Girija Aiswarya, Shotan David, Hacham Yael, Amir Rachel
Department of Plant Science, MIGAL-Galilee Research Center, Kiryat Shmona, Israel.
Department of Biotechnology, Tel-Hai College, Upper Galilee, Israel.
Front Plant Sci. 2020 Aug 5;11:1136. doi: 10.3389/fpls.2020.01136. eCollection 2020.
The low level of methionine, an essential sulfur-containing amino acid, limits the nutritional quality of seeds. Two main factors can control the level of protein-bound methionine: the level of free methionine that limits protein accumulation and the methionine residues inside the storage proteins. To reveal the main limiting factor, we generated transgenic seed-specific plants expressing the methionine-rich sunflower seed storage () protein (A1/A2). The contents of protein-bound methionine in the water-soluble protein fraction that includes the SSA in A1/A2 were 5.3- and 10.5-fold, respectively, compared to control, an empty vector (EV). This suggests that free methionine can support this accumulation. To elucidate if the level of free methionine could be increased further in the protein-bound methionine, these lines were crossed with previously characterized plants having higher levels of free methionine in seeds (called SSE). The progenies of the crosses (A1S, A2S) exhibited the highest level of protein-bound methionine, but this level did not differ significantly from A2, suggesting that all the methionine residues of A2 were filled with methionine. It also suggests that the content of methionine residues in the storage proteins is the main limiting factor. The results also proposed that the storage proteins can change their content in response to high levels of free methionine or SSA. This was assumed since the water-soluble protein fraction was highest in A1S/A2S as well as in SSE compared to EV and A1/A2. By using these seeds, we also aimed at gaining more knowledge about the link between high free methionine and the levels of metabolites that usually accumulate during abiotic stresses. This putative connection was derived from a previous analysis of SSE. The results of metabolic profiling showed that the levels of 29 and 20 out of the 56 metabolites were significantly higher in SSE and A1, respectively, that had higher level of free methionine, compared A1S/A2S, which had lower free methionine levels. This suggests a strong link between high free methionine and the accumulation of stress-associated metabolites.
蛋氨酸是一种必需的含硫氨基酸，其低水平限制了种子的营养品质。有两个主要因素可以控制蛋白质结合型蛋氨酸的水平：限制蛋白质积累的游离蛋氨酸水平以及贮藏蛋白内部的蛋氨酸残基。为了揭示主要的限制因素，我们培育了表达富含蛋氨酸的向日葵种子贮藏蛋白（A1/A2）的转基因种子特异性植物。与对照（空载体，EV）相比，包含A1/A2中SSA的水溶性蛋白组分中蛋白质结合型蛋氨酸的含量分别提高了5.3倍和10.5倍。这表明游离蛋氨酸可以支持这种积累。为了阐明游离蛋氨酸水平是否可以在蛋白质结合型蛋氨酸中进一步提高，将这些株系与先前鉴定的种子中游离蛋氨酸水平较高的植物（称为SSE）杂交。杂交后代（A1S、A2S）表现出最高水平的蛋白质结合型蛋氨酸，但该水平与A2没有显著差异，这表明A2的所有蛋氨酸残基都已被蛋氨酸填满。这也表明贮藏蛋白中蛋氨酸残基的含量是主要限制因素。结果还表明，贮藏蛋白可以响应高水平的游离蛋氨酸或SSA而改变其含量。之所以这样推测，是因为与EV和A1/A2相比，A1S/A2S以及SSE中的水溶性蛋白组分最高。通过使用这些种子，我们还旨在获得更多关于高游离蛋氨酸与非生物胁迫期间通常积累的代谢物水平之间联系的知识。这种假定的联系源自先前对SSE的分析。代谢谱分析结果表明，与游离蛋氨酸水平较低的A1S/A2S相比，在游离蛋氨酸水平较高的SSE和A1中，56种代谢物中的29种和20种水平分别显著更高。这表明高游离蛋氨酸与胁迫相关代谢物的积累之间存在紧密联系。
