Castor oil contains approximately 90% ricinoleic acid (RA) which is stored mainly in the form of tri‐ricinoleic acid containing triacylglycerols (TAG). Ricinoleate is synthesized from oleate (18:1n‐9) esterified to the sn‐2 position of phosphatidylcholine (PtdCho) catalyzed by oleoyl‐12‐hydroxylase. PtdCho‐derived diacylglycerol (DAG) is an important substrate pool for TAG synthesis, and the interconversion between PtdCho and DAG has been shown to play a critical role in channeling hydroxy fatty acids (HFA) to TAG. Although phospholipase D (PLD) has been reported to catalyze the hydrolysis of PtdCho to produce phosphatidic acid which can then be converted to DAG, its potential functions in the channeling of RA from PtdCho to DAG and the assembly of RA on TAG is largely unknown. In the present study, 11 PLD genes were identified from the Castor Bean Genome Database. Gene expression analysis indicated that RcPLD9 is expressed at relatively high levels in developing seeds compared to other plant tissues. Sequence and phylogenetic analyses revealed that RcPLD9 is a homolog of Arabidopsis PLDζ2. Overexpression of RcPLD9 in the Arabidopsis CL7 line producing C18‐HFA resulted in RA content reductions in the polar lipid fraction (mainly PtdCho) and mono‐HFA‐TAG, but increased RA content in di‐HFA‐TAG. Since part of RA in di‐HFA‐TAG is derived from HFA‐DAG, the results indicated that RcPLD9 facilitates the channeling of RA from PtdCho to DAG for its assembly on TAG in developing seeds.

中文翻译：

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来自开发蓖麻籽胚乳的 PLDζ2 同源基因的表征。

蓖麻油含有大约 90% 的蓖麻油酸 (RA)，主要以含有三酰基甘油 (TAG) 的三蓖麻油酸形式储存。蓖麻油酸酯是由油酸（18:1n-9）在油酰-12-羟化酶催化下与磷脂酰胆碱（PtdCho）的sn-2位酯化合成的。PtdCho 衍生的二酰基甘油 (DAG) 是 TAG 合成的重要底物库，PtdCho 和 DAG 之间的相互转化已被证明在将羟基脂肪酸 (HFA) 引导至 TAG 中起着关键作用。尽管已报道磷脂酶 D (PLD) 催化 PtdCho 水解产生磷脂酸，然后可将其转化为 DAG，但其在 RA 从 PtdCho 到 DAG 的通道中的潜在功能以及 RA 在 TAG 上的组装在很大程度上是未知的。在目前的研究中，从蓖麻子基因组数据库中鉴定出 11 个 PLD 基因。基因表达分析表明与其他植物组织相比，RcPLD9在发育种子中的表达水平相对较高。序列和系统发育分析表明RcPLD9是拟南芥PLDζ2的同源物。RcPLD9在产生 C18-HFA 的拟南芥 CL7 系中的过表达导致极性脂质部分（主要是 PtdCho）和单 HFA-TAG 中的 RA 含量减少，但增加了 di-HFA-TAG 中的 RA 含量。由于 di-HFA-TAG 中的部分 RA 来源于 HFA-DAG，结果表明 RcPLD9 促进了 RA 从 PtdCho 到 DAG 的通道，以便在种子发育过程中组装在 TAG 上。