Changes in Ca2+ metabolism in Arabidopsis guard cells in response to blue light

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 Ca(2+), since phototropins increase the cytosolic Ca(2+) in seedlings and mesophyll cells. The potential involvement of Ca(2+) 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 Ca(2+) in guard cells increases in response to blue light. We recently studied changes in the cytosolic Ca(2+) in guard cells in response to blue light using aequorin-transformed Arabidopsis. We detected no increase in Ca(2+) prior to blue light-dependent activation of plasma membrane H(+)-ATPase that depended on phototropins. But we detected a photosynthesis-dependent Ca(2+) increase. Only when the external K(+) concentration was low did blue light induce Ca(2+) influx based on phototropin-mediated membrane hyperpolarization. In this addendum, I discuss Ca(2+) changes in response to blue light in guard cells and phototropin-mediated Ca(2+) signaling.