A Brassinosteroid-Hypersensitive Mutant of BAK1 Indicates That a Convergence of Photomorphogenic and Hormonal Signaling Modulates Phototropism

The phototropic response of Arabidopsis (Arabidopsis thaliana) is induced by the phototropin photoreceptors and modulated by the cryptochrome and phytochrome photoreceptors. Downstream of these photoreceptors, asymmetric lateral redistribution of auxin underlies the differential growth, which results in phototropism. Historical physiological evidence and recent analysis of hormone-induced gene expression demonstrate that auxin and brassinosteroid signaling function interdependently. Similarly, in this study we report evidence that interactions between brassinosteroids and auxin signaling modulate phototropic responsiveness. We found that elongated, a previously identified photomorphogenesis mutant, enhances high-light phototropism and represents a unique allele of BAK1/SERK3, a receptor kinase implicated in brassinosteroid perception. Altogether, our results support the hypothesis that phototropic responsiveness is modulated by inputs that influence control of auxin response factor-mediated transcription.