20 Gibberellin and Abscisic Acid Biosynthesis and Response

Abstract

INTRODUCTION
Evidence for the roles of hormones in plant growth and development has been classically obtained by (1) applying the pertinent hormone and afterwards observing the resulting responses, and (2) correlating endogenous hormone levels with physiological effects. More recently, hormone-deficient mutants have been used for establishing the roles of a particular hormone.

To elicit a response, a certain hormone concentration and a receptor are both needed. Hormone response mutants are supposedly impaired either in the receptor or at a step in the transduction pathway. These mutants are phenotypically similar to hormone-deficient mutants but still contain the hormone and do not respond to that hormone, whether endogenous or applied. Biosynthetic and response mutants for both gibberellins (GA) and abscisic acid (ABA) are known in a number of species, including Arabidopsis.

In this chapter, we describe various physiological functions of GA and ABA in Arabidopsis. In addition, we discuss how mutants for both hormones have been exploited to facilitate further understanding of GA and ABA biosynthesis and action.

Physiological Effects of Gibberellins
The most striking effect of applied GA is an increase in stem length, especially in dwarf mutants of maize, pea, and rice (Phinney 1984), and rosette plants (Zeevaart 1983). The cellular basis for stem elongation is a combination of cell division and cell elongation (Talón et al. 1991). In addition, GAs are involved in flower formation (Zeevaart 1983), male sex expression, fruit set and growth (Pharis and King 1985), and seed germination (Graebe 1987). GAs also induce the synthesis of...