Betulinic Acid Delays Turnip Mosaic Virus Infection by Activating the Phytosulfokine Signalling Pathway in Nicotiana benthamiana.

Plant viral diseases pose a significant threat to agricultural production, and the availability of effective drugs against viral diseases remains limited. In this study, we discovered that betulinic acid (BA), a pentacyclic triterpenoid derived from plants, delays infection by turnip mosaic virus (TuMV) in Nicotiana benthamiana. Transcriptomic analysis revealed that BA treatment specifically induced the expression of N. benthamiana phytosulfokine 3 (NbPSK3), a plant pentapeptide hormone with diverse functions, while TuMV infection suppressed its expression. Further study demonstrated that NbPSK3 positively regulates antiviral defence against TuMV infection. Disruption of PSK signalling by targeting the membrane-bound PSK receptors (PSKRs) promoted viral infection. Additionally, exogenous sulphonated PSK (active form) treatment significantly delayed infection by TuMV in N. benthamiana compared to unmodified PSK peptides (dPSK, inactive form) or control treatments, while silencing the receptor NbPSKR1 abolished the ability of PSK to inhibit TuMV infection. Moreover, the inhibition of TuMV infection by BA is dependent on the PSK-PSKR signalling pathway. Overall, these findings not only underscore the potential of BA as a promising and environmentally friendly agent for modulating plant viral diseases but also emphasise the role of the PSK signalling pathway in promoting at least partial resistance to TuMV, which might have interest for crop breeding.