phot1 and phot2 mediate blue light regulation of stomatal opening

The stomatal pores of higher plants allow for gaseous exchange into and out of leaves. Situated in the epidermis, they are surrounded by a pair of gua rd cells which control their opening in response to many environmental stim uli, including blue light(1,2). Opening of the pores is mediated by K+ accu mulation in guard cells through a K+ channel and driven by an inside-negati ve electrical potential(3). Blue light causes phosphorylation and activatio n of the plasma membrane H+-ATPase that creates this potential(1,2,4-6). Th us far, no blue light receptor mediating stomatal opening has been identifi ed(7), although the carotenoid, zeaxanthin, has been proposed(2,8). Arabido psis mutants deficient in specific blue-light-mediated responses have ident ified(7,9-14) four blue light receptors, cryptochrome 1 (cry1), cryptochrom e 2 (cry2), phot1 and phot2. Here we show that in a double mutant of phot1 and phot2 stomata do not respond to blue light although single mutants are phenotypically normal. These results demonstrate that phot1 and phot2 act r edundantly as blue light receptors mediating stomatal opening.