SAR1B and mitotic cell cycle: Malfunctioning tapetum, leading to male sterility. Microspores in sar1b pollen sacs started to degenerate. The plasma membrane (PM) of microspores in sar1b pollen sacs was detached from the cell wall, and at anther dehiscence, sar1b pollen sacs contained only a pile of cellular debris Microspores aborted at anther developmental stage 10, arrest of pollen development at Pollen Mitosis I