ROS in root gravitropism: the auxin messengers?

Abstract

The role of auxins in gravitropism was discovered during early molecular research into phytohormones. Further research led to the conclusion that auxin re-distribution is responsible for the gravitropic response in roots. This auxin effect, first proven in monocots was found later in dicots, including Arabidopsis. In a recent article, Jung-Hee Joo and colleagues 1xRole of auxin-induced reactive oxygen species in root gravitropism. Joo, J.-H. et al. Plant Physiol. 2001; 126: 1055–1060Crossref | PubMed | Scopus (282)See all References1 investigated the role that reactive oxygen species (ROS) have in this process in the monocot Zea mays. Their results strongly suggest that ROS are key mediators of the auxin effect in gravitropism.First, Joo et al. show that ROS are produced following the gravitropic stimulus. When maize roots are placed horizontally to stimulate the gravitropic response, the generation of ROS is detected in the apex. Production of ROS is asymmetric and occurs in the lower cortex. The production of ROS is responsible for the gravitropic response because the gravitropic response is inhibited by treatment with ROS scavengers, such as N-acetyl cysteine (NAC). Second, the application of auxin to roots stimulates ROS production in root cells in planta and in root protoplast cultures. Further indication of a directional cause–effect relationship between auxin–ROS and gravitropism comes from roots treated with N-1-naphthylphthalamic acid (NPA, an inhibitor of auxin transport). NPA treatment results in inhibition of gravitropism. Adding H2O2 can reverse the effect of NPA.Joo et al. indicate that ROS might work by activating kinase, but they also take into account that other mediators of the gravitropic response might exist, such as calcium and inositol (1,4,5)-trisphosphate.The generation of ROS has already been identified in several plant processes, in particular, their involvement in defence responses. Recently, calcium-dependent protein kinases have been reported to be important in plant defence responses. In recent years, we have witnessed the convergence of transduction chains from diverse plant hormones towards common points. A question provoked by Joo et al.'s work is: do other plant hormones apart from auxin have ROS as mediators of their action?