The phenotype of wild type and GA‐deficient Arabidopsis plants. The GA1 gene encodes a GA biosynthetic enzyme, copalyl diphosphate synthase and the ga1‐3 mutation results in extremely low levels of endogenous GA. In this mutant DELLA proteins are stabilized resulting in its dark‐green dwarf phenotype. Introducing knockout mutations into specific DELLA genes in this background allows their function to be investigated. Each pot contains four plants which were grown under short‐day conditions for 8 weeks. WT=wild type.

Schematic representation of the conserved domains of the DELLA proteins. The conserved domains are indicated by the filled boxes. Boxes infilled in green in the N‐terminal region of the protein indicate domains present only in DELLA proteins, whereas the five GRAS motif designations are shown in blue. The DELLA‐specific domains (DELLA and TVHYNP, also known as domains I and II, respectively) are required for GA signalling whereas the C‐terminal domain has a repressor function. The Arabidopsisgai and wheat Rht‐B1b and Rht‐D1b mutations have, respectively, a 17‐amino acid deletion beginning at the first amino acid (D) of the DELLA motif, or translational stop codons at the end of the DELLA (domain I) domain.

Phenotype of wild type (WT) and quadruple‐DELLAArabidopsis plants. Quadruple DELLA plants have knockout mutations in GAI, RGA, RGL1 and RGL2, but have a functional RGL3 gene. These plants have a phenotype similar to WT plants treated with GA, they show reduced seed dormancy and early flowering, particularly when grown in short‐day photoperiods. In this image, plants were grown for 9 weeks in short‐day conditions. quad‐DELLA=quadruple DELLA mutant.

The phenotype of WT, gain‐of‐function (GoF) and loss‐of‐function (LoF) mutants of barley. GoF mutants containing a mutation within the DELLA domain resulting in stabilization of the DELLA protein, exhibit a dark‐green dwarf phenotype with increased tillering compared to WT plants. LoF mutants exhibit a GA‐constitutive phenotype (plants are light green, tall and slender with reduced tillering); the mutation is located in the C‐terminal (repressor) region of SLN1. The WT and the GoF mutant plants are fertile, but the LoF mutant is male sterile.

The GA‐DELLA signalling mechanism. In the absence of GA, the DELLA proteins are stabilized in the nucleus and repress DELLA‐mediated growth, presumably via modulation of transcription of target genes. In the presence of GA, GA binds to the soluble GID1 receptor. In the nucleus the GA‐GID1 complex associates with the DELLA proteins, promoting a further interaction between DELLA and the SCF^SLY1/GID2 complex. The SCF complex catalyses the polyubiquitination of the DELLA protein, triggering DELLA degradation by the 26S proteasome. Destruction of the DELLA protein derepresses the DELLA‐mediated growth restraint and allows growth to occur.

DELLA proteins are integrators of multiple phytohormone and signalling pathways. The DELLA proteins restrain plant growth; restraint is released by degradation of the protein(s) by GA‐triggered DELLA degradation. The DELLAs integrate information from multiple signalling pathways; all of the pathways and stresses illustrated have been shown to alter the abundance/stability of DELLAs. It is suggested that DELLAs affect biotic stress responses by modulating the jasmonic acid/ethylene‐mediated defence signalling pathways. ? denotes the potential for involvement of additional pathways which may affect GA abundance or impinge directly on DELLA accumulation.