Fertilization relies on pollen mother cells able to transit from mitosis to meiosis to supply gametes. This process involves remarkable changes at the molecular, cellular and physiological levels including (but not limited to) remodelling of the cell wall. During the meiosis onset, cellulose content at the pollen mother cell walls gradually declines with the concurrent deposition of the polysaccharide callose in anther locules. We aim to understand the biological significance of cellulose-to-callose turnover in pollen mother cells walls using electron microscopic analyses of rice flowers. Our observations indicate that in wild type rice anthers, the mitosis-to-meiosis transition coincides with a gradual reduction in the number of cytoplasmic connections called plasmodesmata. A mutant in the Oryza sativa callose synthase GSL5 (Osgsl5-3), impaired in callose accumulation in premeiotic and meiotic anthers, displayed a greater reduction in plasmodesmata frequency among pollen mother cells and tapetal cells suggesting a role for callose in plasmodesmata maintenance. In addition, a significant increase in extracellular distance between pollen mother cells and impaired premeiotic cell shaping was observed in the Osgsl5-3 mutant. The results suggest that callose-to-cellulose turnover during mitosis-meiosis transition is necessary to maintain cell-to-cell connections and optimal extracellular distance among the central anther locular cells. Findings of this study contribute to our understanding of the regulatory influence of callose metabolism during meiosis initiation in flowering plants.
Figure: PD frequency is differentially regulated upon entry to meiosis and in Osgsl5-3 mutant anthers (A) A diagram of the cross section of rice anther (left), and transmission electron microscopic images of PDs (yellow arrows) between neighbouring pollen mother cells (PMCs) in wild type (WT) (middle) and Osgsl5-3 mutant (right). (B) Quantification of PD frequency in premeiotic interphase and early meiosis I for WT and Osgsl5-3 anthers. PD frequency was gradually decreased from premeiosis to early meiosis in WT, but significantly reduced in Osgsl5-3 mutant compared to WT.
