Phytophthora resistant transgenic avocado rootstocks for non GM fruit

Phytophthora root rot is the most serious and important disease of avocado worldwide. The causal agent, Phytophthora cinnamomi, has over 1,000 hosts and is a primary constraint on avocado productivity. We are employing the innovative strategy of RNA silencing to engineer avocado rootstocks for resistance to Phytophthora root rot. RNA silencing is a conserved eukaryotic surveillance mechanism with similar pathways in plants, animals and fungi. A key conserved feature is that it is triggered by double-stranded RNA (dsRNA) that is processed into small interfering (si)RNAs which can degrade RNA sharing a complete homology. It can provide high-level pathogen resistance by specific targeting of essential pathogen genes via siRNAs taken up from transgenic plants.

We have established the proof of concept for the technology to impart resistance to P.cinnamomi using model species Arabidopsis thaliana. The dsRNA constructs targeting essential genes in Phytophthora cinnamomi have been selected for silencing efficiency by soaking the fungal mycelium in dsRNA and selecting for lethal phenotype. Transgenic Arabidopsis thaliana plants generated with selected dsRNA constructs show resistance to P. cinnamomi. The average root growth for plants carrying the RNA silencing based resistance to the fungus was more than double that of wild type control plants when grown in soil heavily infested with P.cinnamomi. The presence of P.cinnamomi specific siRNAs in transgenic A. thaliana plants has been confirmed by RNA blot analysis.

Somatic embryo production form various rootstocks of Avocado has been optimised. The efficiency of regeneration of the somatic embryos has been improved from 5% to 40%. Transformation of avocado somatic embryos of selected rootstocks with these dsRNA constructs is underway. The genetic modification  for imparting resistance will be limited to the rootstock, therefore the grafted scion will remain non-transgenic and produce GM free fruit.