Araji Soha, Grammer Theresa A, Gertzen Ross, Anderson Stephen D, Mikulic-Petkovsek Maja, Veberic Robert, Phu My L, Solar Anita, Leslie Charles A, Dandekar Abhaya M, Escobar Matthew A
Department of Biological Sciences, California State University, San Marcos, California 92096.
Plant Physiol. 2014 Mar;164(3):1191-203. doi: 10.1104/pp.113.228593. Epub 2014 Jan 21.
The enzyme polyphenol oxidase (PPO) catalyzes the oxidation of phenolic compounds into highly reactive quinones. Polymerization of PPO-derived quinones causes the postharvest browning of cut or bruised fruit, but the native physiological functions of PPOs in undamaged, intact plant cells are not well understood. Walnut (Juglans regia) produces a rich array of phenolic compounds and possesses a single PPO enzyme, rendering it an ideal model to study PPO. We generated a series of PPO-silenced transgenic walnut lines that display less than 5% of wild-type PPO activity. Strikingly, the PPO-silenced plants developed spontaneous necrotic lesions on their leaves in the absence of pathogen challenge (i.e. a lesion mimic phenotype). To gain a clearer perspective on the potential functions of PPO and its possible connection to cell death, we compared the leaf transcriptomes and metabolomes of wild-type and PPO-silenced plants. Silencing of PPO caused major alterations in the metabolism of phenolic compounds and their derivatives (e.g. coumaric acid and catechin) and in the expression of phenylpropanoid pathway genes. Several observed metabolic changes point to a direct role for PPO in the metabolism of tyrosine and in the biosynthesis of the hydroxycoumarin esculetin in vivo. In addition, PPO-silenced plants displayed massive (9-fold) increases in the tyrosine-derived metabolite tyramine, whose exogenous application elicits cell death in walnut and several other plant species. Overall, these results suggest that PPO plays a novel and fundamental role in secondary metabolism and acts as an indirect regulator of cell death in walnut.
多酚氧化酶（PPO）催化酚类化合物氧化为高反应性醌类。PPO衍生的醌类聚合导致采后果实切口或碰伤处褐变，但PPO在未受损的完整植物细胞中的天然生理功能尚不清楚。核桃（Juglans regia）产生大量酚类化合物，且仅有一种PPO酶，使其成为研究PPO的理想模型。我们构建了一系列PPO沉默的转基因核桃株系，其PPO活性不到野生型的5%。令人惊讶的是，在没有病原体侵染的情况下（即类病斑表型），PPO沉默的植株叶片上出现了自发坏死病斑。为了更清楚地了解PPO的潜在功能及其与细胞死亡的可能联系，我们比较了野生型和PPO沉默植株的叶片转录组和代谢组。PPO沉默导致酚类化合物及其衍生物（如香豆酸和儿茶素）的代谢以及苯丙烷途径基因的表达发生重大变化。观察到的几种代谢变化表明，PPO在体内酪氨酸代谢和羟基香豆素七叶亭生物合成中起直接作用。此外，PPO沉默的植株中酪氨酸衍生的代谢产物酪胺大量增加（9倍），外源施用酪胺会导致核桃和其他几种植物细胞死亡。总体而言，这些结果表明，PPO在次生代谢中发挥着新的重要作用，并作为核桃细胞死亡的间接调节因子。
