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The Arabidopsis calcium-sensing receptor CAS is a crucial regulator of extracellular calcium-induced stomatal closure. Free cytosolic Ca2+ (Ca2+i) increases in response to a high extracellular calcium (Ca2+o) level through a CAS signalling pathway and finally leads to stomatal closure. Multidisciplinary approaches including histochemical, pharmacological, fluorescent, electrochemical, and molecular biological methods were used to discuss the relationship of hydrogen peroxide (H2O2) and nitric oxide (NO) signalling in the CAS signalling pathway in guard cells in response to Ca2+o. Here it is shown that Ca2+o could induce H2O2 and NO production from guard cells but only H2O2 from chloroplasts, leading to stomatal closure. In addition, the CASas mutant, the atrbohD/F double mutant, and the Atnoa1 mutant were all insensitive to Ca2+o-stimulated stomatal closure, as well as H2O2 and NO elevation in the case of CASas. Furthermore, it was found that the antioxidant system might function as a mediator in Ca2+o and H2O2 signalling in guard cells. The results suggest a hypothetical model whereby Ca2+o induces H2O2 and NO accumulation in guard cells through the CAS signalling pathway, which further triggers Ca2+i transients and finally stomatal closure. The possible cross-talk of Ca2+o and abscisic acid signalling as well as the antioxidant system are discussed.

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We selected six tree species, Pinus massoniana Lamb., Cryptomeria fortunei Hooibr. ex Otto et Dietr., Cunninghamia lanceolata (Lamb.) Hook., Liquidambar formosana Hance, Pinus armandii Franch. and Castanopsis chinensis Hance, which are widely distributed as dominant species in the forest of southern China where acid deposition is becoming more and more serious in recent years. We investigated the effects and potential interactions between simulated acid rain (SiAR) and three calcium (Ca) levels on seed germination, radicle length, seedling growth, chlorophyll content, photosynthesis and Ca content in leaves of these six species. We found that the six species showed different responses to SiAR and different Ca levels. Pinus armandii and C. chinensis were very tolerant to SiAR, whereas the others were more sensitive. The results of significant SiAR × Ca interactions on different physiological parameters of the six species demonstrate that additional Ca had a dramatic rescue effect on the seed germination and seedling growth for the sensitive species under SiAR. Altogether, we conclude that the negative effects of SiAR on seed germination, seedling growth and photosynthesis of the four sensitive species could be ameliorated by Ca addition. In contrast, the physiological processes of the two tolerant species were much less affected by both SiAR and Ca treatments. This conclusion implies that the degree of forest decline caused by long-term acid deposition may be attributed not only to the sensitivity of tree species to acid deposition, but also to the Ca level in the soil.

A proteomic study using 2-D gel electrophoresis and MALDI-TOF MS was performed to characterize the responses of Arabidopsis thaliana plants to simulated acid rain (SiAR) stress, which is a global environmental problem and has become a serious issue in China in recent years. The emphasis of the present study was to investigate the overall protein expression changes when exposed to SiAR. Out of over 1000 protein spots reproducibly resolved, 50 of them changed their abundance by at least 2-fold. Analysis of protein expression patterns revealed that a set of proteins associated with energy production, metabolism, cell rescue, cell defense and protein folding, etc., could play important roles in mediating plant response to SiAR. In addition to this, some proteins involved in stress responses and jasmonic acid pathway are also involved in plant response to SiAR. More interestingly, the expression of several ubiquitination-related proteins changed dramatically after 32-h SiAR treatment, suggesting that they may act as a molecular marker for the injury phenotype caused by SiAR. Based on our results, we proposed a schematic model to explain the mechanisms associated with the systematic response of Arabidopsis plants to SiAR.

It is well known that nitric oxide (NO) enhances salt tolerance of glycophytes. However, the effect of NO on modulating ionic balance in halophytes is not very clear. This study focuses on the role of NO in mediating K+/Na+ balance in a mangrove species, Kandelia obovata Sheue, Liu and Yong. We first analyzed the effects of sodium nitroprusside (SNP), an NO donor, on ion content and ion flux in the roots of K. obovata under high salinity. The results showed that 100 μM SNP significantly increased K+ content and Na+ efflux, but decreased Na+ content and K+ efflux. These effects of NO were reversed by specific NO synthesis inhibitor and scavenger, which confirmed the role of NO in retaining K+ and reducing Na+ in K. obovata roots. Using western-blot analysis, we found that NO increased the protein expression of plasma membrane (PM) H+-ATPase and vacuolar Na+/H+ antiporter, which were crucial proteins for ionic balance. To further clarify the molecular mechanism of NO-modulated K+/Na+ balance, partial cDNA fragments of inward-rectifying K+ channel, PM Na+/H+ antiporter, PM H+-ATPase, vacuolar Na+/H+ antiporter and vacuolar H+-ATPase subunit c were isolated. Results of quantitative real-time PCR showed that NO increased the relative expression levels of these genes, while this increase was blocked by NO synthesis inhibitors and scavenger. Above results indicate that NO greatly contribute to K+/Na+ balance in high salinity-treated K. obovata roots, by activating AKT1-type K+ channel and Na+/H+ antiporter, which are the critical components in K+/Na+ transport system.

Plant calcium sensing receptor (CAS) optimizes photosynthesis by its effect on the formation of photosynthetic electron transport. CAS also regulates transpiration under water stress. A novel correlation between CAS and plant water use efficiency is revealed