Male Germ Line Development in Arabidopsis. duo pollen Mutants Reveal Gametophytic Regulators of Generative Cell Cycle Progression  

Abstract

Male germ line development in flowering plants is initiated with the formation of the generative cell that is the progenitor of the two sperm cells. While structural features of the generative cell are well documented, genetic programs required for generative cell cycle progression are unknown. We describe two novel Arabidopsis (Arabidopsis thaliana) mutants, duo pollen1 (duo1) and duo pollen2 (duo2), in which generative cell division is blocked, resulting in the formation of bicellular pollen grains at anthesis. duo1 and duo2 map to different chromosomes and act gametophytically in a male-specific manner. Both duo mutants progress normally through the first haploid division at pollen mitosis I (PMI) but fail at distinct stages of the generative cell cycle. Mutant generative cells in duo1 pollen fail to enter mitosis at G2-M transition, whereas mutant generative cells in duo2 enter PMII but arrest at prometaphase. In wild-type plants, generative and sperm nuclei enter S phase soon after inception, implying that male gametic cells follow a simple S to M cycle. Mutant generative nuclei in duo1 complete DNA synthesis but bypass PMII and enter an endocycle during pollen maturation. However, mutant generative nuclei in duo2 arrest in prometaphase of PMII with a 2C DNA content. Our results identify two essential gametophytic loci required for progression through different phases of the generative cell cycle, providing the first evidence to our knowledge for genetic regulators of male germ line development in flowering plants.Plant sexual reproduction depends on the timely construction of male and female gametes that are produced by the haploid gametophyte generation. In flowering plants, male gametogenesis is restricted to a simple cell lineage of two cell divisions following meiosis that results in the production of two nonmotile sperm cells. The first division of the microspore at pollen mitosis I (PMI) is asymmetric and gives rise to a large transcriptionally active vegetative cell and a diminutive generative cell with condensed chromatin and fewer organelles. After PMI, the two cells follow different developmental pathways that are characterized by the differential control of the cell cycle and gene expression. Whereas the vegetative cell exits the cell cycle in G1 and differentiates, the generative cell completes a further cell cycle to form the two sperm cells (for review, see Tanaka, 1997;Twell et al., 1998). Although the general pathway leading to sperm cell formation is clear, our knowledge of the genetic and molecular control of generative cell cycle progression and male germ line development is very limited.Gene expression within the male gametes has been explored in some plants. Male gamete specific histones have been identified in isolated generative cells of lily (Lilium longiflorum; Ueda and Tanaka, 1995), and some genes (ERCC1, LGC1, and FtsZ) that are expressed preferentially or specifically in the male gametes have been isolated (Xu et al., 1998(Xu et al., , 1999Mori and Tanaka, 2000). Recently, larg...

Endosperm: an integrator of seed growth and development

“…In Arabidopsis, direct measurements of DNA content suggest that sperm cells enter a new S-phase after PMII, which is presumably completed either before or during pollen tube growth [6,7]. Thus, when delivered to the embryo sac, sperm cells are likely to be in G 2 phase [8].…”

MIKC* MADS Domain Heterodimers Are Required for Pollen Maturation and Tube Growth in Arabidopsis

“…To determine whether there was a consistent cellular basis for the defect, stage 11 anthers (Smyth et al, 1990) were collected from wild-type and double mutant plants and pollen ultrastructure was examined by transmission electron microscopy. At this stage of development, pollen are primarily tricellular (Durbarry et al, 2005) and are near maturity. Many of the pollen showed signs of structural damage, which may have occurred either before or during processing.…”

Female gametophytic cell specification and seed development require the function of the putativeArabidopsisINCENP orthologWYRD

“…In addition to rbr, several mutations in Arabidopsis impairing cell cycling affect cell fate and mitotic progression in the male gametophyte. Most of the mutants arrest at PMII Chen et al, 2008;Durbarry et al, 2005;Iwakawa et al, 2006;Nowack et al, 2006;Rotman et al, 2005) and affect the identity of the generative cell. By contrast, the role of cell cycle regulators in the female gametophyte is less well understood.…”