Systematic survey of the function of ROP regulators and effectors during tip growth in the moss P. patens.

Rho/Rac of Plants (ROP) GTPases are plant-specific small GTPases that regulate cell morphology. ROP activity is controlled and read by several families of regulatory proteins. However, how these diverse regulators contribute to polarized growth remains understudied. In a system-wide approach, we used RNAi to silence each gene family of known ROP regulators in the juvenile tip growing tissues of the moss Physcomitrella patens. We found that the GTPase Activating Proteins (GAPs), but not the ROP Enhancers (RENs), are essential for tip growth. The Guanine Exchange Factors (GEFs), which are comprised of ROPGEFs and Spikes, both contribute to growth. However, silencing Spikes results in less polarized plants as compared to silencing ROPGEFs, suggesting that Spikes contribute more to establishing cell polarity. Silencing the single gene family of Guanine Dissociation Inhibitors (GDIs) also inhibits growth, resulting in small, unpolarized plants. In contrast, silencing the ROP effector RIC, which is encoded by a single gene, results in plants larger than controls, suggesting that RIC functions to inhibit tip growth in moss. Taken together, this systematic loss-of-function survey provides insights into the function of ROP regulators during polarized growth.