Characterization of proteins in seed dormancy: Modulation of enzyme activity in the embryo

Dormancy is the inability of a seed to germinate under conditions favorable for germination. The properties of the seed have been studied extensively in efforts to understand why some seeds remain dormant, however we still do not understand the processes that control germination or dormancy. The hormone abscisic acid (ABA) plays important roles associated with dormancy, but the ABA signal transduction pathway has yet to reveal the mechanisms controlling dormancy. The genetics, physiology and biochemistry of seeds have been analyzed to search for a "dormancy factor," but no such gene or protein has been found. In barley, dormancy can be induced by modulation of growth conditions, allowing the analysis of the processes taking place in dormant embryos when placed under germination conditions.; Dormant barley seeds were used to compare the biochemistry of embryos during imbibition in dormant vs. germinating seeds. Chapter one describes the patterns of protein synthesis, degradation and modification during imbibition, revealing differences between dormant and germinating embryos. Proteolytic degradation in germinating embryos contrasts with the absence of proteolysis in dormant embryos during imbibition. Distinctive patterns of post-translationally modified proteins were evident in dormant vs. germinating seeds: the redox and phosphorylation status of certain proteins differed between germinating and dormant embryos. In a follow-up analysis of proteolytic activitity during germination, a protease was identified that is activated at the onset of germination and is absent or inactive in dormant embryos. This protease was found to be a serine protease, with a molecular mass of 73 kDa. The enzyme was active in the scutellum, shoot and root of germinating embryos with an acidic pH optimum and a pI of 4.6. Its possible role in germination and dormancy is analyzed in chapter 2. Chapter three describes the characterization of the sucrose-cleaving enzyme invertase in embryos of barley. The activity of invertase was directly correlated with germination. Invertase was activated in germinating embryos immediately prior to radicle emergence, while in dormant embryos no invertase activity could be detected throughout the imbibition period. An analysis of the different invertase isoforms revealed that the enzyme is present in dormant embryos at the same levels as in germinating embryos, and preliminary work suggests that phosphorylation could be involved in invertase activation.