Polarized growth: maintaining focus on the tip.

Tip growth, a spatially focused cell expansion, has been best characterized in two plant cell types: pollen tubes and root hairs. It has long been established that both cell types require three intracellular components for this process: a tip-high calcium gradient, a polarized actin cytoskeleton, and tip-directed vesicle trafficking. More recently, additional mechanistic parallels have been observed between the two cell types, including roles for ROP and Rab GTPase signaling, phosphoinositides, calcium-dependent protein kinases, and the exocyst. Uncovering pathways that control the three components is beginning to reveal a highly interconnected network, which we call the tip growth LENS (for localization enhancing network, self-sustaining), that coordinates the required cellular activities to allow regulated tip growth, and to maintain itself as the tip advances.