Risk assessment and resistance mechanism of Pseudomonas syringae pv. actinidiae to streptothricins.

BACKGROUND

Streptothricins (STs) are microbial-derived compounds effective against Pseudomonas syringae pv. actinidiae (Psa), the causal agent of kiwifruit bacterial canker. However, the risk of resistance development and underlying mechanisms in Psa remain unclear. This study aims to establish baseline sensitivity of Psa to STs, characterize resistance-associated phenotypes, and identify molecular resistance mechanisms to inform sustainable STs use in agriculture.

RESULTS

Sensitivity testing of 186 Psa isolates from Chinese kiwifruit-producing regions revealed no naturally resistant strains, with a mean baseline half-maximal effective concentration (EC50) of 1.000 ± 0.240 μg mL-1 for STs. Laboratory-induced STs-resistant mutants (R-Psa) exhibited significant fitness costs, slower growth rates, impaired motility, and complete loss of virulence in planta, with no leaf necrosis observed in inoculated kiwifruit. Cross-resistance assays showed no or low cross-resistance to streptomycin, tetramycin, copper hydroxide, and kasugamycin. Molecular analyses identified a GNAT-family acetyltransferase, P-SatA, as the key resistance determinant: overexpression of P-SatA reduced STs sensitivity by 100-fold, while gene deletion restored wild-type sensitivity. In vitro validation confirmed P-SatA inactivates STs via acetylation, abolishing antibacterial activity.

CONCLUSION

This study establishes that STs resistance in Psa is mediated by P-SatA-dependent acetylation, with resistant strains incurring substantial fitness penalties that limit their environmental competitiveness. The absence of natural resistance and low cross-resistance to other pesticides highlight STs as a sustainable option for kiwifruit canker management. These findings advance our understanding of metabolic resistance mechanisms in plant pathogens and provide a scientific basis for optimizing STs application to mitigate resistance risks. © 2025 Society of Chemical Industry.