The theory that bioactive gibberellins (GAs) act as inhibitors of inhibitors of plant growth was based originally on the slender pea (Pisum sativum) mutant (genotype la cry-s), but the molecular nature of this mutant has remained obscure. Here we show that the genes LA and CRY encode DELLA proteins, previously characterized in other species (Arabidopsis [Arabidopsis thaliana] and several grasses) as repressors of growth, which are destabilized by GAs. Mutations la and cry-s encode nonfunctional proteins, accounting for the fact that la cry-s plants are extremely elongated, or slender. We use the la and cry-s mutations to show that in roots, DELLA proteins effectively promote the expression of GA synthesis genes, as well as inhibit elongation. We show also that one of the DELLAregulated genes is a second member of the pea GA 3-oxidase family, and that this gene appears to play a major role in pea roots.It is well known that primary root growth is strongly influenced by the plant hormone GA (Davies, 2004). For example, the application of bioactive GA to roots treated with the growth inhibitor ancymidol completely restored growth to that of the untreated plants (Tanimoto, 1991). Yaxley et al. (2001) established the importance of GAs for root growth in peas (Pisum sativum) by using a variety of GA-deficient mutant plants. In the na-1 mutant, for example, root GA 1 levels, and root elongation, were significantly reduced compared with wild-type plants, and when the GA 1 content was restored to wild-type levels, so too was root elongation.The GAs act by destabilizing the growth inhibitory DELLA proteins (Peng et al., 1997;Harberd et al., 1998;Silverstone et al., 2001;Alvey and Harberd, 2005). In other words, GA acts as an ''inhibitor of an inhibitor '' (Harberd et al., 1998). Interestingly, there is also evidence that DELLA proteins promote the biosynthesis of active GAs. For example, in the Arabidopsis (Arabidopsis thaliana) DELLA mutant rga, the expression of the biosynthesis gene GA4 is reduced, indicating that high DELLA protein levels are associated with an up-regulation of GA synthesis genes (Silverstone et al., 2001 (Silverstone et al., 1998;Gubler et al., 2002). However, the available evidence indicates greater redundancy in dicots compared with monocots (Ikeda et al., 2001;Thomas and Hedden, 2006). There have been five DELLA genes isolated from Arabidopsis (GAI, RGA, RGL1, RGL2, and RGL3), yet only one in rice (SLR1), barley (SLN1), and maize (Zea mays; D8; Peng et al., 1997Peng et al., , 1999Silverstone et al., 1998;Ikeda et al., 2001;Chandler et al., 2002;Gubler et al., 2002), with the possibility of another DELLA gene in maize (D9; accession no. ABI84225). It should be noted, however, that DELLAs have been studied in fewer dicot model species than in monocot species. To date, DELLA-encoding genes from pea have not been reported, even though observations on the slender phenotype of pea triggered the early suggestion that GA acts an inhibitor of an inhibitor (Brian, 1957). The elongated slender phenotype, con...

Models describing plant hormone interactions are often complex and web-like. Here we assess several suggested interactions within one experimental system, elongating pea internodes. Results from this system indicate that at least some suggested interactions between auxin, gibberellins (GAs), brassinosteroids (BRs), abscisic acid (ABA) and ethylene do not occur in this system or occur in the reverse direction to that suggested. Furthermore, some of the interactions are relatively weak and may be of little physiological relevance. This is especially true if plant hormones are assumed to show a log-linear response curve as many empirical results suggest. Although there is strong evidence to support some interactions between hormones (e.g. auxin stimulating ethylene and bioactive GA levels), at least some of the web-like complexities do not appear to be justified or are overstated. Simpler and more targeted models may be developed by dissecting out key interactions with major physiological effects.

Auxin promotes GA biosynthesis in the aboveground parts of plants. However, it has not been demonstrated previously that this interaction occurs in roots. To understand the interactions between auxin and GAs in these organs, we treated wild-type pea (Pisum sativum L.) roots with the inhibitors of auxin action, p-chlorophenoxyisobutyric acid (PCIB) and yokonolide B (YkB), and with the auxin transport inhibitor N-1-naphthylphthalamic acid (NPA). These compounds generally downregulated GA synthesis genes and upregulated GA deactivation genes, and reduced the level of the bioactive GA1. These effects indicate that in pea roots, auxin at normal endogenous levels stimulates GA biosynthesis. We show also that supra-optimal levels of exogenous auxin reduce the endogenous level of bioactive GA in roots, although the effect appears too small to account for the strong growth-inhibitory effect of high auxin levels.

The recent cloning of the pea genes LA and CRY has historical implications, since the combined effect of null mutations in these genes is the elongated, gibberellin-insensitive "slender" phenotype, which gave rise to the theory that gibberellins (GAs) are inhibitors of inhibitors of growth. Interestingly, the duplication event that produced the second gene (LA or CRY) appears to have occurred more than 100 mya, and yet the two genes have retained essentially similar functions. They both encode DELLA proteins, which inhibit growth while at the same time promoting the synthesis of the growth-promoting hormone, gibberellin (GA). This duality of function is discussed in the context of recent suggestions that DELLAs integrate multiple hormone signals, rather than just the GA signal. We also present new data showing that LA and CRY play a major role in regulating fruit growth.