Plant development: Channelling elongation

signaling update home

SOURCE:

New research has pinpointed the production of reactive oxygen species (ROS) by an NAPDH oxidase in the activation of Ca²⁺ channels in elongating root cells.

Everybody knows that to grow, plants need minerals and water from the soil, which they obtain through roots and root hairs. The formation of these structures requires cell expansion — by way of elongation — which, in turn, needs calcium (Ca²⁺) acquisition. But, until now, what regulated the Ca²⁺ influx wasn't so obvious. Research led by Liam Dolan's group, though, has pinpointed the production of reactive oxygen species (ROS) by an NAPDH oxidase in the activation of Ca²⁺ channels in elongating root cells.

Because Arabidopsis thaliana rhd2 mutants develop very short root hairs and stunted roots, and are defective in Ca²⁺ uptake, the authors decided to clone the gene encoding RHD2. They found that the gene — At5g51060 — had previously been defined as Arabidopsis thaliana respiratory burst oxidase homolog C (AtrbohC). Rather unsurprisingly, as implied by the name, the AtrbohC protein and other Atrbohs are homologous to the gp91^phox subunit of the mammalian NAPDH oxidase that catalyses ROS production.

What, then, is the connection between RHD2/AtrbohC and growth? ROS production was reduced by $sim$ 50% in root apices from rhd2 mutants compared with wild-type apices. Normally, ROS are present as the root hair emerges as a bulge and further increase as the elongation rate goes up. Adding an inhibitor of NADPH oxidase to the apices of wild-type plants prevented ROS accumulation, the elongation of root-hair bulges and the extension rate of the primary root, thereby phenocopying the rhd2 mutant.

The authors then tried the opposite approach. Could ROS applied to rhd2-mutant root-hair bulges induce root-hair growth? Indeed it could. Application of the most reactive ROS, hydroxyl radicals (OH), to rhd2-mutant root-hair bulges restored root-hair growth, although the growth lacked the polarity found in wild-type hairs. Moreover, this was coincident with a rapid increase in the cytoplasmic levels of Ca²⁺ ([Ca²⁺]_c), which was blocked in the presence of 0.1 mM Gd³⁺, a Ca²⁺-channel antagonist.

These data implicated ROS in the increase of [Ca²⁺]_c by Ca²⁺ influx, so the next step was to see if plasma-membrane Ca²⁺ channels could be activated by ROS. Within a few minutes of OH dot treatment, a Ca²⁺-permeable, inwardly rectifying, hyperpolarization-activated conductance was detected in protoplasts from the elongation zone epidermis. This was again blocked by 0.1 mM Gd³⁺, which also decreased the root elongation rate, as did a Ca²⁺ chelator. Because rhd2 mutants and wild-type cells didn't differ significantly in their current amplitudes, the rhd2 mutation seems not to affect the ROS-mediated channel sensitivity or the number of channel proteins. In root-hair apical spheroplasts, OH dot activated a Ca²⁺-, Ba²⁺- and TEA⁺-permeable, inwardly rectifying, hyperpolarization-activated conductance.

So in protoplasts from the elongation zone epidermis and apical spheroplasts, ROS is involved in cell elongation by activating Ca²⁺ channels. The influx of Ca²⁺ is likely to modulate actin dynamics and other growth processes, and this mechanism could well extend to all plant cells. As the mammalian gp91^phox is regulated by Rac, the authors propose that RHD2/AtrbohC could be similarly controlled by Rac-like proteins in plants — ROPs.

Katrin Bussell

References

Foreman, J. et al. Reactive oxygen species produced by NADPH oxidase regulate plant cell growth. Nature 422, 442–446 (2003). | Article |

Links

WEB SITE

Liam Dolan's laboratory