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The movement of guard cells in stomatal complexes controls water loss and CO2 uptake in plants. Examination of the dual-affinity nitrate transporter gene AtNRT1.1 (CHL1) revealed that it is expressed and functions in Arabidopsis guard cells. CHL1 promoter-beta-glucuronidase and CHU promoter-green fluorescent protein constructs showed strong expression in guard cells, and immunolocalization experiments with anti-CHL1 antibody confirmed these results. To assess CHL1 function, chil mutant plants grown in the presence of nitrate were examined. Compared with wild-type plants, chil mutants had reduced stomatal opening and reduced transpiration rates in the light or when deprived of CO2 in the dark. These effects result in enhanced drought tolerance in chl1 mutants. At the cellular level, chl1 mutants showed reduced nitrate accumulation in guard cells during stomatal opening and failed to show nitrate-induced depolarization of guard cells. In wild-type guard cells, nitrate induced depolarization, and nitrate concentrations increased threefold during stomatal opening. These results identify an anion transporter that functions in stomatal opening and demonstrate that CHL1 supports stomatal function in the presence of nitrate.
INTRODUCTION
Stomata act as ports that regulate the uptake of CO2 for photosynthesis and the evaporation of water for transpiration in plants. Gas exchange through stomatal pores in the leaves of plants is regulated by the turgor-driven expansion and contraction of guard cells in response to environmental and internal signals, including light, humidity, CO2, phytohormones, calcium, and reactive oxygen species (reviewed by Blatt, 2000; Assmann and Wang, 2001; Dietrich et al., 2001; Schroeder et al., 2001; Roelfsema and Hedrich, 2002). Changes in guard cell turgor are driven by fluxes of K+ and Cl- and, depending on the growth conditions and the time of day, the accumulation or loss of malate and Suc (reviewed by Talbott and Zeiger, 1998; Blatt, 2000; Assmann and Wang, 2001; Dietrich et al., 2001; Schroeder et al., 2001). For example, light-induced stomatal opening involves activation of the plasma membrane H+-ATPase, which results in the hyperpolarization of the plasma membrane and the opening of inward-rectifying K+ channels. The influx of K+ is accompanied by an influx of Cl- and an accumulation of malate.
The search for ion channel/transporter genes involved in stomatal movement has led to the identification of multiple K channel genes that are expressed and function in...