Plant Immune System Activation Upon Citrus Leprosis Virus C Infection Is Mimicked by the Ectopic Expression of the P61 Viral Protein

Abstract

Citrus leprosis virus C (CiLV-C, genus Cilevirus , family Kitaviridae ) is an atypical virus that does not spread systemically in its plant hosts. Upon its inoculation by Brevipalpus mites, only localized lesions occur, and the infection remains limited to cells around mite feeding sites. Here, we aimed to gain insights into the putative causes of viral unfitness in plants by expanding the limited knowledge of the molecular mechanisms underlying plant/kitavirid interactions. Firstly, we quantified the CiLV-C viral RNAs during the infection in Arabidopsis thaliana plants using RT-qPCR and systematized it by defining three stages of distinguishing subgenomic and genomic RNA accumulation: i) 0–24 h after infestation, ii) 2–4 days after infestation (dai), and iii) 6–10 dai. Accordingly, the global plant response to CiLV-C infection was assessed by RNA-Seq at each period. Results indicated a progressive reprogramming of the plant transcriptome in parallel to the increasing viral loads. Gene ontology enrichment analysis revealed the induction of cell growth-related processes at the early stages of the infection and the triggering of the SA-mediated pathway, ROS burst and hypersensitive response (HR) at the presymptomatic stage. Conversely, infected plants downregulated JA/ET-mediated pathways and processes involved in the primary metabolism including photosynthesis. Marker genes of unfolded protein response were also induced, suggesting a contribution of the endoplasmic reticulum stress to the cell death caused by the viral infection. Finally, we transiently expressed CiLV-C proteins in Nicotiana benthamiana plants to undertake their roles in the elicited plant responses. Expression of the CiLV-C P61 protein consistently triggered ROS burst, upregulated SA- and HR-related genes, increased SA levels, reduced JA levels, and caused cell death. Mimicry of responses typically observed during CiLV-C–plant interaction indicates P61 as a putative viral effector causing the HR-like symptoms associated with the infection. Our data strengthen the hypothesis that symptoms of CiLV-C infection might be the outcome of a hypersensitive-like response during an incompatible interaction. Consequently, the locally restricted infection of CiLV-C, commonly observed across infections by kitavirids, supports the thesis that these viruses, likely arising from an ancestral arthropod-infecting virus, are unable to fully circumvent plant defenses.

Histopathological Patterns of Lesions Caused by Two Dichorhavirus

“…Detection of hydrogen peroxide within the chlorotic region of lesions is an important information that can shed light on the possible signaling effect of defense mechanisms of infected tissues in the chlorotic region. Recently, it was described that CiLV-C infection triggers ROS burst and cell death (ARENA et al, 2020). Based on our observations, we suggest that the ROS in these two examined Dichorhavirus cases may have a different role, as inducing the cell hypertrophy in spongy parenchyma in the chlorotic halo.…”

Molecular Epidemiology of Citrus Leprosis Virus C: A New Viral Lineage and Phylodynamic of the Main Viral Subpopulations in the Americas

“…This virus can be found naturally infecting more than twenty plant species and, notably, symptoms are not lesions as locally restricted as those observed in citrus plants infected by CiLV-C. Since the interaction of CiLV-C with citrus plants is dominated by a hypersensitive-like response ( Arena et al, 2016 , 2020 ), the expansion of the host range to Citrus spp. might have resulted in CiLV-C fitness reduction.…”

“…might have resulted in CiLV-C fitness reduction. Alternatively, based on the low variability of viruses inside each CiLV-C clade, a thought-provoking question is whether CiLV-C can be considered a specialist virus, whose interaction with citrus is carefully selected to act as a helper (effector-like) factor of the mite infestation to suppress the plant defenses ( Arena et al, 2016 , 2018 , 2020 ).…”

“…CiLV-C does not systemically infect its host plants, it only causes local chlorotic and/or necrotic lesions in leaves, fruits, and branches ( Figure 1 ), which may result from an incompatible interaction led by a hypersensitivity-like response ( Arena et al, 2016 , 2020 ). The viral spread, even to different points in an infected plant, is exclusively mediated by viruliferous mites of, mainly, the species Brevipalpus yothersi ( Ramos-González et al, 2016 ).…”

Poorly Conserved P15 Proteins of Cileviruses Retain Elements of Common Ancestry and Putative Functionality: A Theoretical Assessment on the Evolution of Cilevirus Genomes

“…Kitaviridae, order Martellivirales, is a family of heterogeneous plant-infecting viruses displaying two, three, or four segments of single-stranded positive-sense RNA molecules as genomes, which have been assigned into the genera Cilevirus, Higrevirus, and Blunervirus, respectively (Quito-Avila et al, 2021). Taken together, kitavirids have been detected in a relatively narrow range of natural hosts, where they normally produce local infections mainly characterized by chlorotic and/or necrotic lesions that in some cases resemble the outcome of a hypersensitive-like response (Arena et al, 2016(Arena et al, , 2020. However, regardless of their failure to consummate the systemic movement through the plants, kitavirids pose serious threats to major crops such as citrus (Ramos- .…”