Summary Global food production is set to keep increasing despite a predicted decrease in total arable land [ 1 ]. 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 [ 2 ]. 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) [ 3 , 4 ]. PIFs activate the expression of auxin biosynthesis genes [ 4 , 5 ] and enhance auxin sensitivity [ 6 ], 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.

(SP), 0000-0002-9223-9996 (RS), 0000-0001-6738-115X (CT) and 0000-0002-5320-6817 (RP) SummaryGlobal food production is set to keep increasing despite a predicted decrease in total arable land [1].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 [2]. Shade is perceived by a reduction in the ratio of red (R) to (FR) light and results in the stabilisation of a class of transcription factors known as PHYTOCHROME INTERACTING FACTORs (PIFs) [3,4]. PIFs activate the expression of auxin biosynthesis genes [4,5] and enhance auxin sensitivity [6], which promotes cell wall loosening and drives elongation growth. Despite our molecular understanding of shade-induced growth, little is known about how this developmental programme 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) signalling and the canonical ABA signalling pathway. Low R:FR light enhances the expression of a positive regulator of the brassinosteroid (BR) signalling pathway, BRASSINOSTEROID SIGNALLING KINASE 5 (BSK5). We found that ABA inhibits BSK5 up-regulation and interferes with GSK3-like kinase inactivation by the BR pathway, thus leading to a suppression of PIF function. By demonstrating a link between the ABA and BR-signalling pathways this study provides an important step forward in our understanding of how environmental cues are integrated into plant development.Keywords: Plant Photobiology, Salt Response, Phytohormones, Abscisic acid, Brassinosteroids.. CC-BY-NC 4.0 International license not peer-reviewed) is the author/funder. It is made available under aThe copyright holder for this preprint (which was . http://dx.doi.org/10.1101/289124 doi: bioRxiv preprint first posted online Mar. 27, 2018; Results and Discussion Soil salinity is detrimental to plants and often results in a reduction in stem and root biomass accumulation [7]. Most previous studies have, however, focused on the effects of salt at a range of very high concentrations (a median concentration of 150mM NaCl) [8]. We opted for a nuanced approach to investigate how low concentrations of NaCl may affect plant shade-avoidance. Growing plants in tissue culture often masks genes involved in NaCl-sensitivity [9,10], and so plants were grown on soil. Arabidopsis thaliana (arabidopsis) seeds were germinated under white light (Wl) for three days, before transfer to new soil that had been pre-treated with NaCl solution. Following this, plants were returned to Wl for a further day to acclimate, before they were shifted to Wl or Wl with supplementary far-red LEDs (+FR). Hypocotyl lengths were measured on day seven (for a schematic diagram of the treatments see Figure S1A). Water or NaCl solution was applied from below and the soil was kept saturated to avoid dehydration...