Nitrate reallocation to plant roots occurs frequently under adverse conditions and was recently characterized to be actively regulated by Nitrate Transporter1.8 (NRT1.8) in Arabidopsis (Arabidopsis thaliana) and implicated as a common response to stresses. However, the underlying mechanisms remain largely to be determined. In this study, characterization of NRT1.5, a xylem nitrate-loading transporter, showed that the mRNA level of NRT1.5 is down-regulated by salt, drought, and cadmium treatments. Functional disruption of NRT1.5 enhanced tolerance to salt, drought, and cadmium stresses. Further analyses showed that nitrate, as well as Na + and Cd 2+ levels, were significantly increased in nrt1.5 roots. Important genes including Na + /H + exchanger1, Salt overly sensitive1, Pyrroline-5-carboxylate synthase1, Responsive to desiccation29A, Phytochelatin synthase1, and NRT1.8 in stress response pathways are steadily up-regulated in nrt1.5 mutant plants. Interestingly, altered accumulation of metabolites, including proline and malondialdehyde, was also observed in nrt1.5 plants. These data suggest that NRT1.5 is involved in nitrate allocation to roots and the consequent tolerance to several stresses, in a mechanism probably shared with NRT1.8.

(J-M.G).Elevations in extracellular calcium ([Ca 2+ ] o ) are known to stimulate cytosolic calcium ([Ca 2+ ] cyt ) oscillations to close stomata. However, the underlying mechanisms regulating this process remain largely to be determined. Here, through the functional characterization of the calcium underaccumulation mutant cau1, we report that the epigenetic regulation of CAS, a putative Ca 2+ binding protein proposed to be an external Ca 2+ sensor, is involved in this process. cau1 mutant plants display increased drought tolerance and stomatal closure. A mutation in CAU1 significantly increased the expression level of the calcium signaling gene CAS, and functional disruption of CAS abolished the enhanced drought tolerance and stomatal [Ca 2+ ] o signaling in cau1. Map-based cloning revealed that CAU1 encodes the H4R3sme2 (for histone H4 Arg 3 with symmetric dimethylation)-type histone methylase protein arginine methytransferase5/Shk1 binding protein1. Chromatin immunoprecipitation assays showed that CAU1 binds to the CAS promoter and modulates the H4R3sme2-type histone methylation of the CAS chromatin. When exposed to elevated [Ca 2+ ] o , the protein levels of CAU1 decreased and less CAU1 bound to the CAS promoter. In addition, the methylation level of H4R3sme2 decreased in the CAS chromatin. Together, these data suggest that in response to increases in [Ca 2+ ] o , fewer CAU1 protein molecules bind to the CAS promoter, leading to decreased H4R3sme2 methylation and consequent derepression of the expression of CAS to mediate stomatal closure and drought tolerance. INTRODUCTIONCytosolic calcium ([Ca 2+ ] cyt ) plays a pivotal role as a second messenger in plant development and interactions with the environment (Helpler and Wayne, 1985;Poovaiah and Reddy, 1993;Bush, 1995). Various signals, including biotic stress, abiotic stress, hormones, and mechanical disturbance, are sensed by plant cells and elicit repetitive oscillation or spiking of [Ca 2+ ] cyt , which are termed Ca 2+ signatures and regulated by Ca 2+ channels, pumps, or carriers localized at the plasma membrane or in the membranes of organelles (Bush, 1995;Thuleau et al., 1998;Rudd and Franklin-Tong, 1999;Sanders et al., 1999;Allen et al., 2000Allen et al., , 2001Harper, 2001). These Ca 2+ signatures vary in frequency and amplitude in response to different stimuli and thus are believed to encode stimulus-specific information that can be transduced and sensed by downstream Ca 2+ sensors, including calmodulins, calcium-dependent proteins, and calcineurin B-like proteins (Zielinski, 1998;Luan et al., 2002;Sanders et al., 2002). These calcium sensors decode Ca 2+ signatures by binding Ca 2+ ions via conserved EF-hand domains and transduce them into specific cellular responses, such as altered phosphorylation and expression of target genes, to bring about physiological responses corresponding to the original stimuli (Luan et al., 2002;Sanders et al., 2002).In contrast with intracellular Ca 2+ , the role of extracellular Ca 2+ ([Ca 2+ ] o ) remains to ...