ABI1 and ABI2 are two protein serine/threonine phosphatases of type 2C (EC 3.1.3.16) that act as key regulators in the responses of Arabidopsis thaliana (L.) Heynh. to abscisic acid (ABA). They are involved in the control of ABA-mediated seed dormancy, stomatal closure and vegetative growth inhibition. Analysis of the enzymatic properties of ABI2 revealed high sensitivities towards protons and unsaturated fatty acids. Furthermore, the protein phosphatase activity of ABI2 is very sensitive to H2O2, which has recently emerged as a secondary messenger of ABA signalling. Upon H2O2 challenge, ABI2 is rapidly inactivated with an IC50 value of 50 microM in the presence of reduced glutathione. Inhibitor studies with phenylarsine oxide and manipulation of the redox status of ABI2 in vitro indicate that oxidation of critical cysteine residue(s) is responsible for inactivation. The levels of the major cellular thiol compounds cysteine and glutathione in leaves and seedlings of A. thaliana are compatible with a physiological role of H2O2 in regulating ABI2 activity. ABI2 is considered to exert negative regulation on ABA action. Thus, transient inactivation of this protein phosphatase by H2O2 would allow or enhance the ABA-dependent signalling process. In conclusion, ABI2 represents a likely target for redox-regulation of a hormonal signalling pathway in higher plants.

Protein phosphatases 2C (PP2Cs) exhibit diverse regulatory functions in signalling pathways of animals, yeast and plants. ABI1 is a PP2C of Arabidopsis that exerts negative control on signalling of the phytohormone abscisic acid (ABA). Characterisation of the redox sensitivity of ABI1 revealed a strong enzymatic inactivation by hydrogen peroxide (H 2 O 2 ) which has recently been implicated as a secondary messenger of ABA signalling. H 2 O 2 reversibly inhibited ABI1 activity in vitro with an IC 50 of approximately 140 W WM in the presence of physiological concentrations of glutathione. In addition, ABI1 was highly susceptible to inactivation by phenylarsine oxide (IC 50 = 3^4 W WM) indicative for the facile oxidation of vicinal cysteine residues. Thus, H 2 O 2 generated during ABA signalling seems to inactivate the negative regulator of the ABA response. ß

Fibrillins are lipid-binding proteins of plastids that are induced under abiotic stress conditions. In response to environmental stress, plants generate abscisic acid (ABA) as an endogenous signal. We show that ABA treatment and fibrillin accumulation enhance the tolerance of photosystem II toward light stress-triggered photoinhibition in Arabidopsis. ABA induces fibrillin accumulation, and the ABA response regulators ABI1 and ABI2 regulate fibrillin expression. The abundance of fibrillin transcripts was specifically reduced in the ABA-insensitive abi1 mutant but not in the abi2 mutant. However, leaves of abi2 revealed in comparison to WT and abi1 enhanced fibrillin levels, pointing to a posttranscriptional control mechanism. Protein interaction analysis identified the protein phosphatase ABI2 to target the preprotein of fibrillin. Interaction was abrogated either by deleting the signal peptide of prefibrillin or by the single amino acid exchange present in the phosphatase-deficient abi2 protein. Thus, ABI1 and ABI2 seem to control fibrillin expression that is involved in mediating ABAinduced photoprotection.ABI1 ͉ ABI2 ͉ abiotic stress ͉ photoinhibition ͉ light stress F ibrillins are plastid-associated lipid-binding proteins that are ubiquitous in plants (1, 2). They have been primarily characterized from chromoplasts of tomato and pepper fruits and are known to accumulate during abiotic stress in plastids e.g., inflicted by high light, cold, and drought (3-5), and also during pathogen infection (6). The family of fibrillin-like proteins is unique to plants, and the members contain a hydrophobic domain that associates with or anchors within lipids (7). Fibrillins associate with stromal lamellae of thylakoids and fibrillic carotenoid-containing structures of chromoplasts (3,8). A model for the fibrillic structures predicts a layer of fibrillin shielding polar lipids and carotenoids (1). In potato, the fibrillin C40.4 protein is specifically associated with the photosystem II (PSII) light-harvesting complex with a presumptive role in the modulation of photosynthetic efficiency (9). A posttranslational control of fibrillin accumulation by abscisic acid (ABA) was indicated by studies on ABA-deficient tomato that failed to accumulate fibrillin despite normal transcript levels (5). Although fibrillin is induced in response to abiotic stress, its role in stress responses and the molecular mechanism regulating its accumulation are still elusive.ABA plays a major role in regulating plant growth and development and mediating adaptations to environmental stress such as cold, drought, and salinity (10). ABA regulates ion channel activities involved in osmoregulation and stomatal closure and influences gene expression at the transcriptional and posttranscriptional levels (11,12). Key regulators of diverse ABA-mediated responses are ABI1 and ABI2. Both proteins are members of a larger family of plant protein phosphatases 2C (PP2Cs), several of which act negatively in a partially redundant manner on ABA responses (13-15). The...

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