New ideas on root hair growth appear from the flanks.

P lant root hairs are experimental model systems for research on plant cell growth. The involvement of protons and reactive oxygen species (ROS) in root hair growth and development has been the subject of significant research effort (e.g., refs. 1–4). pH specifically has been implicated in the control of plant cell expansion (5), including that in root hair cells (6). ROS have recently come to the fore with the report that a specific respiratory burst oxidase homologue (RBOHC) of Arabidopsis is necessary for the establishment of the tip-based calcium gradient known to be essential for root hair polar growth (1, 7–11). In this issue of PNAS, Monshausen et al. (12) take great strides in furthering understanding of the specific roles played by both protons and ROS in root hair growth in Arabidopsis. Monshausen et al. (12) report that root hair growth is not a constant process; instead it occurs as discrete episodes of growth followed by stasis. By measuring the external pH around the surface of the root hair apex, the researchers demonstrated that each period of growth correlated with alkalization of external pH. This external alkalization coincided with internal cellular acidification, suggesting an influx of protons into the root hair cell. Because this influx was coincidental with the growth phase, the authors advance the hypothesis that acidification of the apoplast leads to the capacity for polar growth during the elongation phase. This growth occurs through the loosening of cell wall components in specific zones of the growing root hair, i.e., just behind the root hair apex. In this way, root hair growth can be limited to only the tip, with the majority of the cell wall at the flanks of the root hair reinforced to prevent expansion. Consistent with this idea was the demonstration that an artificially generated elevation of external pH led to cessation of growth (caused by global rigidification of the cell wall), whereas reduction in pH led to bursting of hair cells (caused by nonlocalized/unregulated cell expansion). These data are consistent with previous work on the pH regulation of cell wall properties (5, 6). The spatiotemporal dissection of dynamic growth and internal/external pH changes are elegantly demonstrated (12) and represent important advances in thinking about root hair growth. Monshausen et al. (12) also reveal a new role for ROS in root hair growth. Here, a situation analogous to that with pH was demonstrated. In this case, oscillations of apoplastic ROS concentration at the top of the flanks of the root hair (not at the apex itself) were detected. The troughs of ROS oscillations were correlated to the growth phases of the root hair. Again, artificial manipulation of ROS concentration revealed the mechanism and causality of ROS action. Increasing or reducing apoplastic ROS levels led to inhibition of root hair growth or bursting, respectively (12). These data are consistent with the reported role for ROS in regulating cell wall extensibility (13, 14). The authors naturally decided to investigate the role of RBOHC in these processes, because it had been previously implicated in ROS-dependent root hair growth (1, 4, 11). They found that, as reported previously, rbohc mutants showed reduced numbers of elongated root hairs under standard conditions (1, 4, 7, 10, 11). Some previous reports suggest that root hair development in rbohc is arrested after initiation (1, 7, 11), but others report that rbohc root hairs burst after initiation (4, 12). Monshausen et al. advance the hypothesis that the bursting of rbohc root hairs that they observed is due to reduced apoplastic ROS levels, in turn leading to uncontrolled/unfocused cell expansion. These observations also lead to a critically important conclusion: