In Arabidopsis, low blue light enhances phototropism by releasing cryptochrome 1-mediated inhibition ofPIF4expression

Abstract

Shade-avoiding plants including Arabidopsis thaliana display a number of growth responses elicited by shade cues including elongation of stem-like structures and repositioning of leaves. Shade also promotes phototropism of de-etiolated seedlings through repression of phytochrome B (phyB) presumably to enhance capture of unfiltered sunlight. Light cues indicative of shade include a reduction in the blue and red portions of the solar spectrum and a low red to far-red ratio. Here we show that in Arabidopsis seedlings both low blue and a low red to far-red ratio are required to rapidly enhance phototropism. However, prolonged low blue treatments through reduced cryptochrome 1 (cry1) activation are sufficient to promote phototropism. The enhanced phototropic response of cry1 mutants in the lab and in response to natural canopies depends on PHYTOCHROME INTERACTING FACTORs (PIFs). In favorable light conditions, cry1 limits the expression of PIF4 while in low blue light PIF4 expression increases, which contributes to phototropic enhancement. The analysis of a quantitative DII auxin reporter indicates that low blue light leads to enhanced auxin levels in the hypocotyl and, upon phototropic stimulation, a steeper auxin gradient across the hypocotyl. We conclude that phototropic enhancement by canopy shade results from the combined activities of phytochrome B and cry1 that converge on PIF regulation.ONE SENTENCE SUMMARYThe persistent depletion of blue light in natural canopy shade relieves the inhibitory effect of cryptochrome 1 on PIF4, enhancing phototropism in de-etiolated Arabidopsis seedlings.Financial supportThis work was supported by the University of Lausanne and a grant from the Swiss National Science foundation (n° 310030B_179558 to C.F.); Human Frontier Science Program organization (HFSP) Grant RPG0054-2013, ANR-12-BSV6-0005 grant (AuxiFlo) to T.V.; The University of Buenos Aires (Grant 20020100100437 to J. J. C.), and Agencia Nacional de Promoción Científica y Tecnológica of Argentina (Grant PICT-2018-01695 to J. J. C.). Alessandra Boccaccini and Martina Legris are funded by Marie Curie fellowships H2020-MSCA-IF-2017 grants CRoSh 796283 and Flat-Leaf 796443 respectively.

True Blue: How Cry1 Inhibits Phototropism in Green Seedlings

“…Previous work had shown that LRFR can promote phototropism in artificial lighting conditions (Goyal et al, 2016), where etiolated seedlings are exposed to a unidirectional source of blue light. However, with a more natural lighting regime, Boccaccini et al (2020) found that LRFR was not sufficient. Only a combination of LBL and LRFR resulted in increased phototropism toward a blue light source within the white growth light (WL; Fig.…”

“…However, plants have another set of blue light photoreceptors, the cryptochromes. Boccaccini et al (2020) discovered that the cry1 mutant has an increased phototropism response in WL/LBL, without a further increase in bending in the LBL/LBL condition, compared with the wild type, which only had a strong response with the LBL pretreatment (LBL/LBL; Fig. 1C).…”

“…In the previous issue of Plant Physiology, Boccaccini et al (2020) showed that in Arabidopsis (Arabidopsis thaliana), a combination of LBL and LRFR promotes phototropism and that LBL alone is sufficient. In addition, they demonstrate that the blue light receptor cryptochrome 1 (cry1) is active in this response.…”

“…wt, Wild type. Figure based on figures 1 and 4 from Boccaccini et al, (2020). phototropic response, showing that these PIFs are necessary for cry1 to inhibit phototropism (Fig.…”

“…In short, this article bridges a long-standing gap between traditional phototropism studies that often use highly artificial lighting regimes and etiolated seedlings exposed to unidirectional monochromatic light. Boccaccini et al (2020) combine these studies with work using green seedlings and growth under regimes with blue light enrichment (Fig. 1A), and they relate these studies to the real world in which a growing plant often experiences canopy shading.…”