Transcriptomics Advancement in the Complex Response of Plants to Viroid Infection

Abstract

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.

Global transcriptomic analysis in avocado nursery trees reveals differential gene expression during asymptomatic infection by avocado sunblotch viroid (ASBVd)

“…Despite differences in diseases causes by pospiviroids and avsunviroids, the cellular pathways affected in ASBVd-infected avocado were comparable to those observed in other viroid-host interactions. One host pathway where DEGs were commonly observed in viroid-infected hosts was plant immune response ( Joubert et al., 2022 ). In this study we observed the upregulation of 14 PR genes and several different RLK s in infected avocado, suggesting that PTI is induced by ASBVd; this result correlates with the findings of other viroid-host transcriptome studies, which showed that PR genes and RLK s are induced in PSTVd-infected tomato, CEVd-infected ‘Etrog’ citron, and in hop ( Humulus lupulus ) infected with citrus bark-cracking viroid (CBCVd) ( Mishra et al., 2018 ; Wang et al., 2019 ; Więsyk et al., 2018 ).…”

“…5 a, b & c). These TFs have been shown to regulate components of phytohormone networks, plant immunity, and plant response to stress ( Ambawat et al., 2013 ; Dröge-Laser et al., 2018 ; Sun et al., 2018 ; Yuan et al., 2019 ) and were differentially expressed in several viroid-host interactions studied to date ( Joubert et al., 2022 ). We hypothesise that altered accumulation of TFs in ASBVd-infected avocado may be responsible for downstream effects on host gene expression observed in this study, though the initial mechanism influencing TF expression remains to be elucidated.…”

“…While the molecular mechanisms underlying pospiviroid pathogenesis are still being explored, there has been significant progress made in understanding how diseased hosts respond to infection by members of the Pospiviroidae family; largely due to investigations into differential gene expression in affected hosts (reviewed by Joubert et al. (2022) ).…”

“…In recent years, next-generation sequencing (NGS) technology has enabled the global transcriptomic analyses of several different hosts infected with PSTVd ( Chi et al., 2022 ; Góra-Sochacka et al., 2019 ; Hadjieva et al., 2021 ; Więsyk et al., 2020 ), HSVd ( Xia et al., 2017 ; Xu et al., 2020 ), citrus exocortis viroid (CEVd) ( Wang et al., 2019 ), CDVd ( Lavagi-Craddock et al., 2022 ) and dendrobium viroid (DVd) ( Li et al., 2022 ). These studies have shown that several host pathways were commonly altered by pospiviroid infection, including host defence signalling, phytohormone networks, secondary metabolism, transport, transcription, as well as protein synthesis and modification ( Joubert et al., 2022 ).…”