An Increase in Pectin Methyl Esterase Activity Accompanies Dormancy Breakage and Germination of Yellow Cedar Seeds

Abstract

Pectin methyl esterase (PME) (EC 3.1.1.11) catalyzes the hydrolysis of methylester groups of cell wall pectins. We investigated the role of this enzyme in dormancy termination and germination of yellow cedar (Chamaecyparis nootkatensis [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. PME activity was positively correlated to the degree of dormancy breakage of yellow cedar seeds. The enzyme produced in different seed parts and in seeds at different times during moist chilling, germination, and early post-germinative growth consisted of two isoforms, both basic with isoelectric points 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, activities of the two basic isoforms of PME that were induced in embryos and in megagametophytes 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 germinative stage. We hypothesize that PME plays a role in weakening of the megagametophyte, allowing radicle emergence and the completion of germination.

Role of Abscisic Acid in Seed Dormancy

“…The pattern of increase in pectin methyl esterase activities in yellow-cedar seeds coincides with the process of dormancy breakage, and the amount of enzyme activity produced is strongly correlated with the ability of seeds to germinate (Ren and Kermode 2000). Furthermore, in the absence of effective dormancy-breaking treatments (that is, when mature seeds are subjected to a 3-day soak or to a control treatment comprised of a 90-day period in warm, moist conditions), no enzyme activity is induced.…”

“…Hydrolytic enzymes implicated in dormancy termination include oxalate oxidase (Grzelezak and others 1985), endo-b-mannanase (Downie and others 1997), and pectin methyl esterase (Ren and Kermode 2000). In tomato seeds, chemical weakening of the surrounding endosperm is caused by enzymes produced under the influence of the embryo: dormant seeds are unable to produce the cell wall degrading enzymes.…”

Demethylesterification of Cell Wall Pectins in Arabidopsis Plays a Role in Seed Germination

“…Concurrently, the embryo exhibits increased turgor and a reduced sensitivity to low osmotic potentials; both the weakening and strengthening processes are positively associated with PME activities (Ren and Kermode, 2000).…”

“…PME activity increases in a tissue-specific manner during seed dormancy release and germination of yellow cedar seeds (Ren and Kermode, 2000), where pectin is the predominant polysaccharide in the cell walls of the megagametophyte (Ren and Kermode, 1999). In the seeds of this highly dormant conifer species, the addition of ABA to intact seeds after dormancy breakage suppresses the activity of two PME isoforms (Ren and Kermode, 2000).…”

“…Very little is known about the role of PMEs in the germination of angiosperm seeds, although one study has characterized their role in the dormancy breakage and germination of conifer seeds (Ren and Kermode, 2000).…”