Guard cells, which form stoma in leaf epidermis, sense and integrate environmental signals to modulate stomatal aperture in response to diverse conditions. Under drought stress, plants synthesize abscisic acid (ABA), which in turn induces a rapid closing of stoma, to prevent water loss by transpiration. However, many aspects of the molecular mechanism for ABAmediated stomatal closure are still not understood. Here, we report a novel negative regulator of guard cell ABA signaling, DOR, in Arabidopsis (Arabidopsis thaliana). The DOR gene encodes a putative F-box protein, a member of the S-locus F-box-like family related to AhSLF-S 2 and specifically interacting with ASK14 and CUL1. A null mutation in DOR resulted in a hypersensitive ABA response of stomatal closing and a substantial increase of drought tolerance; in contrast, the transgenic plants overexpressing DOR were more susceptible to the drought stress. DOR is strongly expressed in guard cells and suppressed by ABA treatment, suggesting a negative feedback loop of DOR in ABA responses. Double-mutant analyses of dor with ABA-insensitive mutant abi1-1 showed that abi1-1 is epistatic to dor, but no apparent change of phospholipase Da1 was detected between the wild type and dor. Affymetrix GeneChip analysis showed that DOR likely regulates ABA biosynthesis under drought stress. Taken together, our results demonstrate that DOR acts independent of phospholipase Da1 in an ABA signaling pathway to inhibit the ABA-induced stomatal closure under drought stress.

Exposure of Arabidopsis callus to microgravity has a significant impact on the expression of proteins involved in stress responses, carbohydrate metabolism, protein synthesis, intracellular trafficking, signaling, and cell wall biosynthesis. Microgravity is among the main environmental stress factors that affect plant growth and development in space. Understanding how plants acclimate to space microgravity is important to develop bioregenerative life-support systems for long-term space missions. To evaluate the spaceflight-associated stress and identify molecular events important for acquired microgravity tolerance, we compared proteomic profiles of Arabidopsis thaliana callus grown under microgravity on board the Chinese spacecraft SZ-8 with callus grown under 1g centrifugation (1g control) in space. Alterations in the proteome induced by microgravity were analyzed by high performance liquid chromatography-electrospray ionization-tandem mass spectrometry with isobaric tags for relative and absolute quantitation labeling. Forty-five proteins showed significant (p < 0.05) and reproducible quantitative differences in expression between the microgravity and 1g control conditions. Of these proteins, the expression level of 24 proteins was significantly up-regulated and that of 21 proteins was significantly down-regulated. The functions of these proteins were involved in a wide range of cellular processes, including general stress responses, carbohydrate metabolism, protein synthesis/degradation, intracellular trafficking/transportation, signaling, and cell wall biosynthesis. Several proteins not previously known to be involved in the response to microgravity or gravitational stimuli, such as pathogenesis-related thaumatin-like protein, leucine-rich repeat extension-like protein, and temperature-induce lipocalin, were significantly up- or down-regulated by microgravity. The results imply that either the normal gravity-response signaling is affected by microgravity exposure or that microgravity might inappropriately induce altered responses to other environmental stresses.