Investigation of molecular mechanisms controlling root growth in rice plants

In this world, food supply insufficiency has been one of the major challenges. Rice is a main crop but it’s growth and yields are severely affected by various stresses. Roots are essential organs during plant growth and responding to different stimuli. Hence, understanding growth mechanisms in rice root is crucial, especially when responding to stresses, which will be beneficial for maintaining high level of rice yields and global food security. In my PhD project, we mainly worked on defining genetic and molecular mechanisms regulating the growth and developmental process of root in rice plants (Oryza sativa, L.), In particular, this project aimed to assess whether the Arabidopsis models (A and B) that explain how RAM is established by cytokinin and ROS respectively, exist in rice. This work allowed for the first time the identification of the Transition Zone (TZ) in rice RAM and showed that TZ positioning and root length were affected by cytokinin and ROS, thus suggesting the validity of the two models also in rice. Through a candidate gene approach we found genes that are putative homologs of genes involved in TZ position in Arabidopsis, for instance genes of the ARR/PIN/GH3 module A. ORR21, ORR22, ORR23, OsPIN1b, OsPIN5b, OsGH3.4 and OsGH3.11 are the best candidates to play a role in the rice ORR/PIN/GH3 module. The analysis of the H2O2 and O2- gradients along the rice root tip was also determined in this work. The results suggest that in rice the size of the RAM can be controlled by the ROS gradient as in Arabidopsis model B. In rice roots, in fact, O2- was mainly present near the root tip (division zone) and H2O2 accumulated mainly far from the root tip (maturation zone). More importantly, endogenous H2O2 distribution in rice root was affected by cytokinin. This strongly support the hypothesis of the existence of an interplay between the two models in rice. Further analysis will focus on in vivo imaging of H2O2 signalling thanks to the use of transgenic rice lines harboring the roGFP2-Orp1 probe, that have been obtained in this PhD project. In conclusion, our work showed some suggestions about molecular mechanisms regulating rice root development and pose a basis of further works aimed to obtain new rice varieties with an improved root architecture and resilience to a changing environment.