Cell wall mechanical strength is partially controlled by the degree of pectin methylesterification that is mediated through the action of pectin methylesterases (PMEs). Changes in cell wall mechanical strength are involved in the endosperm cap weakening and radicle elongation growth during the germination of some endospermic seeds. However, little is known about the roles of the status of pectin methylesterification and PME activities during seed germination, especially in lettuce, a traditional model plant for seed germination research. In the present study, we demonstrated that the rupture of lettuce endosperm cap during seed germination was caused by cell rupture, not by cell separation as formerly reported in other species. Next, we investigated the status of pectin methylesterification of the endosperm cap and radicle during germination, using immunolabeling by specific monoclonal antibodies against methyl-esterified pectins and calcium bridges. We also analyzed the expression patterns of PME activities and an encoding gene LsPME/PMEI51 in the endosperm cap and radicle. It was found that cell walls of the endosperm cap comprise more methyl-esterified pectins than radicle, while PME activities and the abundance of LsPME/PMEI51 transcripts in the endosperm cap are lower than those in the radicle. In addition, the status of pectin methylesterification and PME expression patterns changed during seed germination. Low PME activities and high pectin methylesterification in the endosperm cap is hypothesized to be the reason of why endosperm does not rupture by cell separation. It is concluded that tissue-specific pectin methylesterification and PME activities play a role in lettuce seed germination.

细胞壁机械强度部分受果胶甲酯化程度的控制，果胶甲酯化程度是通过果胶甲酯酶 (PME) 的作用介导的。一些胚乳种子萌发过程中，细胞壁机械强度的变化与胚乳帽弱化和胚根伸长生长有关。然而，关于果胶甲酯化状态和 PME 活性在种子萌发过程中的作用知之甚少，尤其是在种子萌发研究的传统模式植物生菜中。在本研究中，我们证明了种子萌发过程中莴苣胚乳帽的破裂是由细胞破裂引起的，而不是像以前在其他物种中报道的那样是由细胞分离引起的。接下来，我们研究了萌发过程中胚乳帽和胚根的果胶甲酯化状态，使用针对甲基酯化果胶和钙桥的特异性单克隆抗体进行免疫标记。我们还分析了 PME 活动和编码基因的表达模式胚乳帽和胚根中的LsPME/PMEI51。结果表明，胚乳盖细胞壁中甲基酯化果胶的含量高于胚根，而胚乳盖中的PME活性和LsPME/PMEI51转录物的丰度低于胚根。此外，果胶甲酯化状态和 PME 表达模式在种子萌发过程中发生了变化。假设胚乳帽中的低PME活性和高果胶甲基酯化是胚乳不会因细胞分离而破裂的原因。可以得出结论，组织特异性果胶甲酯化和 PME 活性在莴苣种子萌发中起作用。