In vivo Reverse Transcription of A Heterologous RNA Virus by Cauliflower Mosaic Virus Reverse Transcriptase During...

Reverse transcriptase (RT) is central to the replication of retroviruses and pararetroviruses. Cauliflower mosaic virus (CaMV), a plant pararetrovirus, replicates via RT using its own viral RNA. Simultaneous infection with two or more viruses in a plant is a common phenomenon that, in addition to affecting the severity of the symptoms, may affect the concentration of infecting viruses. Interactions between viruses in mixed infections can alter disease dynamics.

This study investigated whether CaMV RT can transcribe complementary DNA (cDNA) from the genomic RNA of a co-infecting RNA virus, Cucumber mosaic virus (CMV), in planta, and explored the consequences of this interaction.

Cauliflower plants were singly infected with CaMV or CMV, or co-infected with both. Symptom development was monitored. Total DNA was extracted from co-infected plants and subjected to PCR using CMV-specific primers, without an in vitro reverse transcription step, to probe for CMV-derived cDNA. Control PCRs on DNA from singly CMV-infected plants and on viral RNA were performed. The identity of PCR products was confirmed by sequencing. The impact of CaMV on CMV accumulation was assessed through dsRNA extraction and quantification at 20 and 45 days post-infection (dpi).

CMV-specific cDNA was successfully amplified directly from the total DNA of the co-infected plants, indicating in vivo reverse transcription of CMV RNA by CaMV RT. This direst PCR-amplification was not due to plant endogenous RTs or Taq polymerase activity because PCR assays on total DNA and CMV RNA from plants with single CMV-infection gave no amplification. Concurrently, in the co-infections, CMV-associated symptoms (vein clearing and banding) diminished after ~45 dpi, and a quantitative dsRNA analysis showed a ~55% reduction in CMV genome concentration by 45 dpi compared to that in single infections.

This study provides the first direct evidence of CaMV RT transcribing a heterologous viral RNA (CMV) into cDNA within plant cells. This interviral activity not only elucidates a mechanism behind the suppression of an RNA virus in a mixed infection but also reveals a novel principle that reflects unexpected genetic interactions of the CaMV RT enzyme substrate due to its flexibility in cDNA synthesis from competitor's RNA, opens new avenues for understanding viral interactions in viral ecology.