Novel bacterial control agent tolprocarb enhances systemic acquired resistance in Arabidopsis and rice as a second mode of action

The fungicide tolprocarb (TPC) is a melanin biosynthesis inhibitor, but it may also have another mode of action. Here in tests of TPC for inducing plant systemic acquired resistance (SAR), TPC induced promoter activity of the tobacco pathogenesis-related gene PR-1a in Arabidopsis thaliana and genes for PBZ1, β-1,3-glucanase, and chitinase 1 in the defense-related salicylic acid (SA) signaling pathway in rice, but not genes for the jasmonate signaling pathway. Probenazole (PBZ), a commercially used plant defense activator, induced genes in both signaling pathways. The antibacterial activity of TPC was equivalent to that of PBZ. Irrigation with 200 μM TPC prevented growth by Pseudomonas syringae pv. maculicola in A. thaliana, and 30 μM TPC inhibited Xanthomonas oryzae pv. oryzae growth in rice. The results of this study suggest that TPC functions not only as a melanin biosynthesis inhibitor but also as an SAR inducer and is applicable as a novel bacterial control agent that induces SAR activity in both A. thaliana and rice.