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 guard cells which control their opening in response to many environmental stimuli, including blue light1,2. Opening of the pores is mediated by K+ accumulation in guard cells through a K+ channel and driven by an inside-negative electrical potential3. Blue light causes phosphorylation and activation of the plasma membrane H+-ATPase that creates this potential1,2,4-6. Thus far, no blue light receptor mediating stomatal opening has been identified7, although the carotenoid, zeaxanthin, has been proposed. Arabidopsis mutants deficient in specific blue-light-mediated responses have identified7,9-14 four blue light receptors, cryptochrome 1 (cry1), cryptochrome 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 redundantly as blue light receptors mediating stomatal opening.
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