In plants, multistep component systems play important roles in signal transduction in response to environmental stimuli and plant growth regulators. Arabidopsis contains six nonethylene receptor histidine kinases, and, among them, AHK1/ATHK1, AHK2, AHK3, and CRE1 were shown to be stress-responsive, suggesting their roles in the regulation of plant response to abiotic stress. Gain-and loss-of-function studies in Arabidopsis indicated that AHK1 is a positive regulator of drought and salt stress responses and abscisic acid (ABA) signaling. Microarray analysis of the ahk1 mutant revealed a down-regulation of many stress-and/or ABA-inducible genes, including AREB1, ANAC, and DREB2A transcription factors and their downstream genes. These data suggest that AHK1 functions upstream of AREB1, ANAC, and DREB2A and positively controls stress responses through both ABA-dependent and ABAindependent signaling pathways. In addition, AHK1 plays important roles in plant growth because the ahk1 ahk2 ahk3 triple mutant showed further reduced growth. Unlike AHK1, loss-offunction analysis of ahk2, ahk3, and cre1 implied that the stressresponsive AHK2, AHK3, and CRE1 act as negative regulators in ABA signaling. AHK2 and AHK3 also negatively control osmotic stress responses in Arabidopsis because ahk2, ahk3, and ahk2 ahk3 mutants were strongly tolerant to drought and salt stress due to up-regulation of many stress-and/or ABA-inducible genes. Last, cytokinin clearly mediates stress responses because it was required for CRE1 to function as a negative regulator of osmotic stress.cross-talk ͉ microarray ͉ osmotic stress ͉ plant growth P lants are constantly exposed to environmental stresses that frequently impose constraints on their growth and productivity. Cells have developed elaborate and sensitive protection systems that enable them to rapidly signal, respond, and properly adapt to various stresses, including drought and high salinity (1-5). Phosphorylation, which is catalyzed by protein kinases, is a key mechanism for intracellular signal transduction in both eukaryotic and prokaryotic cells. Exposure of Saccharomyces cerevisiae to a high-osmolarity environment leads to rapid phosphorylation and activation of the MAPK kinase Hog1 through either the SLN1 or SHO1 branch of the HOG pathway (6). Overexpression of Arabidopsis histidine kinase AHK1/ATHK1 in S. cerevisiae sln1 and sho1 deletion mutants enables the yeast mutant to grow normally under high-salinity conditions, suggesting that the histidine kinase (HK) AHK1 can sense and transduce a signal of external osmolarity to downstream targets (7). Moreover, the AHK1 transcript accumulated in Arabidopsis in response to changes in external osmolarity, suggesting the functional importance of AHK1 for the efficient sensing of environmental signals. However, it is still not known whether AHK1 functions as an osmosensor in plants.Beside the nonethylene receptor AHK1, the Arabidopsis genome encodes 10 other putative HKs: the ethylene receptor (ER) HKs ETR1, ETR2, EIN4, ERS1, and ERS2 and the ...

Full-length cDNAs are essential for functional analysis of plant genes. Using the biotinylated CAP trapper method, we constructed full-length Arabidopsis cDNA libraries from plants in different conditions, such as drought-treated, coldtreated, or unstressed plants, and at various developmental stages from germination to mature seed. We prepared a cDNA microarray using ‫ف‬ 1300 full-length Arabidopsis cDNAs to identify drought-and cold-inducible genes and target genes of DREB1A/CBF3, a transcription factor that controls stress-inducible gene expression. In total, 44 and 19 cDNAs for drought-and cold-inducible genes, respectively, were isolated, 30 and 10 of which were novel stress-inducible genes that have not been reported as drought-or cold-inducible genes previously. Twelve stress-inducible genes were identified as target stress-inducible genes of DREB1A, and six of them were novel. On the basis of RNA gel blot and microarray analyses, the six genes were identified as novel drought-and cold-inducible genes that are controlled by DREB1A. Eleven DREB1A target genes whose genomic sequences have been registered in the GenBank database contained the dehydration-responsive element (DRE) or DRE-related CCGAC core motif in their promoter regions. These results show that our full-length cDNA microarray is a useful material with which to analyze the expression pattern of Arabidopsis genes under drought and cold stresses, to identify target genes of stress-related transcription factors, and to identify potential cis -acting DNA elements by combining the expression data with the genomic sequence data. INTRODUCTIONSequencing projects are producing large quantities of genomic and cDNA sequences for a number of organisms. In the model plant Arabidopsis, the complete genomic sequences of two chromosomes have been determined (Lin et al., 1999;Mayer et al., 1999), and the entire genomic sequence was completed by the end of 2000. Expressed sequence tag (EST) projects have also provided a major contribution with the discovery of expressed genes (Höfte et al., 1993;Newman et al., 1994;Cooke et al., 1996;Asamizu et al., 2000). A recent release of dbEST (the EST database of the National Center for Biotechnology Information, http:// www.ncbi.nlm.nih.gov) contained partial cDNA sequences in which more than half of the total gene complement (i.e., ‫ف‬ 28,000 genes) is represented (as estimated from the gene content of the entirely sequenced chromosome 2 in Arabidopsis [Lin et al., 1999]). A major challenge for the coming decade is the functional analysis of this large set of genes.Recently, microarray technology has become a useful tool for the analysis of genome-scale gene expression (Schena et al., 1995;Eisen and Brown, 1999). This DNA chip-based technology arrays cDNA sequences on a glass slide at a density Ͼ 1000 genes/cm 2 . These arrayed sequences are hybridized simultaneously to a two-color fluorescently labeled cDNA probe pair prepared from RNA samples of different cell or tissue types, allowing direct and large-scale comparati...

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