hormone accumulation with after-ripening.  Higher seed dormancy is generally associated with higher tolerance to preharvest sprouting (PHS), the germination of mature seed on the mother plant when moist conditions occur before harvest.  PHS is a more serious problem in white than red kernelled wheat.  The resulting starch damage leads to economic hardship for farmers when sprouted wheat must be sold as feed.  The semi-dominant ERA8 mutation increased PHS tolerance without compromising emergence.