Identification of the antifungal compound, trans-cinnamic acid, produced by Photorhabdus luminescens, a potential biopesticide against pecan scab

Abstract

Pecan scab (caused by Fusicladium effusum) limits the productivity of pecan in the southeastern USA. Alternatives to conventional fungicides should be biorational, of low environmental risk with a lower risk of fungicide resistance. Research showed that metabolites from the nematode symbiont Photorhabdus luminescens suppress pecan scab, but the bioactive molecules had not been identified. Extracts from P. luminescens were investigated using a bioactivity-directed fractionation approach to identify the constituent(s) responsible for the activity. High throughput antifungal bioautography assays against Colletotrichum gloeosporioides, C. fragariae, and C. acutatum were used to guide the fractionation. One of the metabolites was purified and identified as trans-cinnamic acid (TCA) using silica gel chromatography followed by semi-preparative high-performance liquid chromatography. In vitro tests confirmed toxicity of TCA to C. gloeosporioides, C. fragariae, and C. acutatum at 10 and 100 μg mL−1 using fungal bioautography inhibition screening plates. The antimycotic activity of TCA was tested in vitro against F. effusum. Zone of inhibition tests, and tests with TCA incorporated into agar showed TCA toxicity to F. effusum at concentration 148–200+ μg mL−1. Further tests incorporating TCA into liquid media demonstrated that TCA arrested all growth of F. effusum at a concentration even as low as 64 μg mL−1. Naturally occurring antimicrobial products might offer an alternative to disease control in crops, helping in minimizing the risk of fungicide resistance, while also minimizing any negative impact on the environment. Additional research is warranted to determine the potential to use TCA as a suppressive agent for pecan scab and other diseases.