Plastid terminal oxidase (PTOX) is a chloroplast enzyme that catalyzes oxidation of plastoquinol (PQH2) and reduction of molecular oxygen to water. Its function has been associated with carotenoid biosynthesis, chlororespiration and environmental stress responses in plants. In the majority of plant species, a single gene encodes the protein and little is known about events of PTOX gene duplication and their implication to plant metabolism. Previously, two putative PTOX (PTOX1 and 2) genes were identified in Glycine max, but the evolutionary origin and the specific function of each gene was not explored. Phylogenetic analyses revealed that this gene duplication occurred apparently during speciation involving the Glycine genus ancestor, an event absent in all other available plant leguminous genomes. Gene expression evaluated by RT-qPCR and RNA-seq data revealed that both PTOX genes are ubiquitously expressed in G. max tissues, but their mRNA levels varied during development and stress conditions. In development, PTOX1 was predominant in young tissues, while PTOX2 was more expressed in aged tissues. Under stress conditions, the PTOX transcripts varied according to stress severity, i.e., PTOX1 mRNA was prevalent under mild or moderate stresses while PTOX2 was predominant in drastic stresses. Despite the high identity between proteins (97%), molecular docking revealed that PTOX1 has higher affinity to substrate plastoquinol than PTOX2. Overall, our results indicate a functional relevance of this gene duplication in G. max metabolism, whereas PTOX1 could be associated with chloroplast effectiveness and PTOX2 to senescence and/or apoptosis.

中文翻译：

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在植物发育和胁迫条件下，最近在大豆中最大表达的PTOX基因的差异表达。

质体末端氧化酶（PTOX）是一种叶绿体酶，可催化质体喹诺醇（PQH2）氧化并将分子氧还原为水。其功能与植物中的类胡萝卜素生物合成，氯呼吸作用和环境胁迫反应有关。在大多数植物物种中，单个基因编码蛋白质，而对PTOX基因复制的事件及其对植物代谢的影响知之甚少。以前，在大豆大豆中鉴定了两个推定的PTOX基因（PTOX1和2），但未探究每个基因的进化起源和特定功能。系统发育分析表明，这种基因复制显然发生在涉及甘氨酸属祖先的物种形成过程中，而在所有其他可利用的植物豆科基因组中都没有这种事件。通过RT-qPCR和RNA-seq数据评估的基因表达表明，两个PTOX基因均在G.max组织中普遍表达，但它们的mRNA水平在发育和胁迫条件下会发生变化。在发育过程中，PTOX1在年轻组织中占主导地位，而PTOX2在老年组织中表达更多。在胁迫条件下，PTOX转录本根据胁迫的严重程度而变化，即，PTOX1 mRNA在轻度或中度胁迫下普遍存在，而PTOX2在剧烈胁迫下占主导地位。尽管蛋白质之间具有高度同一性（97％），但分子对接显示PTOX1对底物质体喹诺醇的亲和力高于PTOX2。总的来说，我们的结果表明该基因重复在G.max代谢中具有功能相关性，而PTOX1可能与叶绿体有效性相关，而PTOX2可能与衰老和/或凋亡相关。