The concentration of PIP2 (yellow) at the tip (left) of root hairs is lost in the AtSfh1p mutant (right).

Tiny hairs on Arabidopsis thaliana roots elongate via tip growth, in which new membrane is added specifically at the root hair tip. Vincent et al. (page 801) now show that a phosphatidylinositol transfer protein (PITP) related to yeast Sec14p is critical for this polarized growth, suggesting that PIP2 may be the start of the polarity cascade in this system.The authors show that Arabidopsis has a large family of these PITPs. One such PITP is AtSfh1p, which, along with its downstream product PIP2, localized to the tip plasma membrane and on post-Golgi vesicles that accumulate at the hair tips.

AtSfh1p mutation disrupted several aspects of polarity normally found in wild-type hairs and culminated in the loss of tip-directed membrane secretion. These lost polarity cues include the tip localization of PIP2, a tip-directed F-actin network, strong tip-localized calcium influx, and the microtubule polymerization that normally follows in the wake of high calcium.

In the authors' model, AtSfh1p on post-Golgi vesicles produces PIP2, which links the vesicles (possibly via interactions with motor proteins) to a tip-directed actin network that can be generated on demand. Once they reach the tip, the vesicles deposit PIP2 in the plasma membrane and thereby reinforce tip-directed actin polymerization. Vesicles may also carry and deposit calcium channels, thus establishing the calcium signals at the tip. One insult to this system, such as the loss of AtSfh1p, would result in a domino effect that kills root hair polarity.

AtSfh1p and many other Arabidopsis PITPs also contain coiled-coil nod domains, which may target the PITPs to distinct subcellular locations. Nitrogen-fixing bacteria express nod domains during nodulation; they might use this trick to subvert AtSfh1p localization and thus polarized membrane secretion while they invade the plant cells.