atcog8-2, A New Mutant Allele of the Conserved Oligomeric Golgi Complex 8, Reveals the Need for the COG Complex for Gametophyte Development in Arabidopsis

Angiosperms proliferate through double fertilization mediated by male (pollen) and female (embryo sac) gametophytes. To determine the genes essential for pollen development in Arabidopsis thaliana, we first generated a mutant population using an activation tagging vector with herbicide-resistance gene and screened mature pollen phenotypes. Then, a T-DNA insertional heterozygous line was isolated, initially named AP22-48, which produced high levels of abnormal pollen grains. Reciprocal crosses revealed that the genetic transmission of the mutant allele was completely blocked through the male and was highly limited through the female. Determination of T-DNA flanking sequences and genetic complementation of AP22-48 identified AtCOG8, a subunit of the Conserved Oligomeric Golgi (COG) complex, which is a tethering factor essential for the Golgi architecture and retrograde vesicle trafficking in eukaryotes. We renamed the mutant atcog8-2, with reference to a previous cog8 mutant (atcog8-1). While atcog8-1 induced male-specific defects during pollen tube growth, atcog8-2 mutant failed to produce normal gametophytes in both sexes. Detailed morphological analysis demonstrated aberrant development of the pollen and embryo sac in atcog8-2 mutants. This study, thus, strongly suggests that the COG complex functions are broad and indispensable for accurate gametophyte development, which is a prerequisite for sexual reproduction in Arabidopsis.