Endogenous phytohormones and regulation of morphogenesis of Arabidopsis thaliana by blue light

We studied the effects of blue light (BL) on the levels of endogenous phytohormones (IAA, ABA, gibberellins, and cytokinins) and morphogenesis of the 7-day-old Arabidopsis thaliana (L.) Heynh seedlings of wild type (Ler) and its hy4 mutant with a disturbed synthesis of cryptochrome 1 (CRY1), which is a receptor for BL. In darkness, the mutant contained considerably less free IAA and zeatin, hut much more ABA as compared to the wild-type seedlings. BL retarded the hypocotyl growth in the wild-type seedlings but stimulated it in the mutant. Elongation of mutant hypocotyls was accompanied by accumulation of free IAA and a decrease in the content of free ABA; the level of cytokinins did not change. We believe that the response of the hy4 hypocotyls to BL is mediated by a BL receptor distinct from cryptochrome 1. The conclusion is that light and hormonal signals interact in the control of the hypocotyl growth in A. thaliana seedlings.