Ethylene regulates plant growth, development, and responsiveness to a variety of stresses. Cloning of the Arabidopsis EIN2 gene identifies a central component of the ethylene signaling pathway. The amino-terminal integral membrane domain of EIN2 shows similarity to the disease-related Nramp family of metal-ion transporters. Expression of the EIN2 CEND is sufficient to constitutively activate ethylene responses and restores responsiveness to jasmonic acid and paraquat-induced oxygen radicals to mutant plants. EIN2 is thus recognized as a molecular link between previously distinct hormone response pathways. Plants may use a combinatorial mechanism for assessing various stresses by enlisting a common set of signaling molecules.

Abscisic acid (ABA) signaling is important for stress responses and developmental processes in plants. A subgroup of protein phosphatase 2C (group A PP2C) or SNF1-related protein kinase 2 (subclass III SnRK2) have been known as major negative or positive regulators of ABA signaling, respectively. Here, we demonstrate the physical and functional linkage between these two major signaling factors. Group A PP2Cs interacted physically with SnRK2s in various combinations, and efficiently inactivated ABA-activated SnRK2s via dephosphorylation of multiple Ser/Thr residues in the activation loop. This step was suppressed by the RCAR/PYR ABA receptors in response to ABA. However the abi1-1 mutated PP2C did not respond to the receptors and constitutively inactivated SnRK2. Our results demonstrate that group A PP2Cs act as 'gatekeepers' of subclass III SnRK2s, unraveling an important regulatory mechanism of ABA signaling.A s sessile organisms, plants have to rapidly recognize and adapt to environmental changes. The phytohormone abscisic acid (ABA) plays a central role in such responses (1, 2), and is also involved in many developmental processes (3) and defense systems (4). Thus, ABA functions as a key molecule that unifies and regulates biotic and abiotic stress responses and the developmental status of the plant. Hence, the biological and agricultural importance of ABA has led to extensive studies on its signaling mechanism, and many putative signal transducers have been reported (5). Although it has been difficult to integrate all of the current findings, significant progress was recently made by two independent research groups. They identified the RCAR/ PYR family proteins as ABA receptors that inhibit protein phosphatase 2C (PP2C) in an ABA-dependent manner (6, 7). Among plant PP2Cs, a group A subfamily (e.g., ABI1 and ABI2) is annotated as negative regulators of the ABA response in seeds through to adult plants (5). Such PP2C-dependent negative regulation can be canceled by RCAR/PYR in response to ABA, leading to activation of some positive regulatory pathways (6, 7). Previously, we demonstrated that ABI1 interacts with a protein kinase, SRK2E (OST1/SnRK2.6) (8). SRK2E belongs to the SNF1-related protein kinase 2 (SnRK2) family, which is activated by ABA or osmotic stress and positively regulates the ABA response in various tissues (9 -11). Furthermore, ABAdependent activation of SRK2E was repressed in an abi1-1 mutant, suggesting that SnRK2 functions downstream of PP2C (8, 9). Based on these findings, a model was hypothesized in which RCAR/PYR and PP2C negatively regulate SnRK2 (7). However, there is no direct evidence demonstrating how PP2C regulates SnRK2, and the molecular process between them remains a question in ABA signaling. Our presented data clearly demonstrated the biochemical relation between PP2C and SnRK2 and elucidated an important regulatory mechanism of ABA signaling. Results and DiscussionRequirement of SnRK2 Activity for ABA Responses. In Arabidopsis, subclass III of the SnRK2 family is composed of...

SUMMARYUnderstanding abiotic stress responses in plants is an important and challenging topic in plant research. Physiological and molecular biological analyses have allowed us to draw a picture of abiotic stress responses in various plants, and determination of the Arabidopsis genome sequence has had a great impact on this research field. The availability of the complete genome sequence has facilitated access to essential information for all genes, e.g. gene products and their function, transcript levels, putative cis-regulatory elements, and alternative splicing patterns. These data have been obtained from comprehensive transcriptome analyses and studies using full-length cDNA collections and T-DNA-or transposon-tagged mutant lines, which were also enhanced by genome sequence information. Moreover, studies on novel regulatory mechanisms involving use of small RNA molecules, chromatin modulation and genomic DNA modification have enabled us to recognize that plants have evolved complicated and sophisticated systems in response to complex abiotic stresses. Integrated data obtained with various 'omics' approaches have provided a more comprehensive picture of abiotic stress responses. In addition, research on stress responses in various plant species other than Arabidopsis has increased our knowledge regarding the mechanisms of plant stress tolerance in nature. Based on this progress, improvements in crop stress tolerance have been attempted by means of gene transfer and marker-assisted breeding. In this review, we summarize recent progress in abiotic stress studies, especially in the post-genomic era, and offer new perspectives on research directions for the next decade.