Soil salinity is a major contributor to crop yield losses. To improve our understanding of root responses to salinity, we developed and exploit here a real-time salt-induced tilting assay (SITA). This method follows root growth upon both gravitropic and salt challenges and revealed that root bending is modulated by salinity, but not by osmotic stress. Next, this salt-specific response was measured in 345 natural Arabidopsis accessions and we discovered a genetic locus, encoding for the

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... modifying enzyme Extensin Arabinose Deficient arabinosyltransferase (ExAD), to be associated with root bending in salt. Extensins are a class of structural cell wall glycoproteins [hydroxyproline-rich glycoproteins (HRGPs)] which are post-translationally modified by O-glycosylation mostly in the form of hydroxyproline (Hyp)-arabinosylation. Here we show that salt induces ExAD-dependent Hyp-arabinosylation, influencing root bending responses and cell wall thickness. We report that exad mutants, which lack all cell wall Hyp-Araf4 modifications, display increased root epidermal cell wall thickness, and are impaired in the salt-dependent root bending response. Our study thus demonstrates a requirement for extensin Hyp-arabinosylation as a novel cell wall modification for the salt-specific directional response of roots and provides new insight into the process of how plant roots sense and respond to salinity.