Resumen de Clues to the function of AWR effector proteins by expression on heterologous systems
Crina Mihaela Popa
The soil-borne plant pathogen Ralstonia solanacearum is the causing agent of bacterial wilt, a devastating disease of wide geographical distribution and host range with enormous economic impact worldwide. To cause disease, R. solanacearum injects a suite of type III effector (T3E) proteins into its hosts, from which only a few have been assigned a function, as it is the case for the T3E repertoires of other bacterial pathogens. Using yeast as a model system, we show that expression of the R. solanacearum awr type III effector family caused growth inhibition to different extents, and AWR5 displayed the most dramatic effect on yeast cells. Production of the full-length AWR5 protein in yeast targeted a cellular process leading to a growth inhibition and reduced cell size, but not involving an evident cell cycle arrest or cell death. Furthermore, we demonstrate that AWR5 is an inhibitor of TOR (target of rapamycin), a central regulator in eukaryotes that controls the switch between cell growth and stress responses in response to nutrient availability. Heterologous expression of awr5 in yeast caused autophagy induction coupled to massive transcriptomic changes, unmistakably reminiscent of TOR inhibition by rapamycin or by nitrogen starvation. The observation that deletion of two components of the PP2A heterotrimeric forms -CDC55 and TPD3- and mutation in SCH9 abolished the dramatic growth defect of cells expressing awr5 indicates AWR5 might exert its function by directly or indirectly inhibiting the TOR pathway upstream PP2A and Sch9. We speculate targeting of TOR pathways might be conserved among AWR2, AWR4 and AWR5, since their expression in yeast leads to similar downstream transcriptional responses. Also, we present evidence in planta that AWR5 T3E caused a reduction in TOR-regulated plant nitrate reductase activity and that the bacterial growth inhibition caused by delivery of AWR5 into host cells was mediated by TOR. Our work demonstrates that yeast is a powerful model for T3E function discovery and reveals not only a new mode of action for T3Es but also a new virulence target that might be conserved across kingdoms.
