Febrifugine, a natural alkaloid, exhibits specific anti-phytophthora
activity; however, its mode of action is unclear. In this study, halofuginone,
a synthetic derivative of febrifugine, showed significantly higher
anti-phytophthora activities than those of febrifugine and the commercial
drug metalaxyl against Phytophthora sojae, Phytophthora capsici, and Phytophthora infestans with effective concentration
for 50% inhibition (EC50) values of 0.665, 0.673, and 0.178
μg/mL, respectively. Proline could alleviate the growth inhibition
of halofuginone on P. capsici, implying
that halofuginone might target prolyl-tRNA synthetase (PcPRS). The anti-phytophthora mechanism of halofuginone was then investigated
by molecular docking, fluorescence titration, and enzymatic inhibition
assays. The results revealed that halofuginone could bind to PcPRS and shared a similar binding site with the substrate
proline. Point mutations at Glu316 and Arg345 led to 24.5 and 16.1%
decreases in the enzymatic activity of PcPRS but
816.742- and 459.557-fold increases in the resistance to halofuginone,
respectively. The results further confirmed that halofuginone was
a competitive inhibitor of proline against PcPRS,
and Glu316 and Arg345 played important roles in the binding of halofuginone
and proline. Taken together, the results indicated that halofuginone
is an alternative anti-phytophthora drug candidate and that PcPRS represents a potential target for the development
of new pesticides.