BioDiscovery Institute, University of North Texas, Denton, Texas 76203-5017, USA.
Department of Biological Sciences, University of North Texas, Denton, Texas, USA.
Nat Plants. 2016 Nov 21;2:16182. doi: 10.1038/nplants.2016.182.
Proanthocyanidins (PAs) are the second most abundant plant polyphenolic compounds after lignin. PAs affect taste, mouth feel and astringency of many fruits, wines and beverages, have been associated with reduced risks of cardiovascular disease, cancer and Alzheimer's disease, can improve nutrition and prevent bloat in ruminant animals and enhance soil nitrogen retention. PAs are oligomers and polymers of flavan-3-ols, primarily (-)-epicatechin and (+)-catechin, but the mechanism by which the monomers polymerize and become insoluble is currently unknown. Leucoanthocyanidin reductase (LAR) has been shown to convert leucocyanidin to (+)-catechin. Here, we report that loss of function of LAR in the model legume Medicago truncatula leads unexpectedly to loss of soluble epicatechin-derived PAs, increased levels of insoluble PAs, and accumulation of 4β-(S-cysteinyl)-epicatechin, which provides the 4→8 linked extension units during non-enzymatic PA polymerization. LAR converts 4β-(S-cysteinyl)-epicatechin back to epicatechin, the starter unit in PAs, thereby regulating the relative proportions of starter and extension units and consequently the degree of PA oligomerization.
原花青素（PAs）是仅次于木质素的植物中第二丰富的多酚类化合物。PAs 影响许多水果、葡萄酒和饮料的口感、口感和涩味，与降低心血管疾病、癌症和阿尔茨海默病的风险有关，可改善反刍动物的营养并预防膨胀，增强土壤氮素保持。PAs 是黄烷-3-醇的低聚物和聚合物，主要为（-）-表儿茶素和（+）-儿茶素，但单体聚合并变得不溶的机制目前尚不清楚。已经表明，白细胞素还原酶（LAR）将白细胞素转化为（+）-儿茶素。在这里，我们报告在模式豆科植物苜蓿中 LAR 的功能丧失导致可溶性表儿茶素衍生的 PAs 丧失，不溶性 PAs 水平增加，以及 4β-（S-半胱氨酸）-表儿茶素的积累，这在非酶 PA 聚合过程中提供 4→8 连接的扩展单元。LAR 将 4β-（S-半胱氨酸）-表儿茶素转化回表儿茶素，即 PAs 的起始单元，从而调节起始单元和扩展单元的相对比例，从而调节 PA 低聚度。
