Phytochrome promotion of mannan-degrading enzyme activities in the micropylar endosperm of Datura ferox seeds requires the presence of the embryo and gibberellin synthesis

Abstract

It has been shown that endosperm weakening, a necessary step in the light induction of germination in Datura ferox seeds, is related to the hydrolysis of cell wall mannose polysaccharides. It is demonstrated that the activities of two enzymes involved in mannan degradation (β-mannanase and β-mannosidase) are strongly promoted in the micropylar endosperm, after Pfr formation but long before radicle protrusion. An increase in enzyme activities occurred in dissected seed parts at similar rates to those in whole seeds, but only when at least a portion of the embryonic axis was present during incubation. De-embryonation of seed parts prevented the Pfr-induced increase in hydrolase activities. Exogenous GA3 promoted mannan-degrading enzyme activities both in the micropylar region of far red-treated whole seeds and those of isolated and deembryonated endosperm sections. The response to Pfr and GA3 was far more pronounced in the micropylar portion than in the rest of the endosperm. Pfr promotion of hydrolase activities and germination was blocked by paclobutrazol, an inhibitor of gibberellin synthesis. A close correlation was found between β-mannanase activity measured 45 h after Pfr formation and germination scored 24 h later. These results support the hypothesis that in D. ferox, endosperm softening is promoted by Pfr via the production in the embryonic axis of a factor, probably a gibberellin, which moves to the endosperm where it stimulates cell wall-mannan hydrolytic enzyme activities.

Expression of an Expansin Is Associated with Endosperm Weakening during Tomato Seed Germination

“…Further studies will also be required to understand the differences in hormonal responses between intact and excised endosperm caps. The importance of the embryo in regulating physiological events in the endosperm cap has been clearly demonstrated in Datura ferox seeds (Sánchez and de Miguel, 1997).…”

Gibberellin regulates Arabidopsis seed germination via RGL2, a GAI/RGA-like gene whose expression is up-regulated following imbibition

“…ABA establishes and maintains dormancy, whereas GA breaks dormancy and induces germination (Koornneef and Karssen 1994;McCarty 1995;Bewley 1997;Steber et al 1998). Under favorable environmental conditions, GA biosynthesis is induced and, in turn, the de novo-synthesized GA induces the expression of genes encoding enzymes such as endo-␤-1,3 glucanase (Leubner-Metzger et al 1995), ␤-1,4 mannan endohydrolase (Bewley 1997;Sánchez and Miguel 1997), and the extensin-like protein AtEPR1 (Dubreucq et al 2000). These enzymes hydrolyze the endosperm and release the inhibitory effects of ABA on embryo growth potential (Koornneef and Karssen 1994;McCarty 1995;Bewley 1997).…”

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

“…In tomato, a germination-specific endo-b-mannanase gene is expressed in the micropylar endosperm cap of seeds (Nonogaki et al, 2000). Seeds of Datura ferox produce endo-b-mannanase and b-mannosidase in the micropylar region of the endosperm after red light stimulation, several hours before the radicle protrudes through it (Sanchez and de Miguel, 1997). In seeds of white spruce (Picea glauca), weakening of the micropylar end of the megagametophyte and nucellus precedes radicle protrusion, and this weakening is associated with endo-b-mannanase activity (Downie et al, 1997).…”