Gibberellin Metabolism and Transport During Germination and Young Seedling Growth of Pea (Pisum sativum L.)

The role of gibberellins (GAs) during germination and early seedling growth is examined by following the metabolism and transport of radiolabeled GAs in cotyledon, shoot, and root tissues of pea (Pisum sativum L.) using an aseptic culture system. Mature pea seeds have significant endogenous GA(20) levels that fall during germination and early seedling growth, a period when the seedling develops the capacity to transport GA(20) from the cotyledon to the shoot and root of the seedling. Even though cotyledons at 0-2 days after imbibition have appreciable amounts of GA(20), the cotyledons retain the ability to metabolize labeled GA(19) to GA(20) and express significant levels of PsGA20ox2 message (which encodes a GA biosynthesis enzyme, GA 20-oxidase). The large pool of cotyledonary GA(20) likely provides substrate for GA(1) synthesis in the cotyledons during germination, as well as for shoots and roots during early seedling growth. The shoots and roots express GA metabolism genes (PsGA3ox genes which encode GA 3-oxidases for synthesis of bioactive GA(1), and PsGA2ox genes which encode GA 2-oxidases for deactivation of GAs to GA(29) and GA(8)), and they develop the capacity to metabolize GAs as necessary for seedling establishment. Auxins also show an interesting pattern during early seedling growth, with higher levels of 4-chloro-indole-3-acetic acid (4-Cl-IAA) in mature seeds and higher levels of indole-3-acetic acid (IAA) in young root and shoot tissues. This suggests a changing role for auxins during early seedling development.