Starting with the rhd2 Arabidopsis mutant, which has stunted root growth and very short root hairs, the authors used transposon-tagging methods to identify the RHD2/AtrbohC gene, which encodes an NADPH oxidase that produces ROS. “When we cloned this gene, we were completely dumbfounded,” says Dolan. But at around the same time, other groups were finding that plants use ROS as second messengers in response to pathogen invasion and to control the movement of guard cells to open and close stomata. What is unusual about the new work is that both the activity and the protein controlling it have been identified, whereas in the other systems, the proteins that produce the ROS are not yet known.
In rhd2 mutant plants, ROS production is decreased by 50%, there is little accumulation of intracellular calcium in the root hair tip, and little growth occurs. Upon addition of exogenous ROS to the system, however, the calcium concentration rises and root hair elongation occurs, indicating that an increase in ROS concentration causes the rise in calcium. What is not yet clear is how this occurs. Whether the effect is direct or indirect “is a complete black box as yet,” says Dolan. The group is already looking for genetic suppressors of the rhd2 mutant, which will identify other components in the pathway, including the calcium channel itself.
Plants use two types of cell growth: tip growth, which occurs in the root hair, and diffuse expansion, which occurs in the root itself (and in most other cells in the plant). Dolan thinks that one of the important implications of this study is that ROS appear to be required for the both diffuse growth and tip growth, implying that the systems share some underlying mechanisms, a situation that was not previously apparent. ▪