Phytophthora cinnamomi is a plant pathogenic oomycete that causes Phytophthora root rot of avocado (PRR). Currently, there is a limited understanding of the molecular interactions underlying this disease. Other Phytophthora species employ an arsenal of effector proteins to manipulate host physiology, of which the RxLR effectors contribute to virulence by interfering with host immune responses. The aim of this study was to identify candidate RxLR effectors in P. cinnamomi that play a role in establishing PRR, and to infer possible functions for these effectors. We identified 61 candidate RxLR genes which were expressed during infection of a susceptible avocado rootstock. Several of these genes were present in multiple copies in the P. cinnamomi genome, suggesting that they may contribute to pathogen fitness. Phylogenetic analysis of the manually predicted RxLR protein sequences revealed 12 P. cinnamomi RxLRs that were related to characterised effectors in other Phytophthora spp., providing clues to their functions in planta. Expression profiles of nine more RxLRs point to possible virulence roles in avocado–highlighting a way forward for studies of this interaction. This study represents the first investigation of the expression of P. cinnamomi RxLR genes during the course of avocado infection, and puts forward a pipeline to pinpoint effector genes with potential as virulence determinants, providing a foundation for the future functional characterization of RxLRs that contribute to P. cinnamomi virulence in avocado.

Viroids are the smallest plant pathogens, consisting of a single-stranded circular RNA of less than 500 ribonucleotides in length. Despite their noncoding nature, viroids elicit disease symptoms in many economically important plant hosts, and are, thus, a class of pathogens of great interest. How these viroids establish disease within host plants, however, is not yet fully understood. Recent transcriptomic studies have revealed that viroid infection influences the expression of genes in several pathways and processes in plants, including defence responses, phytohormone signalling, cell wall modification, photosynthesis, secondary metabolism, transport, gene expression and protein modification. There is much debate about whether affected pathways signify a plant response to viroid infection, or are associated with the appearance of disease symptoms in these interactions. In this review, we consolidate the findings of viroid–host transcriptome studies to provide an overview of trends observed in the data. When considered together, changes in the gene expression of different hosts upon viroid infection reveal commonalities and differences in diverse interactions. Here, we discuss whether trends in host gene expression can be correlated to plant defence or disease development during viroid infection, and highlight avenues for future research in this field.