Soil salinity is a major contributor to crop yield losses. To improve our understanding of root responses to salinity, we developed and exploited a real-time salt-induced tilting assay. This assay follows root growth upon both gravitropic and salt challenges, revealing that root bending upon tilting is modulated by Na+ ions, but not by osmotic stress. Next, we measured this salt-specific response in 345 natural Arabidopsis (Arabidopsis thaliana) accessions and discovered a genetic locus, encoding the cell wall–modifying enzyme EXTENSIN ARABINOSE DEFICIENT TRANSFERASE (ExAD) that is associated with root bending in the presence of NaCl (hereafter salt). Extensins are a class of structural cell wall glycoproteins known as hydroxyproline (Hyp)-rich glycoproteins, which are posttranslationally modified by O-glycosylation, mostly involving Hyp-arabinosylation. We show that salt-induced ExAD-dependent Hyp-arabinosylation influences root bending responses and cell wall thickness. Roots of exad1 mutant seedlings, which lack Hyp-arabinosylation of extensin, displayed increased thickness of root epidermal cell walls and greater cell wall porosity. They also showed altered gravitropic root bending in salt conditions and a reduced salt-avoidance response. Our results suggest that extensin modification via Hyp-arabinosylation is a unique salt-specific cellular process required for the directional response of roots exposed to salinity.

中文翻译：

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细胞壁延伸蛋白的阿拉伯基化是根部对盐度的定向响应所必需的

土壤盐分是造成农作物减产的主要原因。为了提高我们对根部对盐度反应的理解，我们开发并利用了实时盐诱导倾斜测定。该测定跟踪了向地性和盐挑战下的根生长，揭示了倾斜时根的弯曲是由Na+离子调节的，而不是由渗透胁迫调节的。接下来，我们在 345 个天然拟南芥 (Arabidopsis thaliana) 种质中测量了这种盐特异性反应，并发现了一个基因位点，编码细胞壁修饰酶延伸阿拉伯糖缺陷转移酶 (ExAD)，该酶与 NaCl 存在下的根部弯曲相关。以下为盐）。延伸蛋白是一类结构细胞壁糖蛋白，称为富含羟脯氨酸 (Hyp) 的糖蛋白，通过 O-糖基化进行翻译后修饰，主要涉及 Hyp-阿拉伯糖基化。我们发现盐诱导的 ExAD 依赖性 Hyp-阿拉伯糖基化会影响根部弯曲反应和细胞壁厚度。 exad1突变体幼苗的根部缺乏延伸蛋白的Hyp-阿拉伯糖基化，表现出根部表皮细胞壁厚度增加和细胞壁孔隙率更大。他们还表现出在盐条件下改变的向地性根弯曲和减少的避盐反应。我们的结果表明，通过Hyp-阿拉伯糖基化进行的延伸蛋白修饰是暴露于盐度的根的定向响应所需的独特的盐特异性细胞过程。