Summary The mode of transport of lignin monomers to the sites of polymerization in the apoplast remains controversial. C‐Lignin is a recently discovered form of lignin found in some seed coats that is composed exclusively of units derived from caffeyl alcohol. RNA‐seq and proteome analyses identified a number of transporters co‐expressed with C‐lignin deposition in the seed coat of Cleome hassleriana. Cloning and influx/efflux analysis assays in yeast identified two low‐affinity transporters, ChPLT3 and ChSUC1, that were active with caffeyl alcohol but not with the classical monolignols p‐coumaryl, coniferyl, and sinapyl alcohols, consistent with molecular modeling and docking studies. Expression of ChPLT3 in Arabidopsis seedlings enhanced root growth in the presence of caffeyl alcohol, and expression of ChPLT3 and ChSUC1 correlated with lignin C‐unit content in hairy roots of Medicago truncatula. We present a model, consistent with phylogenetic and evolutionary considerations, whereby passive caffeyl alcohol transport may be supplemented by hitchhiking on secondary active transporters to ensure the synthesis of C‐lignin, and inhibition of synthesis of G‐lignin, in the apoplast.

中文翻译：

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主要促进子家族转运蛋白特异性增强 C-木质素生物合成过程中咖啡醇的摄取


总结 木质素单体向质外体聚合位点的运输方式仍然存在争议。C-木质素是最近发现的一种木质素形式，存在于一些种皮中，完全由咖啡醇衍生的单元组成。RNA-seq 和蛋白质组分析确定了许多与 Cleome hassleriana 种皮中的 C-木质素沉积共表达的转运蛋白。酵母中的克隆和内流/外排分析测定确定了两种低亲和力转运蛋白 ChPLT3 和 ChSUC1，它们对咖啡醇具有活性，但对经典木质素对香豆酰、针叶酰基和芥子醇没有活性，这与分子建模和对接研究一致。在咖啡醇存在下，拟南芥幼苗中 ChPLT3 的表达促进了根系生长，ChPLT3 和 ChSUC1 的表达与苜蓿毛状根中的木质素 C-单位含量相关。我们提出了一个与系统发育和进化考虑一致的模型，其中被动咖啡醇运输可以通过搭便车在次级活性转运蛋白上补充，以确保 C-木质素的合成，并抑制质外体中 G-木质素的合成。