Salt tolerance enables plants to withstand the toxicity of high concentrations of soluble salts, particularly NaCl. Increasing soil salinity slows plant growth and ultimately affects productivity with varying levels of impact depending on the plant species, duration of exposure, and stage of development. Therefore, engineering plant salt tolerance, defined as maintaining superior growth performance under high soil salinity, would be a valuable trait in response to the global salinisation of arable land (Munns et al., 2020). In a study recently published in New Phytologist, Xiao et al. (2024; doi: 10.1111/nph.20171) developed stable salt-tolerant wheat lines (Triticum aestivum L.) by overexpressing the small ubiquitin-like modifier (SUMO) protease-encoding gene TaDSU.

The ability to quickly reprogram protein functions in response to salt stress becomes crucial to activate cellular protection mechanisms. The covalent attachment of SUMO on target proteins is a well-recognised post-translational modification that can change the localisation, stability and activity of target proteins. In brief, SUMO is ligated by a specialised set of enzymes to lysine residues usually embedded within a canonical consensus of amino acids (Benlloch & Lois, 2018). After SUMO conjugation, SUMO deconjugating enzymes (belonging to different SUMO protease families, each encoded by multiple genes in plants) quickly remove SUMO from their targets to maintain a dynamic equilibrium between SUMOylated and nonSUMOylated target levels (Ghosh et al., 2024). The study by Xiao et al. contributes to the expanding list of abiotic stress responses mediated by SUMO proteases by highlighting the role of TaDSU. The authors observed increased shoot and root growth in wheat lines overexpressing TaDSU (TaDSU OX) compared with the wild-type (WT), specifically under saline soil conditions. High salt concentrations in the soil also cause osmotic stress, which reduces water uptake and impairs growth. Supporting the role of TaDSU in protecting plant cells from the osmotic aspect of salt stress, TaDSU OX lines showed reduced growth inhibition when cultivated in a medium containing mannitol, a nonmetabolisable sugar that induces osmotic stress. Metabolic and cation profiling of plants subject to salt stress revealed that TaDSU OX lines had reduced levels of Na⁺ content in the shoot (accompanied by increased K⁺), increased contents of soluble sugar and proline, and reduced ROS accumulation, which can be regarded as metabolic hallmarks for augmented salt tolerance.

TaDSU, a homologue of OVERLY TOLERANT TO SALT1/2 (OTS1/2) from Arabidopsis and OsOTS1 from rice (Conti et al., 2008; Srivastava et al., 2016), has SUMO protease activity in vitro. Its overexpression in Arabidopsis leads to a global reduction in the SUMOylated proteome under salt stress, indicating deSUMOylation activity also in vivo. However, an important question raised by Xiao et al. concerns the specific target(s) of TaDSU-mediated deSUMOylation, which are responsible for the increased salt tolerance. A yeast two-hybrid screen followed by independent pairwise interaction assays identified the transcription factor TaMYC2 as a bona fide substrate for TaDSU SUMO protease activity. This was shown in transient assays and by using stable Arabidopsis lines, since Arabidopsis MYC2 also appears to interact with TaDSU and TaDSU overexpression confers increased salt tolerance to Arabidopsis. This suggests a conserved mechanism of salt tolerance mediated by TaDSU through MYC2 deSUMOylation. Exposure of WT Arabidopsis seedlings to high salt conditions resulted in increased levels of MYC2 accumulation accompanied by a corresponding increase of MYC2 SUMOylation. Conversely, when TaDSU is overexpressed in Arabidopsis, the MYC2 SUMOylated pool was reduced specifically under salt conditions, with no corresponding reduction in the nonSUMOylated pool of MYC2 relative to the WT. These data support a correlation between the reduced accumulation of SUMOylated MYC2 and the acquisition of salt tolerance traits. TaDSU OX lines (in wheat and Arabidopsis) also displayed increased levels of MYC2 transcripts but limited to salinity conditions. Supporting the role of TaMYC2 in conferring salt tolerance downstream of TaDSU, virus-induced gene-silencing (VIGS) of TaMYC2 significantly weakened the salt-tolerant phenotype of TaDSU OX lines in wheat. Therefore, the enhanced salt tolerance conferred by TaDSU overexpression can be attributed to an increase in TaMYC2 transcript accumulation and a reduced SUMOylation level of TaMYC2.

The study of Xiao et al. also describes a direct transcriptional regulation of TaDSU mediated by MYC2. TaDSU promoter contains multiple MYC2 binding motifs and TaMYC2 can directly bind to these regions to activate TaDSU. The TaMYC2-dependent regulation of TaDSU expression was further investigated in wheat plants in which TaMYC2 was knocked down through VIGS. As expected, VIGS-treated plants had reduced levels of TaMYC2 and correspondingly decreased levels of TaDSU transcript accumulation, specifically under salt stress conditions. Interestingly, a similar MYC2-OTS1/2 direct regulation is conserved in Arabidopsis because OTS1/2 transcripts were upregulated in MYC2 overexpression lines. Additionally, a chromatin immunoprecipitation assay supports the direct binding of MYC2 to the OTS1/2 promoters. Therefore, under salt stress conditions, TaMYC2 accumulation feeds back into the transcriptional activation of TaDSU, which in turn promotes deSUMOylation of TaMYC2 (Fig. 1).

Gains in stress tolerance measured under controlled environments are often less clear under field trials due to the higher complexity of natural stressors. Xiao et al. verified the performance of TaDSU wheat overexpression lines under saline soils through multiyear experiments. Overall, transgenic lines had more spikes per plant than the untransformed background, with significant increases in yield under salt stress conditions. TaDSU OX lines also performed better in another location characterised by saline–alkaline soil, whereas no yield penalty was observed in the presence of low salt.

The study by Xiao et al. thus provides a path to an effective genetic strategy for wheat improvement under an agriculturally relevant scenario. It also raises intriguing questions across three interconnected areas, ranging from broader biological and physiological aspects of hormone signalling to more detailed molecular insights. First, it highlights the utility of SUMO modifications and, more generally, post-translational modification to modify key traits in crops. It also reveals important connections between TaDSU-MYC2 and other hormonal gene networks, mainly abscisic acid (ABA), since TaDSU OX lines had reduced ABA sensitivity. ABA and other phytohormones are known to play important roles in regulating ionic homeostasis and plant growth under salt stress conditions (Achard et al., 2006). Second, the relative paucity of the SUMO conjugation; encoding genes suggests a major role for SUMO deconjugating enzymes in providing specificity to adaptive responses in plants (Ghosh et al., 2024). TaDSU, like OTS1/2, falls in a subclade of cysteine proteases, which evolved mostly in angiosperms, perhaps in conjunction with the expansion and adaptation of flowering plants to an increasingly challenging environment or the evolution of more complex plant structures and functions. However, questions remain about how these proteases are post-translationally activated, particularly in response to salt stress. Third, SUMOylation is a fast and dynamic mode of influencing protein function. Here, the discovery of the TaDSU-TaMYC2 regulation opens new questions regarding the direct role of SUMOylation on MYC2 function. The recruitment of MYC2 to chromatin appears to be reduced in TaDSU OX lines. How does SUMOylation regulate MYC2 DNA binding? Does SUMOylation also influence MYC2-dependent transcriptional activation at its regulated promoters?

Future studies will be required to better understand the TaDSU-TaMYC2 interplay and the broader role of SUMOylation in plant stress responses. Nevertheless, from an agriculture perspective, insights from this study, combined with advancements in detecting SUMOylation targets in vivo (Sang et al., 2024), enhance our ability to engineer SUMOylation for boosting salinity tolerance in crop species.

耐盐性使植物能够承受高浓度可溶性盐（尤其是 NaCl）的毒性。土壤盐度增加会减慢植物生长，并最终影响生产力，影响程度因植物种类、暴露持续时间和发育阶段而异。因此，工程植物耐盐性，定义为在高土壤盐度下保持卓越的生长性能，将是应对全球耕地盐碱化的宝贵特性（Munns 等 ，2020）。在最近发表在《新植物学家》上的一项研究中，Xiao 等 人。（2024; doi： 10.1111/nph.20171） 通过过表达小泛素样修饰剂 （SUMO） 蛋白酶编码基因 TaDSU 开发了稳定的耐盐小麦品系 （Triticum aestivum L.）。

响应盐胁迫快速重编程蛋白质功能的能力对于激活细胞保护机制至关重要。SUMO 在靶蛋白上的共价连接是一种公认的翻译后修饰，可以改变靶蛋白的定位、稳定性和活性。简而言之，SUMO被一组特殊的酶连接到赖氨酸残基上（通常嵌入在氨基酸的规范共识中（Benlloch & Lois，2018）。SUMO 偶联后，SUMO 解偶联酶（属于不同的 SUMO 蛋白酶家族，每个家族都由植物中的多个基因编码）迅速从其靶标中去除 SUMO，以维持 SUMO 化和非 SUMO 化靶标水平之间的动态平衡（Ghosh等 人，2024 年）。Xiao 等 人的研究。通过强调 TaDSU 的作用，为 SUMO 蛋白酶介导的非生物胁迫反应的扩展列表做出贡献。作者观察到，与野生型 （WT） 相比，过表达 TaDSU （TaDSU OX） 的小麦品系的芽和根生长增加，特别是在盐碱土壤条件下。土壤中的高盐浓度也会引起渗透压，从而减少水分吸收并损害生长。TaDSU OX 品系支持 TaDSU 在保护植物细胞免受盐胁迫渗透方面的作用，在含有甘露醇（一种诱导渗透胁迫的不可代谢糖）的培养基中培养时，生长抑制作用降低。 受盐胁迫的植物的代谢和阳离子分析显示，TaDSU OX 品系在地上部的 Na⁺ 含量降低（伴随着 K⁺ 增加），可溶性糖和脯氨酸含量增加，ROS 积累减少，这可以被视为增强耐盐性的代谢标志。

TaDSU，对拟南芥的 SALT1/2 （OTS1/2） 过度耐受和水稻的 OsOTS1 的同源物（Conti等 人，2008 年;Srivastava et al.， 2016） 在体外具有 SUMO 蛋白酶活性。它在拟南芥中的过表达导致盐胁迫下 SUMO 化蛋白质组的整体减少，表明在体内也具有去 SUMOylation 活性。然而，Xiao 等 人提出了一个重要的问题。涉及 TaDSU 介导的 deSUMOylation 的特定靶标，这些靶标是导致耐盐性增加的原因。酵母双杂交筛选，然后进行独立的成对相互作用测定，确定转录因子 TaMYC2 是 TaDSU SUMO 蛋白酶活性的真正底物。这在瞬时测定和使用稳定的拟南芥系中得到证明，因为拟南芥 MYC2 似乎也与 TaDSU 相互作用，并且 TaDSU 过表达赋予对拟南芥更高的盐耐受性。这表明 TaDSU 通过 MYC2 deSUMOylation 介导的耐盐性存在保守机制。WT 拟南芥幼苗暴露于高盐条件下导致 MYC2 积累水平增加，伴随着 MYC2 SUMOylation 的相应增加。相反，当 TaDSU 在拟南芥中过表达时，MYC2 SUMO化池在盐条件下特异性减少，MYC2 的非 SUMO化池相对于 WT 没有相应的减少。这些数据支持 SUMO化 MYC2 积累减少与获得耐盐性状之间的相关性。 TaDSU OX 品系（在小麦和拟南芥中）也显示出 MYC2 转录物水平增加，但仅限于盐度条件。病毒诱导的 TaMYC2 基因沉默 （VIGS） 支持 TaMYC2 在赋予 TaDSU 下游耐盐性中的作用，显著削弱了小麦 TaDSU OX 品系的耐盐表型。因此，TaDSU 过表达赋予的耐盐性增强可归因于 TaMYC2 转录本积累的增加和 TaMYC2 的 SUMOylation 水平降低。

Xiao 等 人的研究。还描述了由 MYC2 介导的 TaDSU 的直接转录调控。TaDSU 启动子包含多个 MYC2 结合基序，TaMYC2 可以直接与这些区域结合以激活 TaDSU。在通过 VIGS 敲低 TaMYC2 的小麦植株中进一步研究了 TaMYC2 对 TaDSU 表达的依赖性调节。正如预期的那样，VIGS 处理的植物降低了 TaMYC2 水平，并相应地降低了 TaDSU 转录本积累的水平，特别是在盐胁迫条件下。有趣的是，类似的 MYC2-OTS1/2 直接调节在拟南芥中是保守的，因为 OTS1/2 转录本在 MYC2 过表达系中上调。此外，染色质免疫沉淀测定支持 MYC2 与 OTS1/2 启动子的直接结合。因此，在盐胁迫条件下，TaMYC2 积累会反馈到 TaDSU 的转录激活中，进而促进 TaMYC2 的去 SUMO 化（图 1）。

由于自然压力源的复杂性更高，在受控环境中测量的抗压性增益在田间试验中通常不太明显。Xiao 等 人。通过多年试验验证了盐碱土壤下 TaDSU 小麦过表达系的性能。总体而言，转基因品系每株植物的刺突比未转化的背景多，在盐胁迫条件下产量显著增加。TaDSU OX 品系在另一个以盐碱土壤为特征的地方也表现更好，而在低盐存在的情况下没有观察到产量损失。

Xiao 等 人的研究。从而为在农业相关情景下小麦改良的有效遗传策略提供了一条途径。它还在三个相互关联的领域提出了有趣的问题，从激素信号转导更广泛的生物学和生理方面到更详细的分子见解。首先，它强调了 SUMO 修饰的效用，更一般地说，强调了翻译后修饰对作物关键性状的修饰。它还揭示了 TaDSU-MYC2 与其他激素基因网络（主要是脱落酸 （ABA））之间的重要联系，因为 TaDSU OX 系降低了 ABA 敏感性。已知 ABA 和其他植物激素在盐胁迫条件下调节离子稳态和植物生长中起重要作用（Achard et al.， 2006）。其次，SUMO 偶联的相对缺乏;编码基因表明 SUMO 解离酶在为植物的适应性反应提供特异性方面起重要作用（Ghosh等人 ，2024 年）。TaDSU 与 OTS1/2 一样，属于半胱氨酸蛋白酶的一个亚分支，该亚分支主要在被子植物中进化，可能与开花植物对日益具有挑战性的环境的扩张和适应或更复杂的植物结构和功能的进化有关。然而，关于这些蛋白酶如何被翻译后激活，特别是响应盐胁迫，仍然存在疑问。第三，SUMOylation 是一种影响蛋白质功能的快速动态模式。在这里，TaDSU-TaMYC2 调节的发现开启了关于 SUMOylation 对 MYC2 功能的直接作用的新问题。在 TaDSU OX 系中，MYC2 向染色质的募集似乎减少。 SUMOylation 如何调节 MYC2 DNA 结合？SUMOylation 是否也影响其调节启动子的 MYC2 依赖性转录激活？

需要进一步的研究来更好地了解 TaDSU-TaMYC2 相互作用以及 SUMOylation 在植物胁迫反应中的更广泛作用。然而，从农业的角度来看，这项研究的见解，结合体内检测 SUMOylation 靶标的进步（Sang等 人，2024 年），增强了我们设计 SUMOylation 以提高作物物种耐盐性的能力。