Glc has hormone-like functions and controls many vital processes through mostly unknown mechanisms in plants. We report here on the molecular cloning of GLUCOSE INSENSITIVE1 (GIN1) and ABSCISIC ACID DEFICIENT2 (ABA2) which encodes a unique Arabidopsis short-chain dehydrogenase/reductase (SDR1) that functions as a molecular link between nutrient signaling and plant hormone biosynthesis. SDR1 is related to SDR superfamily members involved in retinoid and steroid hormone biosynthesis in mammals and sex determination in maize. Glc antagonizes ethylene signaling by activating ABA2/GIN1 and other abscisic acid (ABA) biosynthesis and signaling genes, which requires Glc and ABA synergistically. Analyses of aba2/gin1 null mutants define dual functions of endogenous ABA in inhibiting the postgermination developmental switch modulated by distinct Glc and osmotic signals and in promoting organ and body size and fertility in the absence of severe stress. SDR1 is sufficient for the multistep conversion of plastid- and carotenoid-derived xanthoxin to abscisic aldehyde in the cytosol. The surprisingly restricted spatial and temporal expression of SDR1 suggests the dynamic mobilization of ABA precursors and/or ABA.

The colonization of land by plants was a key event in the evolution of life. Here we report the draft genome sequence of the filamentous terrestrial alga Klebsormidium flaccidum (Division Charophyta, Order Klebsormidiales) to elucidate the early transition step from aquatic algae to land plants. Comparison of the genome sequence with that of other algae and land plants demonstrate that K. flaccidum acquired many genes specific to land plants. We demonstrate that K. flaccidum indeed produces several plant hormones and homologues of some of the signalling intermediates required for hormone actions in higher plants. The K. flaccidum genome also encodes a primitive system to protect against the harmful effects of high-intensity light. The presence of these plant-related systems in K. flaccidum suggests that, during evolution, this alga acquired the fundamental machinery required for adaptation to terrestrial environments.

Movement of the plant hormone abscisic acid (ABA) within plants has been documented; however, the molecular mechanisms that regulate ABA transport are not fully understood. By using a modified yeast two-hybrid system, we screened Arabidopsis cDNAs capable of inducing interactions between the ABA receptor PYR/PYL/RCAR and PP2C protein phosphatase under low ABA concentrations. By using this approach, we identified four members of the NRT1/PTR family as candidates for ABA importers. Transport assays in yeast and insect cells demonstrated that at least one of the candidates ABA-IMPORTING TRANSPORTER (AIT) 1, which had been characterized as the low-affinity nitrate transporter NRT1.2, mediates cellular ABA uptake. Compared with WT, the ait1 / nrt1 . 2 mutants were less sensitive to exogenously applied ABA during seed germination and/or postgermination growth, whereas overexpression of AIT1 / NRT1 . 2 resulted in ABA hypersensitivity in the same conditions. Interestingly, the inflorescence stems of ait1 / nrt1 . 2 had a lower surface temperature than those of the WT because of excess water loss from open stomata. We detected promoter activities of AIT1 / NRT1 . 2 around vascular tissues in inflorescence stems, leaves, and roots. These data suggest that the function of AIT1 / NRT1 . 2 as an ABA importer at the site of ABA biosynthesis is important for the regulation of stomatal aperture in inflorescence stems.