AP1/2ß-mediated exocytosis of tapetum-specific transporters is required for pollen development in Arabidopsis thaliana.

ABSTRACT

AP-1 and AP-2 adaptor protein (AP) complexes mediate clathrin-dependent trafficking at the trans-Golgi network (TGN) and the plasma membrane, respectively. Whereas AP-1 is required for trafficking to plasma membrane and vacuoles, AP-2 mediates endocytosis. These AP complexes consist of four subunits (adaptins) two large subunits (ß1 and γ for AP-1 and ß2 and α for AP-2), a medium subunit µ, and a small subunit σ. In general, adaptins are unique to each AP complex, with the exception of ß subunits that are shared by AP-1 and AP-2 in some invertebrates. Here, we show that the two putative Arabidopsis thaliana AP1/2ß adaptins co-assemble with both AP-1 and AP-2 subunits and regulate exocytosis and endocytosis in root cells, consistent with their dual localization at the TGN and plasma membrane. Deletion of both ß adaptins is lethal in plants. We identified a critical role of ß adaptins in pollen wall formation and reproduction, involving the regulation of membrane trafficking in the tapetum and pollen germination. In tapetal cells, ß adaptins localize almost exclusively to the TGN and mediate exocytosis of the plasma membrane transporters such as ATP-binding cassette (ABC)G9 and ABCG16. This study highlights the essential role of AP1/2ß adaptins in plants and their specialized roles in specific cell types.