The BEACH Domain-Containing Protein SPIRRIG Modulates Actin-Dependent Root Hair Development in Coordination with the WAVE/SCAR and ARP2/3 Complexes

Root hairs are single cell protrusions that enable roots to optimize nutrient and water acquisition. They attain their tubular shapes by confining growth to the cell apex, a process called tip growth. The actin cytoskeleton and endomembrane systems are essential for tip growth; however, little is known about how these cellular components coordinate their activities during this process. Here, we show that SPIRRIG (SPI), a BEACH domain-containing protein involved in membrane trafficking, and BRK1 and SCAR2, subunits of the WAVE/SCAR (W/SC) actin nucleating promoting complex, display polarized localizations to root hairs at distinct developmental stages. SPI accumulates at the root hair apex via post-Golgi vesicles and positively regulates tip growth by maintaining tip-focused vesicle secretion and filamentous-actin integrity. BRK1 and SCAR2 on the other hand, mark the root hair initiation domain to specify the position of root hair emergence. Consistent with the localization data, tip growth was reduced in spi and the position of root hair emergence was disrupted in brk1 and scar1234. BRK1 depletion coincided with SPI accumulation as root hairs transitioned from initiation to tip growth. Taken together, our work uncovers a role for SPI in facilitating actin-dependent root hair development through pathways that might intersect with W/SC.