New ideas on root hair growth appear from the flanks

Abstract

Plant 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 … *E-mail: m.r.knight{at}durham.ac.uk