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Hypocotyl phototropism of etiolated Arabidopsis seedlings is primarily mediated by the blue-light receptor kinase phototropin 1 (phot1). Phot1-mediated curvature to continuous unilateral blue light irradiation (0.5 μmol m −2  s −1 ) is enhanced by overhead pre-treatment with red light (20 μmol m −2  s −1  for 15 min) through the action of phytochrome (phyA). Here, we show that pre-treatment with blue light is equally as effective in eliciting phototropic enhancement and is dependent on phyA. Although blue light pre-treatment was sufficient to activate early phot1 signaling events, phot1 autophosphorylation  in vivo  was not found to be saturated, as assessed by subsequently measuring phot1 kinase activity  in vitro . However, enhancement effects by red and blue light pre-treatment were not observed at higher intensities of phototropic stimulation (10 μmol m −2  s −1 ). Phototropic enhancement by red and blue light pre-treatments to 0.5 μmol m −2  s −1  unilateral blue light irradiation was also lacking in transgenic Arabidopsis where  PHOT1  expression was restricted to the epidermis. Together, these findings indicate that phyA-mediated effects on phot1 signaling are restricted to low intensities of phototropic stimulation and originate from tissues other than the epidermis.

Phytochrome A Mediates Blue-Light Enhancement of Second-Positive Phototropism in Arabidopsis