Abstract
Photoreceptors, phytochromes and cryptochromes regulate hypocotyl growth under specific conditions by suppressing negative-gravitropism, modulating phototropism and inhibiting elongation. Although those effects seem to be partially caused via the regulation of the phytohormone auxin, the underlying molecular mechanisms are still poorly understood. Here we demonstrate that photoreceptors in Arabidopsis have at least two distinct effects: the inhibition of straight growth of hypocotyl depending on the ABC-type auxin transporter PGP19 and the induction of differential growth of hypocotyl independently of PGP19. Our genetic and gene-expression analyses reveal that the components involved in auxin transport, PGP19, is the suppressor of the straight growth in hypocotyl, and that activation of phytochromes and cryptochromes reduce the PGP19 expression and suppresses the basipetal auxin transport in hypocotyls. These results suggest that the control of auxin transport by phytochromes and cryptochromes is a critical component of the regulation of hypocotyl growth in response to light.
