SummaryAbscisic acid (ABA), a plant hormone, is involved in responses to environmental stresses such as drought and high salinity, and is required for stress tolerance. ABA is synthesized de novo in response to dehydration. 9-cis-epoxycarotenoid dioxygenase (NCED) is thought to be a key enzyme in ABA biosynthesis. Here we demonstrate that the expression of an NCED gene of Arabidopsis, AtNCED3, is induced by drought stress and controls the level of endogenous ABA under drought-stressed conditions. Overexpression of AtNCED3 in transgenic Arabidopsis caused an increase in endogenous ABA level, and promoted transcription of drought-and ABA-inducible genes. Plants overexpressing AtNCED3 showed a reduction in transpiration rate from leaves and an improvement in drought tolerance. By contrast, antisense suppression and disruption of AtNCED3 gave a drought-sensitive phenotype. These results indicate that the expression of AtNCED3 plays a key role in ABA biosynthesis under drought-stressed conditions in Arabidopsis. We improved drought tolerance by gene manipulation of AtNCED3 causing the accumulation of endogenous ABA.

Gibberellins (GAs) are phytohormones that are essential for many developmental processes in plants. It has been postulated that plants have both membrane-bound and soluble GA receptors; however, no GA receptors have yet been identified. Here we report the isolation and characterization of a new GA-insensitive dwarf mutant of rice, gid1. The GID1 gene encodes an unknown protein with similarity to the hormone-sensitive lipases, and we observed preferential localization of a GID1-green fluorescent protein (GFP) signal in nuclei. Recombinant glutathione S-transferase (GST)-GID1 had a high affinity only for biologically active GAs, whereas mutated GST-GID1 corresponding to three gid1 alleles had no GA-binding affinity. The dissociation constant for GA4 was estimated to be around 10(-7) M, enough to account for the GA dependency of shoot elongation. Moreover, GID1 bound to SLR1, a rice DELLA protein, in a GA-dependent manner in yeast cells. GID1 overexpression resulted in a GA-hypersensitive phenotype. Together, our results indicate that GID1 is a soluble receptor mediating GA signalling in rice.

SummaryRaf®nose family oligosaccharides (RFO) accumulating during seed development are thought to play a role in the desiccation tolerance of seeds. However, the functions of RFO in desiccation tolerance have not been elucidated. Here we examine the functions of RFO in Arabidopsis thaliana plants under drought-and cold-stress conditions, based on the analyses of function and expression of genes involved in RFO biosynthesis. Sugar analysis showed that drought-, high salinity-and cold-treated Arabidopsis plants accumulate a large amount of raf®nose and galactinol, but not stachyose. Raf®nose and galactinol were not detected in unstressed plants. This suggests that raf®nose and galactinol are involved in tolerance to drought, high salinity and cold stresses. Galactinol synthase (GolS) catalyses the ®rst step in the biosynthesis of RFO from UDP-galactose. We identi®ed three stress-responsive GolS genes (AtGolS1, 2 and 3) among seven Arabidopsis GolS genes. AtGolS1 and 2 were induced by drought and high-salinity stresses, but not by cold stress. By contrast, AtGolS3 was induced by cold stress but not by drought or salt stress. All the GST fusion proteins of GST-AtGolS1, 2 and 3 expressed in Escherichia coli had galactinol synthase activities. Overexpression of AtGolS2 in transgenic Arabidopsis caused an increase in endogenous galactinol and raf®nose, and showed reduced transpiration from leaves to improve drought tolerance. These results show that stress-inducible galactinol synthase plays a key role in the accumulation of galactinol and raf®nose under abiotic stress conditions, and that galactinol and raf®nose may function as osmoprotectants in drought-stress tolerance of plants.

The chronic food shortage that was feared after the rapid expansion of the world population in the 1960s was averted largely by the development of a high-yielding semi-dwarf variety of rice known as IR8, the so-called rice 'green revolution'. The short stature of IR8 is due to a mutation in the plant's sd1 gene, and here we identify this gene as encoding an oxidase enzyme involved in the biosynthesis of gibberellin, a plant growth hormone. Gibberellin is also implicated in green-revolution varieties of wheat, but the reduced height of those crops is conferred by defects in the hormone's signalling pathway.

Since their discovery as cell-division factors in plant tissue culture about five decades ago, cytokinins have been hypothesized to play a central role in the regulation of cell division and differentiation in plants. To test this hypothesis in planta, we isolated Arabidopsis plants lacking one, two, or three of the genes encoding a subfamily of histidine kinases (CRE1, AHK2, and AHK3) that function as cytokinin receptors. Seeds were obtained for homozygous plants containing mutations in all seven genotypes, namely single, double, and triple mutants, and the responses of germinated seedlings in various cytokinin assays were compared. Both redundant and specific functions for the three different cytokinin receptors were observed. Plants carrying mutations in all three genes did not show cytokinin responses, including inhibition of root elongation, inhibition of root formation, cell proliferation in and greening of calli, and induction of cytokinin primary-response genes. The triple mutants were small and infertile, with a reduction in meristem size and activity, yet they possessed basic organs: roots, stems, and leaves. These results confirm that cytokinins are a pivotal class of plant growth regulators but provide no evidence that cytokinins are required for the processes of gametogenesis and embryogenesis.S ince the discovery of kinetin in 1956 as a degradation product of DNA that promotes cell division in plants (1), a considerable amount of biochemical, physiological, and, most recently, genetic research has focused on elucidating the diverse roles that cytokinins play in plant growth and development. Perturbations of cytokinin levels in plants via over-expression of bacterial cytokinin synthesis genes (2-4), recovery of mutant plants with a higher-than-normal cytokinin content (5), and characterization of loss-of-function mutants of the cytokinin receptor CYTOKININ RESPONSE 1 (CRE1) (6-9) have implicated cytokinins in a wide variety of processes, including cell division, organ formation and regeneration, senescence, apical dominance, vascular development, response to pathogens, and nutrient mobility. These numerous roles for cytokinins, coupled with the failure of mutant screens to yield plants with nondetectable cytokinin levels, led to the longstanding belief that cytokinins are essential for plant growth and development.Plants respond to cytokinin through a multistep phosphorelay system, consisting of sensor histidine kinase (HK) proteins, histidine phosphotransfer (HPt) proteins, and effector response regulator (RR) proteins. Over-expression and loss-of-function analyses of particular HK, HPt, and RR proteins in Arabidopsis (8-13), combined with transient expression assays in protoplasts (14), have led to a model for cytokinin signaling (for a review, see refs. 15 and 16), beginning with perception of cytokinins by HK proteins.The Arabidopsis genome encodes six nonethylene receptor HKs: CRE1͞WOL͞AHK4, AHK2, AHK3, AtHK1, CKI1, and CKI2͞AHK5. Among them, CRE1, Arabidopsis HK2 (AHK2), and Arabidopsis HK3 (A...