Takashi Aoyama

Bio: Takashi Aoyama is an academic researcher from Kyoto University. The author has contributed to research in topics: Arabidopsis & Arabidopsis thaliana. The author has an hindex of 32, co-authored 69 publications receiving 5810 citations. Previous affiliations of Takashi Aoyama include Sapienza University of Rome & Laboratory of Molecular Biology.

A glucocorticoid-mediated transcriptional induction system in transgenic plants

Abstract: A novel chemical induction system for transcription in plants has been developed, taking advantage of the regulatory mechanism of vertebrate steroid hormone receptors. A chimeric transcription of the DNA-binding domain of the yeast transcription factor GAL4, the transactivating domain of the herpes viral protein VP16, and the receptor domain of the rat glucocorticoid receptor (GR). The GVG gene was introduced into transgenic tobacco and Arabidopsis together with a luciferase (Luc) gene which was transcribed from a promoter containing six tandem copies of the GAL4 upstream activating sequence. Induction of luciferase activity was observed when the transgenic tobacco plants were grown on an agar medium containing dexamethasone (DEX), a strong synthetic glucocorticoid. Induction levels of the luciferase activity were well correlated with DEX concentrations in the range from 0.1 to 10 microM and the maximum expression level was over 100 times that of the basal level. Analysis of the induction kinetics by Northern blot analysis showed that the Luc mRNA was first detected 1 h after DEX treatment and increased to the maximum level in 4 h. The stationary induction level and the duration of the induction varied with the glucocorticoid derivative used. The GVG gene activity can also be regulated by DEX in transgenic Arabidopsis plants. The results indicate that a stringent chemical control of transcription can be achieved in plants with the GVG system. Advantages and potential uses of this system are also discussed.

A genetic framework for the control of cell division and differentiation in the root meristem.

Abstract: Plant growth and development are sustained by meristems. Meristem activity is controlled by auxin and cytokinin, two hormones whose interactions in determining a specific developmental output are still poorly understood. By means of a comprehensive genetic and molecular analysis in Arabidopsis, we show that a primary cytokinin-response transcription factor, ARR1, activates the gene SHY2/IAA3 (SHY2), a repressor of auxin signaling that negatively regulates the PIN auxin transport facilitator genes: thereby, cytokinin causes auxin redistribution, prompting cell differentiation. Conversely, auxin mediates degradation of the SHY2 protein, sustaining PIN activities and cell division. Thus, the cell differentiation and division balance necessary for controlling root meristem size and root growth is the result of the interaction between cytokinin and auxin through a simple regulatory circuit converging on the SHY2 gene.

ARR1, a transcription factor for genes immediately responsive to cytokinins.

Abstract: Cytokinins are a class of phytohormones involved in various physiological events of plants. The Arabidopsis sensor histidine kinase CRE1 was recently reported to be a cytokinin receptor. We used a steroid-inducible system to show that the transcription factor-type response regulator ARR1 directs transcriptional activation of the ARR6 gene, which responds to cytokinins without de novo protein synthesis. This fact, together with characteristics of ARR1-overexpressing plants and arr1 mutant plants, indicates that the phosphorelay to ARR1, probably from CRE1, constitutes an intracellular signal transduction occurring immediately after cytokinin perception.

Arabidopsis ARR1 and ARR2 response regulators operate as transcriptional activators.

Abstract: The genes coding for the response regulators ARR1 and ARR2 have previously been identified by in silico screening of an expression sequence tag database and subsequent cloning from both Arabidopsis cDNA and genomic libraries Their structures, in which the N-terminal signal receiver domain is followed by the output domain, are characteristic of typical bacterial response regulators of the two-component regulatory systems that control responses to a variety of environmental stimuli Here we present evidence that these response regulators actually work as transcription factors ARR1 and ARR2 were localized in the nuclei of plant cells regardless of the presence or absence of their signal receiver domain Their middle segments, which faintly resemble the mammalian oncogene product Myb, were capable of binding double-stranded DNA in a sequence-specific manner in vitro Their C-terminal halves functioned as transactivation domains in plant cells when combined with the DNA-binding domain of yeast GAL4 They thus possess all the essential components of a transcriptional activator Both ARR1 and ARR2 promoted expression of a reporter gene in plant cells through their own target sequence Truncation of their N-terminal signal receiver domain led to an increase in transactivation An as yet unidentified phospho-relay signal may modulate the capability for transactivation and/or DNA binding through the signal receiver domain

Shade avoidance responses are mediated by the ATHB-2 HD-zip protein, a negative regulator of gene expression.

Abstract: The ATHB-2 gene encoding an homeodomain-leucine zipper protein is rapidly and strongly induced by changes in the ratio of red to far-red light which naturally occur during the daytime under the canopy and induce in many plants the shade avoidance response. Here, we show that elevated ATHB-2 levels inhibit cotyledon expansion by restricting cell elongation in the cotyledon-length and -width direction. We also show that elevated ATHB-2 levels enhance longitudinal cell expansion in the hypocotyl. Interestingly, we found that ATHB-2-induced, as well as shade-induced, elongation of the hypocotyl is dependent on the auxin transport system. In the root and hypocotyl, elevated ATHB-2 levels also inhibit specific cell proliferation such as secondary growth of the vascular system and lateral root formation. Consistent with the key role of auxin in these processes, we found that auxin is able to rescue the ATHB-2 lateral root phenotype. We also show that reduced levels of ATHB-2 result in reciprocal phenotypes. Moreover, we demonstrate that ATHB-2 functions as a negative regulator of gene expression in a transient assay. Remarkably, the expression in transgenic plants of a derivative of ATHB-2 with the same DNA binding specificity but opposite regulatory properties results in a shift in the orientation of hypocotyl cell expansion toward radial expansion, and in an increase in hypocotyl secondary cell proliferation. A model of ATHB-2 function in the regulation of shade-induced growth responses is proposed.

Plant pathogens and integrated defence responses to infection.

Abstract: Plants cannot move to escape environmental challenges. Biotic stresses result from a battery of potential pathogens: fungi, bacteria, nematodes and insects intercept the photosynthate produced by plants, and viruses use replication machinery at the host's expense. Plants, in turn, have evolved sophisticated mechanisms to perceive such attacks, and to translate that perception into an adaptive response. Here, we review the current knowledge of recognition-dependent disease resistance in plants. We include a few crucial concepts to compare and contrast plant innate immunity with that more commonly associated with animals. There are appreciable differences, but also surprising parallels.

Cell Signaling during Cold, Drought, and Salt Stress

Abstract: Low temperature, drought, and high salinity are common stress conditions that adversely affect plant growth and crop production. The cellular and molecular responses of plants to environmental stress have been studied intensively ([Thomashow, 1999][1]; [Hasegawa et al., 2000][2]). Understanding the

Plant nitrogen assimilation and use efficiency.

Abstract: Crop productivity relies heavily on nitrogen (N) fertilization. Production and application of N fertilizers consume huge amounts of energy, and excess is detrimental to the environment; therefore, increasing plant N use efficiency (NUE) is essential for the development of sustainable agriculture. Plant NUE is inherently complex, as each step—including N uptake, translocation, assimilation, and remobilization—is governed by multiple interacting genetic and environmental factors. The limiting factors in plant metabolism for maximizing NUE are different at high and low N supplies, indicating great potential for improving the NUE of current cultivars, which were bred in well-fertilized soil. Decreasing environmental losses and increasing the productivity of crop-acquired N requires the coordination of carbohydrate and N metabolism to give high yields. Increasing both the grain and N harvest index to drive N acquisition and utilization are important approaches for breeding future high-NUE cultivars.