Action mechanism of the novel rice blast fungicide tolprocarb distinct from that of conventional melanin biosynthesis inhibitors

Abstract

The target site of tolprocarb, a novel systemic fungicide used for controlling rice blast, was investigated. Tolprocarb decolorized the mycelia of Magnaporthe grisea; the decolorization was reversed by adding scytalone or 1, 3,6,3,6,. This result suggested that the target site of tolprocarb was polyketide synthase (PKS), which regulated polyketide synthesis and pentaketide cyclization in melanin biosynthesis. Further, we produced a transgenic Aspergillus oryzae, which possessed the PKS gene of M. grisea, and performed in vitro assays of PKS using membrane fractions from the transgenic A. oryzae. Compared with some conventional melanin biosynthesis inhibitors (cMBIs), tolprocarb only inhibited PKS activity in vitro. These results indicated that tolprocarb's target protein in M. grisea was PKS, which differentiates this fungicide from other cMBIs.

Synthesis and activities of tolprocarb derivatives against <i>Pyricularia oryzae</i>: relationships among the activities for polyketide synthase, melanin biosynthesis, and rice blast

“…16) Tolprocarb developed by Mitsui Chemicals Agro, Inc., showed a high controlling effect on rice blast by both nursery-box application and broadcasting on paddy rice. 19) In our previous paper, 20) we demonstrated that tolprocarb inhibited PKS activity with an IC 50 =0.03 µM. In the present study, a series of tolprocarb derivatives were evaluated for PKS-A, MBI-A, and RBC-E. A linear relationship with r 2 =0.90 between PKS-A and MBI-A was evident.…”

“…MICs for MBI-A were determined by visually observing the mycelial color change because melanin synthesized in the mycelia can be observed as dark gray, and the inhibition of melanin biosynthesis, such as the addition of tolprocarb, yields colorless mycelia. 20) A distinct growth inhibition was not observed in the melanin biosynthesis inhibitory assay with all compounds studied here. …”

“…20,21) The cell-free assay of PKS was performed using the previously described procedure 20,21) with some modifications. A reaction mixture (final volume, 200 µL) containing 50 mM potassium phosphate (pH 7.2), 500 µM malonyl-CoA, 450 µg/mL protein, and a tolprocarb derivative was incubated for 2 hr at 25°C.…”

“…[17][18][19] In our previous report, the target site of tolprocarb was found to be PKS, which is different from that of conventional fungicides. 20) As a result, tolprocarb was classified into a new mode-of-action category in the Fungicide Resistance Action Committee code list. In the present study, we evaluated a series of tolprocarb derivatives for P. oryzae polyketide synthase inhibitory activity (PKS-A), melanin biosynthesis inhibitory activity (MBI-A), and rice blast control efficacy (RBC-E) to verify their target sites and to investigate the structure-activity relationships (SARs) and activity-activity relationships.…”

Effect of Various Compounds Blocking the Colony Pigmentation on the Aflatoxin B1 Production by Aspergillus flavus

“…The blocking of the melanin biosynthesis in pathogenic fungi can result in the loss of their pathogenicity and an increased susceptibility of fungi to biotic and abiotic stresses [7,8,9]. For example, a defect in one of the PKS-encoding genes of A. flavus resulting in a formation of non-pigmented sclerotia significantly increased the susceptibility of the fungus to the deleterious effect of UV light and heat; in addition, the melanin-deficient mutant was less resistant to insect predation as compared with the wild-type strain generating melanized sclerotia [10].…”