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Seed germination is an elaborate process during life cycle and regulated by numerous intrinsic and environmental cues. The role of amino acids in seed germination under salt stress has been reported, however the underlying mechanisms by which amino acids mitigate salt stress-induced seed germination remain largely elusive. Here, we reported that seed germination of Arabidopsis thaliana wild-type was suppressed by exposure to salt stress, and that the inhibition was markedly alleviated by exogenous application of those amino acids including Gly, Cys, Ser and Met. Calcium channel blocker La 3+ suppressed the mitigating effect of those amino acids on seed germination under salt stress. We further demonstrated that germination of wild-type seeds was suppressed by the glutamate receptor (GLR) antagonist 6,7-dinitriquinoxaline-2,3-dinoe (DNQX) under salt stress and the alleviative effects of amino acid on seed germination under salt stress were suppressed by DNQX. In addition, mutation of clade 3 GLRs led to lower seed germination rates of atglr3.4 and atglr3.7 mutants than those of wild-type seeds when exposed to NaCl and the mitigating effects of amino acids on seed germination rates of atglr3.4 and atglr3.7 were less effective than those of wild-type seeds. Moreover, seed germination of atglr3.4-1 , atglr3.4-2 and atglr3.7 mutants were less sensitive to CaCl 2 and DNQX than wild-type under salt stress. Taken together, these findings highlight the involvement of GLR-dependent Ca 2+ influx in alleviation of salt stress-induced inhibition of seed germination by exogenous amino acids.

Iron deficiency is one of the major limiting factors affecting quality and production of crops in calcareous soils. Numerous signaling molecules and transcription factors have been demonstrated to play a regulatory role in adaptation of plants to iron deficiency. However, the mechanisms underlying the iron deficiency-induced physiological processes remain to be fully dissected. Here, we demonstrated that the protein kinase CIPK23 was involved in iron acquisition. Lesion of CIPK23 rendered Arabidopsis mutants hypersensitive to iron deficiency, as evidenced by stronger chlorosis in young leaves and lower iron concentration than wild-type plants under iron-deficient conditions by down-regulating ferric chelate reductase activity. We found that iron deficiency evoked an increase in cytosolic Ca(2+) concentration and the elevated Ca(2+) would bind to CBL1/CBL9, leading to activation of CIPK23. These novel findings highlight the involvement of calcium-dependent CBL-CIPK23 complexes in the regulation of iron acquisition. Moreover, mutation of CIPK23 led to changes in contents of mineral elements, suggesting that CBL-CIPK23 complexes could be as "nutritional sensors" to sense and regulate the mineral homeostasis in Arabisopsis.

To understand the role of small GTPases in response to abiotic stress, we isolated a gene encoding a small GTPase, designated MfARL1, from a subtracted cDNA library in Medicago falcata, a native legume species in semi-arid grassland in northern China. The function of MfARL1 in response to salt stress was studied by expressing MfARL1 in Arabidopsis. Wild-type (WT) and transgenic plants constitutively expressing MfARL1 showed comparable phenotype when grown under control conditions. Germination of seeds expressing MfARL1 was less suppressed by salt stress than that of WT seeds. Transgenic seedlings had higher survival rate than WT seedlings under salt stress, suggesting that expression of MfARL1 confers tolerance to salt stress. The physiological and molecular mechanisms underlying these phenomena were elucidated. Salt stress led to a significant decrease in chlorophyll contents in WT plants, but not in transgenic plants. Transgenic plants accumulated less amounts of H(2)O(2) and malondialdehyde than their WT counterparts under salt stress, which can be accounted for by the higher catalase activities, lower activities of superoxide dismutase, and peroxidase in transgenic plants than in WT plants. Transgenic plants displayed lower Na(+)/K(+) ratio due to less accumulation of Na(+) than wild-type under salt stress conditions. The lower Na(+)/K(+) ratio may result from less accumulation of Na(+) due to reduced expression of AtHKT1 that encodes Na(+) transporter in transgenic plants under salt stress. These findings demonstrate that MfARL1 encodes a novel stress-responsive small GTPase that is involved in tolerance to salt stress.

Plants have developed numerous strategies to cope with phosphorus (P) deficiency resulting from low availability in soils. Evolution of ethylene and up-regulation of root secreted acid phosphatase activity are common for plants in response to P deficiency. To determine the role of ethylene in response of plants to P deficiency, we investigated the effects of ethylene precursor (1-amino cyclopropane-1-carboxylic acid, ACC) and ethylene synthesis antagonists (aminoethoxyvinylglycine AVG, cobalt, Co 2+ ) on P con