The resistance of sour orange to Citrus tristeza virus is mediated by both the salycilic acid and the RNA silencing defense pathways

Citrus tristeza virus (CTV), induces in the field decline and death of citrus varieties grafted on sour orange (SO) rootstock, which has forced the use of alternative decline-tolerant rootstocks in affected countries, despite the highly desirable agronomic features of the SO rootstock. Declining citrus plants display phloem necrosis below the bud union. Also, sour orange is minimally susceptible to CTV compared to other citrus varieties, suggesting partial resistance of SO to CTV. Here, by silencing different citrus genes with a citrus leaf blotch virus-based vector, we have examined the implication of the RNA silencing and salicylic acid (SA) defense pathways in the resistance of SO to CTV. Silencing of genes RDR1, NPR1 or DCL2-DCL4, associated with these defense pathways, enhanced virus spread and accumulation in SO plants in comparison with non-silenced controls, whereas silencing genes NPR3-NPR4, associated with the hypersensitive response, produced a slight decrease of CTV accumulation and reduced stunting of SO grafted on CTV-infected rough lemon plants. We have additionally found that the CTV RNA silencing suppressors p20 and p23 suppress also the SA signalling defense, with the suppressor activity being higher in the most virulent isolates. This article is protected by copyright. All rights reserved.