Stimulation of the blue light phototropic receptor NPH1 causes a transientincrease in cytosolic Ca2+

Blue light regulates plant growth and development, and three photoreceptors , CRY1, CRY2, and NPH1, have been identified. The transduction pathways of these receptors are poorly understood. Transgenic plants containing aequori n have been used to dissect the involvement of these three receptors in the regulation of intracellular Ca2+. Pulses of blue light induce cytosolic Ca 2+ transients lasting about 80 s in Arabidopsis and tobacco seedlings. Use of organelle-targeted aequorins shows that Ca2+ increases are limited to th e cytoplasm. Blue light treatment of cry1, cry2, and nph1 mutants showed th at NPH1, which regulates phototropism, is largely responsible for the Ca2transient. The spectral response of the Ca2+ transient is similar to that o f phototropism, supporting NPH1 involvement. Furthermore, known interaction s between red and blue light and between successive blue light pulses on ph ototropic sensitivity are mirrored in the blue light control of cytosolic C a2+ in these seedlings. Our observations raise the possibility that physiol ogical responses regulated by NPH1, such as phototropism, may be transduced through cytosolic Ca2+.