Soil Salinity Limits Plant Shade Avoidance

Global food production is set to keep increasing despite a predicted decrease in total arable land [1Ausubel J.H.J. Wernick I.K.I. Waggoner P.E.P. Peak Farmland and the Prospect for Land Sparing.Popul. Dev. Rev. 2013; 38: 221-242Crossref Scopus (94) Google Scholar]. To achieve higher production, denser planting will be required on increasingly degraded soils. When grown in dense stands, crops elongate and raise their leaves in an effort to reach sunlight, a process termed shade avoidance [2Fraser D.P. Hayes S. Franklin K.A. Photoreceptor crosstalk in shade avoidance.Curr. Opin. Plant Biol. 2016; 33: 1-7Crossref PubMed Scopus (113) Google Scholar]. Shade is perceived by a reduction in the ratio of red (R) to far-red (FR) light and results in the stabilization of a class of transcription factors known as PHYTOCHROME INTERACTING FACTORS (PIFs) [3Lorrain S. Allen T. Duek P.D. Whitelam G.C. Fankhauser C. Phytochrome-mediated inhibition of shade avoidance involves degradation of growth-promoting bHLH transcription factors.Plant J. 2008; 53: 312-323Crossref PubMed Scopus (537) Google Scholar, 4Li L. Ljung K. Breton G. Schmitz R.J. Pruneda-Paz J. Cowing-Zitron C. Cole B.J. Ivans L.J. Pedmale U.V. Jung H.S. et al.Linking photoreceptor excitation to changes in plant architecture.Genes Dev. 2012; 26: 785-790Crossref PubMed Scopus (350) Google Scholar]. PIFs activate the expression of auxin biosynthesis genes [4Li L. Ljung K. Breton G. Schmitz R.J. Pruneda-Paz J. Cowing-Zitron C. Cole B.J. Ivans L.J. Pedmale U.V. Jung H.S. et al.Linking photoreceptor excitation to changes in plant architecture.Genes Dev. 2012; 26: 785-790Crossref PubMed Scopus (350) Google Scholar, 5Hornitschek P. Kohnen M.V. Lorrain S. Rougemont J. Ljung K. López-Vidriero I. Franco-Zorrilla J.M. Solano R. Trevisan M. Pradervand S. et al.Phytochrome interacting factors 4 and 5 control seedling growth in changing light conditions by directly controlling auxin signaling.Plant J. 2012; 71: 699-711Crossref PubMed Scopus (399) Google Scholar] and enhance auxin sensitivity [6Hersch M. Lorrain S. de Wit M. Trevisan M. Ljung K. Bergmann S. Fankhauser C. Light intensity modulates the regulatory network of the shade avoidance response in Arabidopsis.Proc. Natl. Acad. Sci. USA. 2014; 111: 6515-6520Crossref PubMed Scopus (89) Google Scholar], which promotes cell-wall loosening and drives elongation growth. Despite our molecular understanding of shade-induced growth, little is known about how this developmental program is integrated with other environmental factors. Here, we demonstrate that low levels of NaCl in soil strongly impair the ability of plants to respond to shade. This block is dependent upon abscisic acid (ABA) signaling and the canonical ABA signaling pathway. Low R:FR light enhances brassinosteroid (BR) signaling through BRASSINOSTEROID SIGNALING KINASE 5 (BSK5) and leads to the activation of BRI1 EMS SUPPRESSOR 1 (BES1). ABA inhibits BSK5 upregulation and interferes with GSK3-like kinase inactivation by the BR pathway, thus leading to a suppression of BES1:PIF function. By demonstrating a link between light, ABA-, and BR-signaling pathways, this study provides an important step forward in our understanding of how multiple environmental cues are integrated into plant development.