Modulation of viral infection in plants by exogenous guanosine

To evaluate the role of purines in antiviral treatments in plants, ribavirin (RB) and tiazofurin (TZ) were applied in combination with guanosine (GS) or adenosine (AS) in in vitro grapevine or tobacco explants infected by Grapevine leafroll associated virus 3 (GLRaV-3) and Cucumber mosaic virus (CMV), respectively. Using a microelectrochemical (trans-plasma membrane electron transport, t-PMET) technique, in vivo assay of free reduced nicotinamide adenine dinucleotide (NADH) was also carried out to estimate the inosine monophosphate dehydrogenase inhibition caused by drugs. Antiviral effectiveness of TZ, evaluated as virus-free explants or virus copies, was significantly hindered by GS in both species, while AS did not interfere with the drugs. GS, but not AS, slightly hindered the antiviral effectiveness of RB. With regard to NADH tests, t-PMET inhibition caused by RB and TZ was dose dependent and the interference of drugs with the NAD+/NADH conversion was confirmed by NADH content. Findings indicate that exogenous GS up to 0.50 mM replenished the GS pool depleted by drugs, contrasting antiviral action. At higher doses of GS, the TZ antiviral action was completely inhibited and exogenous GS caused a feedback that reduced t-PMET activity. The reversal was partially against RB, suggesting that the reduction of the GS pool contributed to the antiviral activity of RB, but it was not the only cause of antiviral effectiveness.