Shade Promotes Phototropism through Phytochrome B-Controlled Auxin Production

• Published in: Current biology Vol. 26; no. 24; pp. 3280 - 3287
• Main Authors: Goyal, Anupama, Karayekov, Elizabeth, Galvão, Vinicius Costa, Ren, Hong, Casal, Jorge J., Fankhauser, Christian
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 19.12.2016
• Subjects: Arabidopsis - physiology; Arabidopsis Proteins - genetics; Arabidopsis Proteins - metabolism; Arabidopsis thaliana; Gene Expression Regulation, Plant - physiology; Indoleacetic Acids - metabolism; Light; photoreceptor crosstalk; phototropin 1; phototropism; Phototropism - physiology; phytochrome B; Phytochrome B - physiology; PHYTOCHROME-INTERACTING FACTORs; Seedlings - physiology; shade avoidance; YUCCAs; Arabidopsis thaliana; YUCCAs; shade avoidance; PHYTOCHROME-INTERACTING FACTORs; phytochrome B; phototropin 1; photoreceptor crosstalk; phototropism
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/j.cub.2016.10.001

Phototropism is an asymmetric growth response enabling plants to optimally position their organs. In flowering plants, the phototropin (phot) blue light receptors are essential to detect light gradients. In etiolated seedlings, the phototropic response is enhanced by the red/far-red (R/FR)-sensing phytochromes (phy) with a predominant function of phyA. In this study, we analyzed the influence of the phytochromes on phototropism in green (de-etiolated) Arabidopsis seedlings. Our experiments in the laboratory and outdoors revealed that, in open environments (high R/FR ratio), phyB inhibits phototropism. In contrast, under foliar shade, where access to direct sunlight becomes important, the phototropic response was strong. phyB modulates phototropism, depending on the R/FR ratio, by controlling the activity of three basic-helix-loop-helix (bHLH) transcription factors of the PHYTOCHROME INTERACTING FACTORs (PIFs) family. Promotion of phototropism depends on PIF-mediated induction of several members of the YUCCA gene family, leading to auxin production in the cotyledons. Our study identifies PIFs and YUCCAs as novel molecular players promoting phototropism in photoautotrophic, but not etiolated, seedlings. Moreover, our findings reveal fundamental differences in the phytochrome-phototropism crosstalk in etiolated versus green seedlings. We propose that in natural conditions where the light environment is not homogeneous, the uncovered phytochrome-phototropin co-action is important for plants to adapt their growth strategy to optimize photosynthetic light capture. •Shade promotes phototropism in green seedlings•Phytochrome B negatively regulates phototropism in green seedlings•PIFs are essential for robust phototropism in green, but not etiolated, seedlings•The developmental stage controls the crosstalk between phototropins and phytochromes Goyal et al. show that foliar shade promotes growth toward the light (phototropism). This response involves the crosstalk between two classes of photoreceptors, the blue-light-sensing phototropin and red-/far-red-sensing phytochromes. The authors propose that this mechanism enhances access of plants to light and hence photosynthetic light capture.