Becana Manuel, Wienkoop Stefanie, Matamoros Manuel A
Estación Experimental de Aula Dei, Consejo Superior de Investigaciones Científicas, Zaragoza, Spain.
Department of Ecogenomics and Systems Biology, University of Vienna, Vienna, Austria.
Front Plant Sci. 2018 Oct 10;9:1434. doi: 10.3389/fpls.2018.01434. eCollection 2018.
Sulfur is an essential nutrient in plants as a constituent element of some amino acids, metal cofactors, coenzymes, and secondary metabolites. Not surprisingly, sulfur deficiency decreases plant growth, photosynthesis, and seed yield in both legumes and non-legumes. In nodulated legumes, sulfur supply is positively linked to symbiotic nitrogen fixation (SNF) and sulfur starvation causes three additional major effects: decrease of nodulation, inhibition of SNF, and slowing down of nodule metabolism. These effects are due, at least in part, to the impairment of nitrogenase biosynthesis and activity, the accumulation of nitrogen-rich amino acids, and the decline in leghemoglobin, ferredoxin, ATP, and glucose in nodules. During the last decade, some major advances have been made about the uptake and metabolism of sulfur in nodules. These include the identification of the sulfate transporter SST1 in the symbiosomal membrane, the finding that glutathione produced in the bacteroids and host cells is essential for nodule activity, and the demonstration that sulfur assimilation in the whole plant is reprogrammed during symbiosis. However, many crucial questions still remain and some examples follow. In the first place, it is of paramount importance to elucidate the mechanism by which sulfur deficiency limits SNF. It is unknown why homoglutahione replaces glutathione as a major water-soluble antioxidant, redox buffer, and sulfur reservoir, among other relevant functions, only in certain legumes and also in different tissues of the same legume species. Much more work is required to identify oxidative post-translational modifications entailing cysteine and methionine residues and to determine how these modifications affect protein function and metabolism in nodules. Likewise, most interactions of antioxidant metabolites and enzymes bearing redox-active sulfur with transcription factors need to be defined. Solving these questions will pave the way to decipher sulfur-dependent mechanisms that regulate SNF, thereby gaining a deep insight into how nodulated legumes adapt to the fluctuating availability of nutrients in the soil.
硫是植物必需的营养元素，是某些氨基酸、金属辅因子、辅酶和次生代谢产物的组成元素。毫不奇怪，硫缺乏会降低豆类和非豆类植物的生长、光合作用和种子产量。在结瘤豆类中，硫供应与共生固氮（SNF）呈正相关，硫饥饿会产生另外三个主要影响：结瘤减少、SNF抑制和根瘤代谢减缓。这些影响至少部分是由于固氮酶生物合成和活性受损、富含氮的氨基酸积累以及根瘤中豆血红蛋白、铁氧还蛋白、ATP和葡萄糖的减少。在过去十年中，关于根瘤中硫的吸收和代谢取得了一些重大进展。这些进展包括在共生体膜中鉴定出硫酸盐转运蛋白SST1，发现类菌体和宿主细胞中产生的谷胱甘肽对根瘤活性至关重要，以及证明共生期间全株的硫同化被重新编程。然而，许多关键问题仍然存在，以下是一些例子。首先，阐明硫缺乏限制SNF的机制至关重要。目前尚不清楚为什么高半胱氨酸仅在某些豆类以及同一豆类物种的不同组织中取代谷胱甘肽，成为主要的水溶性抗氧化剂、氧化还原缓冲剂和硫库以及其他相关功能。需要开展更多工作来鉴定涉及半胱氨酸和甲硫氨酸残基的氧化翻译后修饰，并确定这些修饰如何影响根瘤中的蛋白质功能和代谢。同样，大多数带有氧化还原活性硫的抗氧化代谢物和酶与转录因子之间的相互作用也需要明确。解决这些问题将为破译调节SNF的硫依赖性机制铺平道路，从而深入了解结瘤豆类如何适应土壤中养分供应的波动。
