Abstract
Calcium is involved in the signal transduction pathway from phototropins, the blue light photoreceptor kinases which mediate chloroplast movements. The chloroplast accumulation response in low light is controlled by both phot1 and phot2, while only phot2 is involved in avoidance movement induced by strong light. Phototropins elevate cytosolic Ca(2+) after activation by blue light. In higher plants, both types of chloroplast responses depend on Ca(2+), and internal calcium stores seem to be crucial for these processes. Yet, the calcium signatures generated after the perception of blue light by phototropins are not well understood. To characterize the localization of calcium in Arabidopsis mesophyll cells, loosely bound (exchangeable) Ca(2+) was precipitated with potassium pyroantimonate and analyzed by transmission electron microscopy followed by energy-dispersive X-ray microanalysis. In dark-adapted wild-type Arabidopsis leaves, calcium precipitates were observed at the cell wall, where they formed spherical structures. After strong blue light irradiation, calcium at the apoplast prevailed, and bigger, multilayer precipitates were found. Spherical calcium precipitates were also detected at the tonoplast. After red light treatment as a control, the precipitates at the cell wall were smaller and less numerous. In the phot2 and phot1phot2 mutants, calcium patterns were different from those of wild-type plants. In both mutants, no elevation of calcium after blue light treatment was observed at the cell periphery (including the cell wall and a fragment of cytoplasm). This result confirms the involvement of phototropin2 in the regulation of Ca(2+) homeostasis in mesophyll cells.
Highlights
Calcium ions are considered to be an extremely versatile secondary messenger, a key element of many responses to biotic and abiotic factors
The KPA precipitation method does not depict the overall distribution of calcium within cells, but only that fraction which is susceptible to precipitation
Calcium imaging in mesophyll cells after light treatment presents a number of difficulties
Summary
Calcium ions are considered to be an extremely versatile secondary messenger, a key element of many responses to biotic and abiotic factors (for a review, see Dodd et al, 2010). Specific ‘calcium signatures’ are generated, mainly in the cytosol, and in the nucleus, mitochondria, and chloroplasts (Stael et al, 2012). Translocations between organelles and the apoplast generates spatially and temporally distinct cytosolic calcium patterns and produces stimulus-specific responses (McAinsh and Pittman, 2009). In the case of blue light signaling, calcium elevation follows the activation of phototropins. Blue light photoreceptor kinases which mediate plant movements and rapid growth responses. The two phototropins of Arabidopsis thaliana, phot and phot, are characterized by different light sensitivities, though they share highly redundant functions. They both control phototropism, leaf expansion, stomatal opening, and the chloroplast accumulation response. Phot mediates the inhibition of the hypocotyl growth reaction, and only phot mediates chloroplast avoidance and dark positioning (for a review, see Christie, 2007)
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