Ultrastructure analysis reveals sporopollenin deposition and nexine formation at early stage of pollen wall development in Arabidopsis

Abstract

In angiosperm, pollen wall formation is a critical step for male gametophyte development. Pollen wall constitutes of the outer layer exine and the inner layer intine. Exine is further divided into sexine and nexine. In Arabidopsis, the general process of pollen wall formation has been reported. However, the nexine formation has not been revealed. Here, we observed the process of pollen wall formation in Arabidopsis thaliana using transmission electron microscope. After callose wall is formed, the primexine is present between plasma membrane and the callose layer in the tetrad. With plasma membrane undulation, sporopollenin precursors accumulated on the peak of undulated membrane which is further developed into probacula. The primexine determines plasma membrane undulation and sporopollenin accumulation based on previous analysis of an undulation-deficient mutant. Some materials obviously different from sporopollenin are filled between the primexine and plasma membrane. These materials cover all the surface of plasma membrane and gradually develop into nexine. After microspore is released from tetrad, the nexine layer is formed and the probacula is further developed into sexine with continued accumulation of sporopollenin. Based on these observations, we proposed a developmental model of early pollen wall formation.

概要

花粉壁由外壁和内壁组成,外壁又分为外壁外层和外壁内层。外壁由绒毡层控制,而内壁由小孢子自身控制。自从有了电子显微镜,就了解到花粉壁有三层结构。花粉外壁内层是高度保守的结构,只有在电镜下能观察到,但是对其如何形成并不清楚。最近报道了模式植物拟南芥中TEK基因特异调控外壁内层的形成,在该突变体中外壁内层特异缺失而外壁外层能正常形成(Nature communications 5:3855)。本文在该项工作的基础上,对模式植物拟南芥花粉壁形成过程进行全面深入分析。发现在四分体的细胞周质中存在的深灰色物质可能是外壁内层成分的前体,其组成成分可能与孢粉素有所不同,当小孢子从四分体中释放时,这些前体物质能够迅速组装成外壁内层。在此基础上,提出了一个拟南芥花粉外壁的发育模型: 在减数分裂形成的四分体中,胼胝质壁和小孢子质膜间形成初生外壁。随后小孢子质膜显示出波浪型结构,绒毡层分泌的孢粉素沉积在波浪型质膜顶端发育成外壁外层结构,而绒毡层分泌的外壁内层物质积累在小孢子质膜表面发育成外壁内层结构。四分体胼胝质壁完全降解释放小孢子后,在外壁内层和小孢子质膜间形成内壁。这一包含花粉壁三层结构的模型不仅有助于其他植物物种花粉壁结构和形成过程的了解,也有助于花粉壁分子机理的深入研究。