An increase in pectin methyl esterase activity accompanies dormancy breakage and germination of yellow cedar seeds

Pectin methyl esterase (PME) (EC 3.1.1.11) catalyzes the hydrolysis of meth ylester groups of cell wall pectins. We investigated the role of this enzym e in dormancy termination and germination of yellow cedar (Chamaecyparis no otkatensis [D. Don] Spach) seeds. PME activity was not detected in dormant seeds of yellow cedar but was induced and gradually increased during moist chilling; high activity coincided with dormancy breakage and germination. P ME activity was positively correlated to the degree of dormancy breakage of yellow cedar seeds. The enzyme produced in different seed parts and in see ds at different times during moist chilling, germination, and early post-ge rminative growth consisted of two isoforms, both basic with isoelectric poi nts of 8.7 and 8.9 and the same molecular mass of 62 kD. The pH optimum for the enzyme was between 7.4 and 8.4. In intact yellow cedar seeds, activiti es of the two basic isoforms of PME that were induced in embryos and in meg agametophytes following dormancy breakage were significantly suppressed by abscisic acid. Gibberellic acid had a stimulatory effect on the activities of these isoforms in embryos and megagametophytes of intact seeds at the ge rminative stage. We hypothesize that PME plays a role in weakening of the m egagametophyte, allowing radicle emergence and the completion of germinatio n.