Elleman C J, Dickinson H G
Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, UK.
New Phytol. 1996 Jun;133(2):197-205. doi: 10.1111/j.1469-8137.1996.tb01886.x.
When components of the mature pollen grain of Brassica oleracea are applied to the cuticularised surfaces of the stigmatic papilla, a number of changes can take place in the architecture of the subjacent cell wall. Treatment of the papillar surface with isolated pollen coating evokes a rapid and extensive expansion of the outer of the two stigmatic wall layers. This response occurs within 4 h but is restricted to those regions where no callose is formed and where the coating exhibits a characteristic increase in electron opacity. This study establishes that the coating alone is responsible for initiating the expansion of the outer wall, which is considered to be an essential step preceding penetration of the stigma surface by the pollen tube. Vesicle-like inclusions, some staining intensely, occasionally occur in regions of the wall expanded by isolated coating, sometimes fusing to form a subcuticular 'boundary layer'. These structures are not observed under compatible grains in vivo and their presence is regarded as artefaetual. However, close examination of the plasma membrane at sites beneath areas of expanded cell wall reveals membrane-bound structures resembling the vesicles being generated by the cytoplasm and moving into the apoplast. These data indicate that signals from the coating initiate the loosening of the wall matrix by stimulating an unusual form of localized secretion which appears to be an essential prerequisite for stigmatic penetration. Isolated pollen coating contaminated with fragments of the grain itself engender a very different response, including the formation of densely-staining vesicles accompanied by extensive accumulations of callose; wall expansion never occurs under these circumstances. Self-pollinations in B. oleracea are frequently accompanied by the synthesis of stigmatic callose, the presence of which has, in the past, been interpreted as forming a structural barrier to incompatible pollen tubes. However, callose elicited both by self-pollinations and coating supplemented by killed grains is independent of protein synthesis and occurs in the presence of the phosphatase inhibitor, okadaic acid. Since both these inhibitors also overcome self-incompatibility (SI) in B. oleracea, these data strongly suggest that callose synthesis is unrelated to the operation of SI, and it is proposed that callose, when formed, is elicited by molecules released from the necrotic pollen protoplast. Interestingly, isolated pollen coating of self- and cross-genotypes does not readily elicit callose. The significance of these changes in the stigmatic wall is discussed in the perspective of current views on pollination and self-incompatibility in Brassica.
当将甘蓝型油菜成熟花粉粒的成分应用于柱头乳突的角质化表面时，其下方细胞壁的结构会发生一些变化。用分离出的花粉包被处理乳突表面，会引起两个柱头壁层中外层的快速且广泛的扩张。这种反应在4小时内发生，但仅限于那些不形成胼胝质且包被电子不透明度呈现特征性增加的区域。这项研究证实，仅包被就负责启动外壁的扩张，这被认为是花粉管穿透柱头表面之前的一个关键步骤。囊泡状内含物，有些染色很深，偶尔出现在因分离的包被而扩张了的壁区域，有时融合形成一个皮下“边界层”。在体内的亲和性花粉粒下未观察到这些结构，其存在被认为是人为造成的。然而，在扩张细胞壁区域下方的位点仔细检查质膜时，会发现膜结合结构类似于由细胞质产生并移入质外体的囊泡。这些数据表明，来自包被的信号通过刺激一种不寻常的局部分泌形式来启动壁基质的松弛，而这种分泌形式似乎是柱头穿透的一个必要前提。被花粉粒自身碎片污染的分离花粉包被会引发非常不同的反应，包括形成染色深的囊泡并伴有大量胼胝质积累；在这些情况下，壁不会扩张。甘蓝型油菜的自花授粉经常伴随着柱头胼胝质的合成，过去人们一直认为其存在是对不亲和花粉管形成的一种结构屏障。然而，自花授粉和用灭活花粉粒补充的包被引发的胼胝质合成与蛋白质合成无关，且在磷酸酶抑制剂冈田酸存在的情况下也会发生。由于这两种抑制剂也能克服甘蓝型油菜的自交不亲和性（SI），这些数据强烈表明胼胝质合成与SI作用无关，有人提出，当形成胼胝质时是由坏死花粉原生质体释放的分子引发的。有趣的是，自交和杂交基因型的分离花粉包被不容易引发胼胝质。本文从当前关于甘蓝型油菜授粉和自交不亲和性的观点出发，讨论了柱头壁这些变化的意义。
