ABSTRACT
Timely progression of the meiotic cell cycle and synchronized establishment of male meiosis in anthers are key to ascertaining plant fertility. With the discovery of novel regulators of the plant cell cycle, the mechanisms underlying meiosis initiation and progression appear to be more complex than previously thought, requiring the conjunctive action of cyclins, cyclin-dependent kinases, transcription factors, protein-protein interactions, and several signaling components. Broadly, cell cycle regulators can be classified into two categories in plants based on the nature of their mutational effects: (1) those that completely arrest cell cycle progression; and (2) those that affect the timing (delay or accelerate) or synchrony of cell cycle progression but somehow complete the division process. Especially the latter effects reflect evasion or obstruction of major steps in the meiosis but have sometimes been overlooked due to their subtle phenotypes. In addition to meiotic regulators, very few signaling compounds have been discovered in plants to date. In this review, we discuss the current state of knowledge about genetic mechanisms to enter the meiotic processes, referred to as the mitosis-meiosis fate decision, as well as the importance of callose (ß-1,3 glucan), which has been unsung for a long time in male meiosis in plants.
ABSTRACT
Callose is a plant cell wall polysaccharide whose deposition is spatiotemporally regulated in various developmental processes and environmental stress responses. The appearance of callose in premeiotic anthers is a prominent histological hallmark for the onset of meiosis in flowering plants; however, the biological role of callose in meiosis remains unknown. Here, we show that rice (Oryza sativa) GLUCAN SYNTHASE LIKE5 (OsGSL5), a callose synthase, localizes on the plasma membrane of pollen mother cells (PMCs) and is responsible for biogenesis of callose in anther locules through premeiotic and meiotic stages. In Osgsl5 mutant anthers mostly lacking callose deposition, aberrant PMCs accompanied by aggregated, unpaired, or multivalent chromosomes were frequently observed and, furthermore, a considerable number of mutant PMCs had untimely progress into meiosis compared to that of wild-type PMCs. Immunostaining of meiosis-specific protein HOMOLOGOUS PAIRING ABERRATION IN RICE MEIOSIS2 in premeiotic PMCs revealed precocious meiosis entry in Osgsl5 anthers. These findings provide insights into the function of callose in controlling the timing of male meiosis initiation and progression, in addition to roles in microsporogenesis, in flowering plants.
