GLUCAN SYNTHASE-LIKE 5 (GSL5) Plays an Essential Role in Male Fertility by Regulating Callose Metabolism During Microsporogenesis in Rice

Callose plays an important role in pollen development in flowering plants. In rice, 10 genes encoding putative callose synthases have been identified; however, none of them has been functionally characterized. In this study, a rice Glucan Synthase-Like 5 (GSL5) knock-out mutant was isolated that exhibited a severe reduction in fertility. Pollen viability tests indicated that the pollen of the mutant was abnormal while the embryo sac was normal. Further, GSL5-RNA interference transgenic plants phenocopied the gsl5 mutant. The RNA expression of GSL5 was found to be knocked out in the gsl5 mutant and knocked down in GSL5-RNA interference transgenic plants by real-time reverse transcripion-PCR (RT-PCR) analysis. The male sterility of the mutant was due to abnormal microspore development; an analysis of paraffin sections of the mutant anthers at various developmental stages revealed that abnormal microspore development began in late meiosis. Both the knock-out and knock-down of GSL5 caused a lack of callose in the primary cell wall of meiocytes and in the cell plate of tetrads. As a result, the callose wall of the microspores was defective. This was demonstrated by aniline blue staining and an immunogold labeling assay; the microspores could not maintain their shape, leading to premature swelling and even collapsed microspores. These data suggest that the callose synthase encoded by GSL5 plays a vital role in microspore development during late meiosis and is essential for male fertility in rice.