In vivo imaging uncovers an abundant but rarely active pool of plant ARP2/3 and its link to exocytosis and endocytosis

The ARP2/3 complex generates branched actin networks that regulate membrane dynamics across eukaryotes. In plants, ARP2/3 is activated primarily by the WAVE/SCAR complex and is essential for cell morphogenesis, yet its spatiotemporal behavior in living cells remains poorly understood. Using high-resolution live-cell microscopy, we found that, in addition to their previously reported stable accumulation at three-way cell junctions and peroxisomes, WAVE/SCAR and ARP2/3 subunits also form abundant, highly dynamic, short-lived assemblies in the cortical cytoplasm, with lifetimes of only a few seconds. Genetic and colocalization analyses further revealed that only a minority of observed complexes are fully assembled and active, indicating that plant cells maintain a large pool of partially assembled or inactive ARP2/3 structures. Our data indicate that microtubules influence the abundance of cortical assemblies, whereas actin primarily affects their dynamics. Importantly, our analysis demonstrated a spatial and functional association between dynamic ARP2/3 foci and exocytotic events at the plasma membrane. Together, our findings suggest that the cytoplasm of plant epidermal cells contains a large reservoir of ARP2/3 complexes whose localized activation is tightly regulated and associated with membrane trafficking.