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.