Chromatogram of representative plant samples

Plant glutathione transferases (GSTs) constitute a large and diverse family of enzymes that are involved in plant stress response, metabolism and defense, yet their physiological functions remain largely elusive. Consistent with the traditional view on GSTs across organisms as detoxification enzymes, <i>in vitro</i> most plant GSTs catalyze glutathionylation, conjugation of the tripeptide glutathione (GSH; γ-Glu-Cys-Gly) onto reactive molecules. However, when it comes to elucidating GST functions it remains a key challenge that the endogenous plant GS-conjugates that would result from such glutathionylation reactions are rarely reported. Furthermore, GSTs often display high substrate promiscuity, and their proposed substrates are prone to spontaneous chemical reactions with GSH, hence single gene knockouts rarely provide clear chemo- or phenotypes. In a few cases, GS-conjugates are demonstrated to be biosynthetic intermediates that are rapidly further metabolized towards a pathway end-product, explaining their low abundance and rare detection. In this review we summarize the current knowledge of plant GST functions, and how and possibly why evolution has resulted in a broad and extensive expansion of the plant GST family. Finally, we demonstrate that endogenous GS-conjugates are more prevalent in plants than assumed, and suggest they are overlooked as clues towards identification of plant GST functions.

植物谷胱甘肽转移酶（glutathione transferases, GSTs）构成一个庞大且多样的酶家族，参与植物的应激响应、代谢过程与防御机制，但其生理功能在很大程度上仍有待阐明。与学界对各类生物中GSTs作为解毒酶的传统认知一致，体外实验中多数植物GSTs可催化谷胱甘肽化反应，即将三肽谷胱甘肽（glutathione, GSH；γ-谷氨酰-半胱氨酰-甘氨酸）结合至活性分子上。然而，在阐明GST功能时仍存在一项核心挑战：此类谷胱甘肽化反应所产生的植物内源性谷胱甘肽结合物（GS-conjugates）鲜有报道。此外，GSTs通常表现出极高的底物混杂性，且其推测底物易与GSH发生自发化学反应，因此单基因敲除实验往往难以获得明确的化学表型或表型特征。在少数案例中，谷胱甘肽结合物被证实为生物合成中间体，可快速进一步代谢为通路终产物，这也解释了其丰度低下且难以被检测到的原因。本综述总结了当前关于植物GST功能的研究进展，以及进化如何且可能为何推动了植物GST家族的广泛扩张。最后，我们证实植物内源性谷胱甘肽结合物的普遍程度远超此前假设，并指出这类结合物作为鉴定植物GST功能的关键线索，长期以来被研究者所忽视。