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The haploid multicellular male gametophyte of plants, the pollen grain, is a terminally differentiated structure whose function ends at fertilization.Unlike pollen grains, the immature gametophyte retains its capacity for totipotent growth when cultured in vitro.Haploid embryo production from cultured immature male gametophytes is a widely used plant breeding and propagation technique that was described nearly 50 years ago, but one that is poorly understood at the mechanistic level.