Pollen Development

Abstract

Pollen is source and transport unit for the male gametes (or their progenitor cell). The unicellular pollen grain represents the microspore of seed plants, the multicellular pollen grain the male gametophytic generation. The development of a pollen grain includes microsporogenesis and microgametogenesis (Figs. 1 and 2, Gomez et al. 2015; Keijzer and Willemse 1988). Microsporogenesis starts with the differentiation of microspore mother cells (MMC) respectively pollen mother cells (PMC). These diploid cells become enclosed by a thick callose wall and undergo meiosis, forming a tetrad of four haploid microspores, each encased in another callose wall insulating them from each other and from the surrounding diploid tapetal cells (Figs. 1 C-E, and 2). Cytokinesis following meiotic nuclear divisions is accompanied by the formation of cleavage planes determined by the configuration and orientation of the meiotic spindle axes. In the case of successive cytokinesis, planes are formed after the first and second meiotic divisions leading to the formation of various microspore tetrad types (see “Pollen Morphology and Ultrastructure”). During simultaneous cytokinesis the cleavage planes are formed simultaneously after the second meiotic division and microspores become arranged in a tetrahedral tetrad (Furness and Rudall 1999, 2001).