Leaf bleaching is associated with extensive transcriptional reprogramming in avocado trees with sunblotch disease

Avocado sunblotch viroid (ASBVd), the type member of the family Avsunviroidae, causes avocado sunblotch disease; marked by the appearance of distinct chlorotic symptoms on fruit, stems and/or leaves of infected trees. Despite the economic impact of the disease, limited information is available regarding the molecular interactions underlying ASBVd infection of avocado. However, studies of other host-viroid pathosystems have revealed large-scale changes to plant gene expression in response to viroid infection. To elucidate molecular changes associated with sunblotch symptom development, we used next-generation sequencing to investigate the global gene expression differences associated with leaf bleaching symptoms in avocado. A total of 3169 genes were differentially expressed in yellow (chlorotic) tissues of bleached leaves compared to asymptomatic leaves from the same ASBVd-infected trees, with most of these genes being upregulated in chlorotic samples. Grouping of genes by predicted function identified substantial alterations to expression of genes involved in genetic information processing, host immune responses, and photosynthesis. Notably, downregulation of chloroplast-associated genes was linked to the manifestation of leaf bleaching. This is the first study to report on global transcriptome changes associated with a symptomatic avsunviroid infection, and provides new insights into the molecular mechanisms underpinning ASBVd-induced chlorosis.