Villin Controls the Formation and Enlargement of Punctate Actin Foci in Pollen Tubes

Self-incompatibility (SI) in Papaver rhoeas triggers dramatic actin alterations in pollen. However, how actin alterations in SI pollen tubes are mechanistically achieved remains largely unexplored. Here we have used treatment with the calcium ionophore A23187 to mimic the SI-induced elevation in cytosolic Ca2+ and trigger the formation of the distinctive F-actin foci. Live-cell imaging reveals that this remodeling involves F-actin fragmentation and depolymerization, accompanied by the rapid formation of punctate actin foci and subsequent increase in their size. We establish that actin foci are generated and enlarged from crosslinking of fragmented actin filament structures. Moreover, we show that villins associate with actin structures and are involved in this actin reorganization process. Notably, we demonstrate that Arabidopsis villin5 promotes actin depolymerization and formation of actin foci by fragmenting actin filaments, and controlling the enlargement of actin foci via bundling actin filaments. Our study thus uncovers important, novel insights about the molecular players and mechanisms involved in forming the distinctive actin foci in pollen tubes.