Exogenous RNAi to control rust fungi

Abstract

Exogenous RNA interference (RNAi) is an effective non-genetically modified (GM) control strategy for plant viruses, insect pests, fungal and oomycete pathogens. It involves the exogenous application of doublestranded RNA (dsRNA) to plants to trigger the silencing of targeted pest/pathogen genes. We have been exploring exogenous RNAi as a sustainable control for rust fungi, one the largest groups of plant pathogens, across natural and agricultural systems. A particular focus has been on Austropuccinia psidii, the causal agent of myrtle rust, an aggressive disease of Myrtaceae plants. A. psidii arrived in Australia in 2010 and poses a serious threat to native plant biodiversity. The pathogen has a large host range of >350 species and serious species decline is already underway. Myrtle rust is not only destroying natural ecosystems but is also impacting native plant industries. We synthesised dsRNA molecules targeting rust fungi genes and tested their fungicidal effects in vitro and in planta. We found that urediniospores can take up dsRNA during the early stages of germination and that dsRNA targeting essential rust genes significantly reduced germination and development of infection structures such as appressoria and infection pegs. Exogenous RNAi was also effective in planta, significantly reducing myrtle rust symptoms on one-year-old trees. Our comparative genomics study showed that genes in the RNAi pathway are conserved across eight families of rust fungi, indicating that exogenous RNAi has potential for broad-scale management of rust fungi.