Identification of New Mutant Alleles of Augmin Subunits Broadens Spectrum of Augmin Function During Sexual Reproduction in Arabidopsis

In flowering plants, production of functional gametophytes is prerequisite for the double fertilization that leads to successful proliferation. Pollen grains as male gametophytes in Arabidopsis thaliana are uniquely patterned with two sperm cells inside a vegetative cell. To better understand the genetic regulation underlying pollen development, we morphologically screened DAPI-stained mature pollen in a mutant population generated using an activation tagging vector in this study. As a result, we identified two independent lines, AL318 and AL434, exhibiting similar mutant phenotypes regarding nucleus number and pollen pattern. Genetic analysis showed that the two mutants are maintained in heterozygotes but not in homozygotes, due to highly reduced genetic transmission from both sexes. Developmental analysis revealed that mutant microspores at the polarized stage either completely fail to enter pollen mitosis I, or abnormally divide with altered division asymmetry, resulting in mature pollen without the male germline. We found that these defects arise from genetic lesions in the AUG2 and AUG4 genes, members of the augmin complex that mediates microtubule (MT)-dependent MT nucleation in a broad range of species. Collectively, our study provides genetic evidence that the augmin complex plays a critical role in mitotic entry and proper execution for pollen mitosis I, broadening the spectrum of augmin function during sexual reproduction in Arabidopsis.