Differentiation of generative and vegetative cells by unequal cell division

Abstract

Summary: Cellular differentiation of a generative and a vegetative cell during pollen development is requisite for sexual plant reproduction and is initiated by an unequal(asymmetric)cell division. The smaller, generative cell produces two sperm cells, or male gametes, whereas the larger, vegetative cell produces an elongated pollen tube, agametophytic cell, to deliver the male gametes to the embryo sac. The distinct differences between two kinds of cell are found in organelle composition and cytoskeletal organization in their cytoplasm. Recent transcriptome analysis also revealed the difference in gene expression profiles between both cells. We pay attention to the difference of theirchromatin configuration, that is assumed to be associated with the difference in gene activity between the two types of nucleus. Namely, the generative nucleus has highly condensed chromatin, as do the nuclei of animal sperm, whereas the vegetative nucleus contains diffuse chromatin. For this reason, by developing an efficient method to isolate the generative and vegetative nuclei from lily pollen protoplasts, their histone composition was investigated biochemically and immunocytochemically. Expression analysis of isolated histone genes was also done. As a result, we detected, in addition to 1)replication-dependent, somatic types of histones common to both nuclei, 2)replication-independent, generative and sperm nuclei-specific histone variants(gH 2 A, gH 2 B and gH 3)and 3)replication-independent, vegetative nucleus-specific histone variant(H 3. 3). The significance of this discovery will be discussed with respect to the function and developmental fates of both cells in the process of sexual plant reproduction.