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• Pulses of blue light induce cytosolic Ca2+ transients lasting about 80 s in Arabidopsis and tobacco seedlings. Use of organelle-targeted aequorins shows that Ca2+ increases are limited to the cytoplasm. Blue light treatment of cry1, cry2, and nph1 mutants showed that NPH1, which regulates phototropism, is largely responsible for the Ca2+ transient.
• Furthermore, known interactions between red and blue light and between successive blue light pulses on phototropic sensitivity are mirrored in the blue light control of cytosolic Ca2+ in these seedlings. Our observations raise the possibility that physiological responses regulated by NPH1, such as phototropism, may be transduced through cytosolic C...
• B signaling controls important plant processes such as phototropism, suppression of stem extension, chloroplast movement, circadian timing, and expression of numerous genes.
• Lewis et al. (12) concluded that either other signal transduction processes such as phosphorylation were the primary pathway or Ca2+ changes were so (undetectably) small as to be irrelevant to signal transduction.
• We have detected both adaptation and crosstalk between the B and R signal transduction pathways. These data raise the possibility that cytosolic Ca2+ ([Ca2+]c) may regulate tropic bending mediated by NPH1

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