No abstract

We investigated the physiological function of three Arabidopsis thaliana homologs of the gibberellin (GA) receptor GIBBERELLIN-INSENSITIVE DWARF1 (GID1) by determining the developmental consequences of GID1 inactivation in insertion mutants. Although single mutants developed normally, gid1a gid1c and gid1a gid1b displayed reduced stem height and lower male fertility, respectively, indicating some functional specificity. The triple mutant displayed a dwarf phenotype more severe than that of the extreme GA-deficient mutant ga1-3. Flower formation occurred in long days but was delayed, with severe defects in floral organ development. The triple mutant did not respond to applied GA. All three GID1 homologs were expressed in most tissues throughout development but differed in expression level. GA treatment reduced transcript abundance for all three GID1 genes, suggesting feedback regulation. The DELLA protein REPRESSOR OF ga1-3 (RGA) accumulated in the triple mutant, whose phenotype could be partially rescued by loss of RGA function. Yeast two-hybrid and in vitro pull-down assays confirmed that GA enhances the interaction between GID1 and DELLA proteins. In addition, the N-terminal sequence containing the DELLA domain is necessary for GID1 binding. Furthermore, yeast three-hybrid assays showed that the GA-GID1 complex promotes the interaction between RGA and the F-box protein SLY1, a component of the SCF SLY1 E3 ubiquitin ligase that targets the DELLA protein for degradation.

No abstract

The shoot apical meristem (SAM) is a pluripotent group of cells that gives rise to the aerial parts of higher plants. Class-I KNOTTED1-like homeobox (KNOX) transcription factors promote meristem function partly through repression of biosynthesis of the growth regulator gibberellin (GA). However, regulation of GA activity cannot fully account for KNOX action. Here, we show that KNOX function is also mediated by cytokinin (CK), a growth regulator that promotes cell division and meristem function. We demonstrate that KNOX activity is sufficient to rapidly activate both CK biosynthetic gene expression and a SAM-localized CK-response regulator. We also show that CK signaling is necessary for SAM function in a weak hypomorphic allele of the KNOX gene SHOOTMERISTEMLESS (STM). Additionally, we provide evidence that a combination of constitutive GA signaling and reduced CK levels is detrimental to SAM function. Our results indicate that CK activity is both necessary and sufficient for stimulating GA catabolic gene expression, thus reinforcing the low-GA regime established by KNOX proteins in the SAM. We propose that KNOX proteins may act as general orchestrators of growth-regulator homeostasis at the shoot apex of Arabidopsis by simultaneously activating CK and repressing GA biosynthesis, thus promoting meristem activity.

The GAs (gibberellins) comprise a large group of diterpenoid carboxylic acids that are ubiquitous in higher plants, in which certain members function as endogenous growth regulators, promoting organ expansion and developmental changes. These compounds are also produced by some species of lower plants, fungi and bacteria, although, in contrast to higher plants, the function of GAs in these organisms has only recently been investigated and is still unclear. In higher plants, GAs are synthesized by the action of terpene cyclases, cytochrome P450 mono-oxygenases and 2-oxoglutarate-dependent dioxygenases localized, respectively, in plastids, the endomembrane system and the cytosol. The concentration of biologically active GAs at their sites of action is tightly regulated and is moderated by numerous developmental and environmental cues. Recent research has focused on regulatory mechanisms, acting primarily on expression of the genes that encode the dioxygenases involved in biosynthesis and deactivation. The present review discusses the current state of knowledge on GA metabolism with particular emphasis on regulation, including the complex mechanisms for the maintenance of GA homoeostasis.