Data from: Autoinhibitory calcium ATPases regulate the calcium gradient required for rapid polarized growth

Polarized growth drives the morphogenesis of elongated cellular
structures. In plants, polarized growth depends on actin and a tip-focused
ionic calcium gradient. How the calcium gradient is maintained remains
unclear. We discovered that autoinhibitory calcium ATPases (ACAs)
redundantly contribute to the steepness of the calcium gradient. ACA1 and
ACA2 localize to the subapical plasma membrane and ACA5 to the vacuole
membrane, providing spatial regulation of calcium efflux. Tip-growing
plant cells also exhibit apical calcium fluctuations. Even though
Δaca1/2/5 cells have a diminished calcium gradient, they exhibit normal
fluctuations and actin but have significantly reduced apical secretion.
Furthermore, cells lacking apical actin retain a strong calcium gradient
but have reduced apical secretion. Suppression of both the calcium
gradient and apical actin dramatically impairs growth, supporting a model
where two independent and parallel processes, the calcium gradient and
apical actin, promote rapid polarized growth.