2 GENETICALLY SEPARABLE PHASES OF GROWTH-INHIBITION INDUCED BY BLUE-LIGHT IN ARABIDOPSIS SEEDLINGS

High fluence-rate blue light (BL) rapidly inhibits hypocotyl growth in Arabidopsis, as in other species, after a lag time of 30 s. This grow th inhibition is always preceded by the activation of anion channels. The membrane depolarization that results from the activation of anion channels by BL was only 30% of the wild-type magnitude in hy4, a mutan t lacking the HY4 BL receptor. High-resolution measurements of growth made with a computer-linked displacement transducer or digitized image s revealed that BL caused a rapid inhibition of growth in wild-type an d hy4 seedlings. This inhibition persisted in wild-type seedlings duri ng more than 40 h of continuous BL. By contrast, hy4 escaped from the initial inhibition after approximately 1 h of BL and grew faster than wild type for approximately 30 h. Wild-type seedlings treated with 5-n itro-2-(3-phenylpropylamino)-benzoic acid, a potent blocker of the BL- activated anion channel, displayed rapid growth inhibition, but, simil ar to hy4, these seedlings escaped from inhibition after approximately 1 h of BL and phenocopied the mutant for at least 2.5 h. The effects of 5-nitro-2-(3-phenylpropylamino)-benzoic acid and the HY4 mutation w ere not additive. Taken together, the results indicate that BL acts th rough HY4 to activate anion channels at the plasma membrane, causing g rowth inhibition that begins after approximately 1 h. Neither HY4 nor anion channels appear to participate greatly in the initial phase of i nhibition.