The formation of mature pollen grains from a diploid microspore mother cell (MMC) involves a specific sequence of divisions. First, the MMC undergoes one meiotic division to produce four haploid microspores . Subsequently, each microspore undergoes its first mitotic division to form a large vegetative cell and a smaller generative cell (the 2-celled stage) . In many plants, including most grasses, the generative cell undergoes a second mitotic division to produce two male gametes before the pollen is shed, resulting in a 3-celled mature pollen grain .