Peter Hedden

Genetic Characterization and Functional Analysis of the GID1 Gibberellin Receptors in Arabidopsis

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 SCFSLY1 E3 ubiquitin ligase that targets the DELLA protein for degradation.

The Cold-Inducible CBF1 Factor–Dependent Signaling Pathway Modulates the Accumulation of the Growth-Repressing DELLA Proteins via Its Effect on Gibberellin Metabolism

Plants have evolved robust mechanisms to respond and adapt to unfavorable environmental conditions, such as low temperature. The C-repeat/drought-responsive element binding factor CBF1/DREB1b gene encodes a transcriptional activator transiently induced by cold that controls the expression of a set of genes responding to low temperature (the CBF regulon). Constitutive expression of CBF1 confers freezing tolerance but also slows growth. Here, we propose that low temperature–induced CBF1 expression restrains growth at least in part by allowing the accumulation of DELLAs, a family of nuclear growth-repressing proteins, the degradation of which is stimulated by gibberellin (GA). We show that cold/CBF1 enhances the accumulation of a green fluorescent protein (GFP)–tagged DELLA protein (GFP-RGA) by reducing GA content through stimulating expression of GA-inactivating GA 2-oxidase genes. Accordingly, transgenic plants that constitutively express CBF1 accumulate less bioactive GA and as a consequence exhibit dwarfism and late flowering. Both phenotypes are suppressed when CBF1 is expressed in a line lacking two DELLA proteins, GA-INSENSITIVE and REPRESSOR OF GA1-3. In addition, we show that DELLAs contribute significantly to CBF1-induced cold acclimation and freezing tolerance by a mechanism that is distinct from the CBF regulon. We conclude that DELLAs are components of the CBF1-mediated cold stress response.

Leaf-Induced Gibberellin Signaling Is Essential for Internode Elongation, Cambial Activity, and Fiber Differentiation in Tobacco Stems

In plants, the directional flow and sites of hormone accumulation facilitate organ development. Identifying leaves as the source of a mobile signal that induces gibberellin accumulation and signaling revealed the hormone’s roles in plant primary and secondary growth. It is demonstrated that leaf-induced gibberellin is required for stem elongation, cambial activity, and fiber formation. The gibberellins (GAs) are a group of endogenous compounds that promote the growth of most plant organs, including stem internodes. We show that in tobacco (Nicotiana tabacum) the presence of leaves is essential for the accumulation of bioactive GAs and their immediate precursors in the stem and consequently for normal stem elongation, cambial proliferation, and xylem fiber differentiation. These processes do not occur in the absence of maturing leaves but can be restored by application of C19-GAs, identifying the presence of leaves as a requirement for GA signaling in stems and revealing the fundamental role of GAs in secondary growth regulation. The use of reporter genes for GA activity and GA-directed DELLA protein degradation in Arabidopsis thaliana confirms the presence of a mobile signal from leaves to the stem that induces GA signaling.

Annual Plant Reviews, Volume 49: Gibberellins, The

This timely book covers the substantial and impressive recent advances in our understanding of the gibberellins and their roles in plant development, including the biosynthesis, inactivation, transport, perception and signal transduction of these important hormones. An introductory chapter traces the history of gibberellin research, describing the many discoveries that form the basis for the recent progress. The exciting emerging evidence for the interaction of gibberellin signalling with that of the other hormones is critically evaluated. The occurrence of gibberellins in fungal, bacterial and lower plant species is also discussed, with emphasis on evolution. Manipulation of gibberellin metabolism and signal transduction through chemical or genetic intervention has been an important aspect of crop husbandry for many years. The reader is presented with important information on the advances in applying gibberellin research in agriculture and horticulture.

Annual Plant Reviews Volume 24: Plant Hormone Signaling: Hedden/Plant

This volume provides an overview of the biosynthesis catabolism perception and signal transduction of the individual hormone classes followed by chapters on hormone distribution and transport and the roles of hormone signaling in specific developmental processes. Particular attention is paid to the regulation of hormone signaling by environmental and developmental cues sites of hormone metabolism and action and interactions between hormone signaling pathways. The book is directed at researchers and professionals in plant biochemistry and molecular biology.

CHAPTER 16:Gibberellin Metabolism

The gibberellin (GA) plant hormones are diterpenoid carboxylic acids that regulate growth and development throughout the life cycle of flowering plants, but are also present in some species of lower plants, fungi and bacteria. The latter stages of their biosynthesis in flowering plants involve the activities of two main families of 2-oxoglutarate-dependent dioxygenases (ODDs): GA 20-oxidases, which oxidize the C-10 methyl group to the aldehyde and then cleave it to form the C19-GAs, and GA 3-oxidases, which introduce a 3β-hydroxyl group as the final step in the formation of the biologically active hormones. Oxidation on C-2 by other ODD families (GA 2-oxidases) that act on C19-GAs or their C20-GA precursors serves as an inactivation mechanism. A further family of ODDs with a restricted species distribution, the GA 7-oxidases, convert the early precursor GA12-aldehyde to GA12, a reaction that is also catalysed by cytochrome P450 monooxygenases. Members of the ODD gene families are major sites of regulation for GA biosynthesis and catabolism, responding to developmental and environmental signals. The paralogues have distinct but often overlapping expression domains, and differ also in levels of expression and regulation. These enzymes have proved to be useful targets for the introduction of beneficial traits into crop species, while the acylcyclohexanedione inhibitors of the GA 3-oxidases have found important application in agriculture as growth retardants.