Update on Development Genetic Analysis of Gibberellin Biosynthesis 1

In his studies on the nature of inheritance, Gregor Mendel (1866) examined seven pairs of traits in pea, including one he called “difference in the length of the stem” (Fig. 1). The tall character dominated the dwarf, segregating three tall to one dwarf. When Mendel’s work was rediscovered at the beginning of this century, Orland White introduced the term Le, for Length, to represent this trait and recognized that the Le/le pair of alleles controlled the presence or absence of a factor for tallness (White, 1917). As described below, this factor was later identified as GA, and dwarf peas that are homozygous for le became extremely important in establishing GAs as natural regulators of plant growth. The term “gibberellin” was first used in 1935 to describe a substance produced by the fungus Gibberella fujikuroi that caused overgrowth symptoms in rice, which was termed the bakanae disease (Yabuta, 1935). This substance proved to be a mixture of GAs, with GA1 and GA3 being the active factors (Takahashi et al., 1955). The early work on GAs remained undiscovered outside of Japan until after World War II, when there was an explosion of interest among plant physiologists and horticulturists. P.W. Brian, working at the Imperial Chemical Industries’ Akers Laboratories in the United Kingdom, induced dwarf pea plants to elongate as normal, tall plants by applying GA3, which he obtained from cultures of G. fujikuroi (Brian and Hemming, 1955). On the basis of these experiments, he proposed that GA was the natural growth factor that was deficient in the dwarf plants. This hypothesis was confirmed by Margaret Radley from the same laboratory, who reported that purified extracts from tall pea seedlings induced stem elongation in dwarf peas, thereby providing evidence that GAs were indeed naturally occurring in higher plants (Radley, 1956). Similar experiments were carried out in B.O. Phinney’s laboratory at the University of California, Los Angeles, where dwarf maize was used as a bioassay to show that many plant extracts contained substances with GA-like activity (Phinney et al., 1957). Nearly 30 years passed before the relationship between the Le alleles and GA content was confirmed on a firm chemical basis. During this time, the development of GC linked to MS facilitated the qualitative and quantitative analysis of GAs. Many GAs had been identified in plants, and their metabolic relationships (see below) were established using radiolabeled compounds. On the basis of the bioactivity of potential intermediates in the GA-biosynthetic pathway on GA-deficient dwarf mutants of maize, Phinney (1984) proposed that only GA1 was active in this species and that many of the other GAs were biosynthetic precursors of GA1 or were products of its catabolism. At approximately the same time, Ingram et al. (1984) demonstrated that the le pea mutant was defective in the conversion of GA20 to GA1 and suggested that the Le gene encodes the 3b-hydroxylase responsible for this conversion (see Fig. 2). This conclusion was supported by analyses of the GA content of tall (Le) and dwarf (le) pea; shoots of the latter contain approximately 10% of the GA1 concentration in tall plants and have a highly elevated GA20 content (Ross et al., 1989). In 1997, in the final chapter of this story, the Le gene was cloned, allowing the genetic lesion in the mutant le gene to be identified and thus providing the molecular basis for Mendel’s observed difference in stem length (Lester et al., 1997; Martin et al., 1997). In this Update we will describe this and other recent successes in isolating genes involved in GA biosynthesis. These advances are providing new insights into GA biosynthesis and its regulation.