Abstract
Phototropins are blue light receptors that mediate responses such as phototropism, chloroplast movement, stomatal opening, and leaf expansion. One candidate signaling molecule from phototropins is cytosolic Ca2+, since phototropins increase the cytosolic Ca2+ in seedlings and mesophyll cells. The potential involvement of Ca2+ in the blue light–dependent activation of plasma membrane H+-ATPase in guard cells has been reported, but it is not yet known whether cytosolic Ca2+ in guard cells increases in response to blue light. We recently studied changes in the cytosolic Ca2+ in guard cells in response to blue light using aequorin-transformed Arabidopsis. We detected no increase in Ca2+ prior to blue light–dependent activation of plasma membrane H+-ATPase that depended on phototropins. But we detected a photosynthesis-dependent Ca2+ increase. Only when the external K+ concentration was low did blue light induce Ca2+ influx based on phototropin-mediated membrane hyperpolarization. In this addendum, I discuss Ca2+ changes in response to blue light in guard cells and phototropin-mediated Ca2+ signaling.