Disruption of chloroplast function through downregulation of phytoene desaturase enhances the systemic accumulation of an aphid-borne, phloem-restricted virus

Authors: Deblasio, Stacy; REBELO, ANA RITA - Boyce Thompson Institute; PARKS, KATHERINE - Boyce Thompson Institute; Gray, Stewart; Heck, Michelle

Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/11/2018
Publication Date: 5/16/2018
Citation: DeBlasio, S.L., Rebelo, A., Parks, K., Gray, S.M., Heck, M.L. 2018. Disruption of chloroplast function through downregulation of phytoene desaturase enhances the systemic accumulation of an aphid-borne, phloem-restricted virus. Molecular Plant-Microbe Interactions. https://doi.org/10.1094/MPMI-03-18-0057-R.
DOI: https://doi.org/10.1094/MPMI-03-18-0057-R

Interpretive Summary: In addition to converting light into chemical energy, plant chloroplasts produce molecules that are active in the defense against various types of plant pathogens including viruses. When the plant is attacked by pathogens the chloroplasts will deliver these defense related compounds to other organelles in the plant by generating tunnel-like projections through which these molecules move. In this study we silenced or turned off the production of phytoene desaturase, a key enzyme regulating chloroplast development and function, to assess the role chloroplasts play in defending the plant against infection by Potato leafroll virus (PLRV) and also against delivery of the virus by its aphid (insect) vector. Impairing the development of chloroplasts and the production of the phytohormone gibberellic acid, enhanced the accumulation of PLRV and reduced the typical symptoms of virus infection; leaf yellowing and plant dwarfism. Disease symptoms of PLRV infection are most likely a consequence of deliberate deactivation of these pathways by the virus to increase its own propagation within susceptible plant hosts and not merely an indirect result of competition for host resources. Furthermore, alterations of chloroplast development and function in these plants increased the number of aphids that developed on the plant relative to a normal plant, suggesting that the virus and insect vector manipulate overlapping plant biochemical pathways that may be used to coordinately benefit the transmission of the virus by insects. This is the first report to show that chloroplast function negatively regulates disease progression and positively regulates aphid reproduction.

Technical Abstract: Chloroplasts lie at the interface of plant defense and pathogen infection. Viruses in the Luteoviridae are aphid-transmitted, single stranded, positive sense RNA viruses that replicate in the cytoplasm of phloem companion and parenchyma cells. Previously, our lab has shown that Potato leafroll virus (PLRV) forms extensive interaction networks with chloroplast-localized proteins and membranes, some of which are specifically mediated by the PLRV readthrough domain (RTD), a structural component of the viral capsid involved in symptom development, systemic virus movement, phloem limitation and aphid transmission. Here, we used virus-induced gene silencing to disrupt chloroplast function and assess the effects on the systemic accumulation of PLRV. Silencing of phytoene desaturase (PDS), a key enzyme in carotenoid, chlorophyll and gibberellic acid (GA) biosynthesis, resulted in a dramatic and sustained increase in the systemic accumulation of wild-type and 'RTD virions and viral RNA in Nicotiana benthamiana. A similar trend could be observed in Solanum sarrachoides, a natural host of PLRV. Comparison of virus levels in PDS-silenced sectors and non-silenced sectors within the same leaf demonstrated that the effect on PLRV was cell autonomous. Addition of exogenous GA3 could partially suppress the effects of PDS-silencing on PLRV accumulation, suggesting that GA signaling downstream of PDS plays a role in limiting PLRV infection. Although the increase in PLRV levels had no effect on the transmission of virus by aphids, silencing of PDS significantly increased the fecundity of N. benthamiana-reared Myzus persicae. To our knowledge, this is the first report to show that chloroplast function negatively regulates the systemic infection of a phloem-limited pathogen.