The role of abscisic acid metabolism in Western White Pine (Pinus monticola Dougl. Ex D. Don) seed dormancy

摘要

Western white pine seeds exhibit deep dormancy at maturity and require several months of moist chilling to reach maximal germination capacities. Toward increasing the efficiency of dormancy-breakage and improving the rate and synchronicity of germination, the effectiveness of various modified pre-chilling and chilling treadments were determined. A key parameter that defined an efficient dormancy-breaking protocol included a lengthy higher temperature soak prior to moist chilling; high moisture content and air exchange during moist chilling were also important. Dormancy of this species is primarily u27coat-enhancedu27, imposed by the seed coat, nucellar membrane, and megagametophyte. However, other physiological aspects of the dormancy mechanism have not been explored - in particular the contribution of abscisic acid (ABA) and its metabolism. To achieve this, ABA and the metabolites, phaseic acid, dihydroph~aseic acid, 7u27-hydroxy ABA, and ABA-glucose ester, were quantified in western white pine seeds during three treatments that terminated dormancy. These treatments were (1) a 98- d moist chilling period, (2) exposure to the ABA-biosynthesis inhibitor fluridone and gibberellic acid, and (3) the removal of the hard seed coat. Overall, any treatment that terminated dormancy and increased germination capacity also resulted in dramatic decreases in ABA content in both embryo and megagametophyte tissues. The catabolism of ABA occurred via several routes, depending on the stage and the seed tissue; the pathways included 8u27- and 7u27-hydroxylation of AHA, and ABA conjugation. In seed populations that remained dormant, ABA was maintained or returned to high levels after a transient decrease. Thus, it has become evident that ABA biosynthesis (i.e. a higher or equal capacity for ABA biosynthesis versus catabolism) is important for dormancy maintenance. As seeds transition to a germinable state, changes in ABA flux - i.e. shifts in the ratio between biosynthesis and catabolism -- occur to support a catabolic state. Further evidence to support this contention comes from expression studies of zeaxthanin epoxidase (ZEP), 9-cis epoxycarotenoid dioxygenase (NCED), and abscisic acid 8u27- hydroxylase (CYP707A) genes during moist-chilling-induced dormancy termination. As seeds transitioned from a dormant to germinable state, expression of ABA biosynthetic genes (ZEP and NCED) decreased while an ABA catabolic gene (CYP707A) increased.