Serving as an important second messenger, calcium ion has unique properties and universal ability to transmit diverse signals that trigger primary physiological actions in cells in response to hormones, pathogens, light, gravity, and stress factors. Being a second messenger of paramount significance, calcium is required at almost all stages of plant growth and development, playing a fundamental role in regulating polar growth of cells and tissues and participating in plant adaptation to various stress factors. Many researches showed that calcium signals decoding elements are involved in ABA-induced stomatal closure and plant adaptation to drought, cold, salt and other abiotic stresses. Calcium channel proteins like AtTPC1 and TaTPC1 can regulate stomatal closure. Recently some new studies show that Ca 2+ is dissolved in water in the apoplast and transported primarily from root to shoot through the transpiration stream. The oscillating amplitudes of [Ca 2+ ] o and [Ca 2+ ] i are controlled by soil Ca 2+ concentrations and transpiration rates. Because leaf water use efficiency (WUE) is determined by stomatal closure and transpiration rate, so there may be a close relationship between Ca 2+ transporters and stomatal closure as well as WUE, which needs to be studied. The selection of varieties with better drought resistance and high WUE plays an increasing role in bio-watersaving in arid and semi-arid areas on the globe. The current paper reviews the relationship between calcium signals decoding elements and plant drought resistance as well as other abiotic stresses for further study.
As toxic divalent heavy metal ions, Zn 2þ , Cu 2þ , Mn 2þ , Cd 2þ , Hg 2þ , and Pb 2þ exert certain negative effects on plant growth and development, which has been paid huge amount of attention to in the management of heavy metal pollution. As a novel kind of endogenous plant hormone, salicylic acid (SA) is considered to exert beneficial physiological effects on plants. In this paper, the activities of catalase (CAT) and superoxide dismutase (SOD) in seedling leaves of a malting barley variety (called Ganpi No.4) cultured under hydroponic cultivation was measured under different treatments of Zn 2þ , Cu 2þ , Mn 2þ , Cd 2þ , Hg 2þ , and Pb 2þ and under different treatments of SA alleviation, respectively, and such values were then compared and analyzed. The results showed that all treatments of different ions and the activities of CAT and SOD and all SA-alleviated treatments and the activities of CAT and SOD exhibited nonlinear relationships. Zn 2þ , Cu 2þ , Mn 2þ exerted less severe stresses on the activities of CAT and SOD at lower concentrations, and led to significantly inhibitive effects on the activities of CAT and SOD at higher concentrations. And Cd 2þ , Hg 2þ , and Pb 2þ exerted significantly repressing effects on the activities of CAT and SOD. As for SA's alleviative effects, such effects were salient for most concentrations of Mn 2þ , Zn 2þ , and Cu 2þ treatments and lower concentrations of Cd 2þ , Hg 2þ , and Pb 2þ treatments, while were not significant for higher concentrations of Cd 2þ , Hg 2þ , and Pb 2þ treatments. These data provided theoretical references for studies on physiological mechanism of cereal crops' resistance or tolerance to heavy metal stress as biomarkers.
Considerable progresses have taken place, both in the methodology available to study changes in intracellular cytosolic calcium and in our understanding of calcium signaling cascades, but how calcium signals function in plant drought resistance is questionable. In plant cells, calcium plays roles as a second messenger coupling a wide range of extracellular stimuli with intracellular responses. Different extracellular stimuli trigger specific calcium signatures: dynamics, amplitude and duration of calcium transients specify the nature, implication and intensity of stimuli. Calcium-binding proteins (sensors) play a critical role in decoding calcium signatures and transducing signals by activating specific targets and corresponding metabolic pathways. Calmodulin is a calcium sensor known to regulate the activity of many mammalian proteins, whose targets in plants are now being identified. Higher plants possess a rapidly growing list of calmodulin targets with a variety of cellular functions. Nevertheless, many targets appear to be unique to higher plants and remain characterized, calling for a concerted effort to elucidate their functions. To date, three major classes of plant calcium signals, including calcium permeable ion channels, Ca 2+ /H + antiporters and Ca 2+ -ATPases, have been responsible for drought-stress signal transduction. This review summarizes the current knowledge of calcium signals involved in plant anti-drought and plant water use efficiency (WUE) and presents suggestions for future focus of study.
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