Introduction

Survival of plant seedlings is affected by many environmental parameters such as available water and soil salinity. Salt has a negative impact on the plant both through its osmotic effect, which may result in reduced ability for water uptake, and because of ion toxicity (Munns & Tester, 2008). It affects many important processes including photosynthesis, respiration, ion uptake and membrane integrity (West et al., 2004; Tavakkoli et al., 2011; Talei et al., 2012; Mansour, 2013; Awlia et al., 2021; Zhao et al., 2021). Salt stress tolerance is expected to involve avoidance mechanisms and reduced uptake and transport of salt ions (Møller et al., 2009). The initial response to saline conditions is a growth arrest of both primary and lateral roots followed by a temporally dynamic acclimation process in which growth is restored and salt tolerance mechanisms activated (Geng et al., 2013). Therefore, it is conceivable that salt stress also affects the development of the water transporting tissue, the xylem, as that would impact salt uptake. However, how salt affects xylem development is not well known.

The xylem harbours vessel strands of hollow cells reinforced with lignified secondary cell walls (SCW). In the Arabidopsis root the xylem forms an axis traversing the stele. The two outer strands of the xylem axis differentiate as protoxylem with annular or helical SCW, whereas metaxylem with pitted SCW occupies the central positions of the axis (Fig. 1a). The diameter and shape of the SCWs are thought to influence hydraulic conductance, and the xylem shape correlates with drought resistance in many different species (Arend & Fromm, 2007; Awad et al., 2010; Tang et al., 2018; Yu et al., 2021). Recently, we and others showed that xylem formation is plastic and responds to water availability. Under conditions of reduced water availability, extra protoxylem strands form, and metaxylem differentiates closer to the root tip (Ramachandran et al., 2018, 2021; Bloch et al., 2019). The hormone abscisic acid (ABA) mediates these developmental responses by at least two different mechanisms. Firstly, ABA promotes the production of miR165 in the endodermis (Ramachandran et al., 2018; Bloch et al., 2019). This miRNA moves into the stele to reduce homeodomain leucine zipper class III (HD-ZIP III) mRNA levels, eventually leading to protoxylem formation in place of metaxylem (Carlsbecker et al., 2010; Miyashima et al., 2011). Secondly, ABA directly promotes the expression of VASCULAR RELATED NAC DOMAIN (VND) transcription factors within the immature xylem cells, where VND7 promotes protoxylem formation, and VND2 and VND3 metaxylem differentiation closer to the root tip (Ramachandran et al., 2021).

Salt stress also triggers ABA signalling, but recent research has in addition highlighted the importance of gibberellin (GA) levels and signalling. Reduced GA levels or signalling may result in salt stress tolerance (Colebrook et al., 2014), whereas the absence of the DELLA repressors of GA signalling make Arabidopsis less salt tolerant (Achard et al., 2006). In line with these findings, salt stress leads to a reduction in bioactive GA levels, which in turn stabilises DELLAs (Achard et al., 2006; Magome et al., 2008). Under normal conditions, GAs affect xylem lignification in both primary and secondary development in several different species (Eriksson et al., 2000; Mauriat & Moritz, 2009; Gou et al., 2011; Ragni et al., 2011; Guo et al., 2015; Wang et al., 2017; Singh et al., 2019), GAs promote xylem formation in secondary development and are important for fibre development (Mauriat & Moritz, 2009; Ragni et al., 2011; Felipo-Benavent et al., 2018). Furthermore, DELLAs are implicated in the regulation of cell wall synthesis and remodelling in Arabidopsis (Locascio et al., 2013; Felipo-Benavent et al., 2018).

Here, we analyse the effect of salt stress on Arabidopsis seedling root xylem development. We show that salt stress results in discontinuous differentiation of the protoxylem strands generating protoxylem gaps. The capacity to form protoxylem gaps correlates with salt tolerance, indicating that gaps promote survival under high salinity. We also show that the formation of protoxylem gaps under salt stress requires DELLA-mediated repression of GA signalling. Under salt stress, DELLAs promote the expression of VND6 and genes encoding SCW enzymes, as well as genes encoding multiple cell wall modifying enzymes including alpha-expansins such as EXP1, also called EXPA1. The loss of VND6 or EXP1 consequently results in less protoxylem gaps forming under salt stress.

中文翻译：

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盐度通过促进拟南芥幼苗耐盐性的 DELLA 依赖性机制诱导不连续的原木素

介绍

植物幼苗的存活受许多环境参数的影响，例如可用水和土壤盐分。盐通过其渗透作用（可能导致吸水能力降低）和离子毒性对植物产生负面影响（Munns & Tester，  2008 年）。它影响许多重要过程，包括光合作用、呼吸作用、离子吸收和膜完整性（West et al .,  2004 ; Tavakkoli et al .,  2011 ; Talei et al .,  2012 ; Mansour,  2013 ; Awlia et al .,  2021 ; Zhao et al., 2012）等，  2021）。预计盐胁迫耐受性涉及避免机制和减少盐离子的吸收和运输（Møller等人，  2009 年）。对盐碱条件的最初反应是初级和侧根的生长停滞，随后是时间动态的适应过程，在该过程中恢复生长并激活耐盐机制（Geng et al .,  2013）。因此，可以想象，盐胁迫也会影响水分运输组织木质部的发育，因为这会影响盐的吸收。然而，盐如何影响木质部发育尚不清楚。

木质部含有用木质次生细胞壁 (SCW) 增强的空心细胞的血管链。在拟南芥的根中，木质部形成一个横穿石碑的轴。木质部轴的两条外链区分为具有环形或螺旋形 SCW 的原木木质部，而具有凹坑 SCW 的后木质部占据轴的中心位置（图 1a）。SCW 的直径和形状被认为会影响导水率，并且木质部形状与许多不同物种的抗旱性相关（Arend & Fromm，  2007；Awad等，  2010；Tang等，  2018；Yu等.,  2021）。最近，我们和其他人表明，木质部的形成是可塑性的，并且对水的可用性有反应。在水分利用率降低的条件下，形成额外的原木聚糖链，后木木质部分化更接近根尖（Ramachandran等人，  2018 年，2021 年；布洛赫等人，  2019 年）。脱落酸激素 (ABA) 通过至少两种不同的机制介导这些发育反应。首先，ABA 促进内皮层中 miR165 的产生（Ramachandran等人，  2018 年；Bloch等人，  2019 年））。该 miRNA 进入石碑以降低同源域亮氨酸拉链 III 类 (HD-ZIP III) mRNA 水平，最终导致异木素形成代替间木质部（Carlsbecker等人，  2010； Miyashima 等人，2011）。其次，ABA 直接促进未成熟木质部细胞内血管相关 NAC 结构域 (VND) 转录因子的表达，其中 VND7 促进原木质部形成，而 VND2 和 VND3 后木质部分化更接近根尖 (Ramachandran et al .,  2021 )。

盐胁迫也会触发 ABA 信号，但最近的研究还强调了赤霉素 (GA) 水平和信号的重要性。降低的 GA 水平或信号可能导致耐盐胁迫（Colebrook等人，  2014 年），而缺乏 GA 信号传导的 DELLA 阻遏物使拟南芥的耐盐性降低（Achard等人，  2006 年）。与这些发现一致，盐胁迫导致生物活性 GA 水平降低，进而稳定 DELLA (Achard et al .,  2006 ; Magome et al .,  2008）。在正常情况下，GAs 会影响几个不同物种的初级和次级发育中的木质部木质化（Eriksson等，  2000；Mauriat & Moritz，  2009；Gou等，  2011；Ragni等，  2011；Guo等。 ,  2015 ; Wang et al .,  2017 ; Singh et al .,  2019 )，GAs 促进木质部在二次发育中的形成，对纤维发育很重要 (Mauriat & Moritz,  2009 ; Ragni et al .,  2011 ; Felipo-Benavent等人，  2018 年）。此外，DELLA 与拟南芥细胞壁合成和重塑的调节有关（Locascio等人，2013 年；Felipo-Benavent等人，  2018 年）。

在这里，我们分析了盐胁迫对拟南芥幼苗根木质部发育的影响。我们表明盐胁迫导致 protoxylem 链的不连续分化，从而产生 protoxylem 间隙。形成 protoxylem 间隙的能力与耐盐性相关，表明间隙促进高盐度下的存活。我们还表明在盐胁迫下形成原木聚糖间隙需要 DELLA 介导的 GA 信号抑制。在盐胁迫下，DELLA 促进VND6和编码 SCW 酶的基因的表达，以及编码多种细胞壁修饰酶的基因的表达，包括 α-伸展蛋白，如 EXP1，也称为 EXPA1。因此，VND6或EXP1的缺失导致盐胁迫下形成的原木聚糖间隙减少。