VILLUM Research Center for Plant Plasticity, Department of Plant and Environmental Sciences, University of Copenhagen, Frederiksberg, 1871, Denmark.
Plant Biochemistry Laboratory, Department of Plant and Environmental Sciences, University of Copenhagen, Frederiksberg, 1871, Denmark.
Plant J. 2018 Jun;94(6):1109-1125. doi: 10.1111/tpj.13923. Epub 2018 May 19.
Cyanogenic glucosides are nitrogen-containing specialized metabolites that provide chemical defense against herbivores and pathogens via the release of toxic hydrogen cyanide. It has been suggested that cyanogenic glucosides are also a store of nitrogen that can be remobilized for general metabolism via a previously unknown pathway. Here we reveal a recycling pathway for the cyanogenic glucoside dhurrin in sorghum (Sorghum bicolor) that avoids hydrogen cyanide formation. As demonstrated in vitro, the pathway proceeds via spontaneous formation of a dhurrin-derived glutathione conjugate, which undergoes reductive cleavage by glutathione transferases of the plant-specific lambda class (GSTLs) to produce p-hydroxyphenyl acetonitrile. This is further metabolized to p-hydroxyphenylacetic acid and free ammonia by nitrilases, and then glucosylated to form p-glucosyloxyphenylacetic acid. Two of the four GSTLs in sorghum exhibited high stereospecific catalytic activity towards the glutathione conjugate, and form a subclade in a phylogenetic tree of GSTLs in higher plants. The expression of the corresponding two GSTLs co-localized with expression of the genes encoding the p-hydroxyphenyl acetonitrile-metabolizing nitrilases at the cellular level. The elucidation of this pathway places GSTs as key players in a remarkable scheme for metabolic plasticity allowing plants to reverse the resource flow between general and specialized metabolism in actively growing tissue.
氰苷是一类含氮的特殊代谢物，通过释放有毒的氰化氢为植物提供化学防御，以抵御草食动物和病原体的侵害。有观点认为，氰苷也是氮的储存库，可以通过一条未知的途径重新用于一般代谢。本文揭示了高粱中氰苷类物质（dhurrin）的一种再循环途径，该途径可避免氰化氢的形成。如体外实验所示，该途径通过 dhurrin 衍生的谷胱甘肽缀合物的自发形成进行，该缀合物可被植物特异性的 λ 类谷胱甘肽转移酶（GSTL）还原裂解，产生对羟基苯乙腈。然后，腈水解酶将其进一步代谢为对羟基苯乙酸和游离氨，然后糖基化为形成对葡糖酰氧基苯乙酸。高粱中 4 种 GSTL 中的 2 种对谷胱甘肽缀合物表现出高立体特异性催化活性，并在高等植物 GSTL 的系统发育树中形成一个亚分支。相应的 2 个 GSTL 的表达与细胞水平上编码对羟基苯乙腈代谢腈水解酶的基因表达共定位。该途径的阐明将 GST 作为代谢可塑性中关键的参与者，使植物能够在活跃生长的组织中逆转一般和特殊代谢之间的资源流动。
