-Derived Secondary Metabolites Reduce Root Infection by in Cowpea.

Root-knot nematodes (RKNs) cause significant economic damage to crop plants, spurring demand for safe, affordable, and sustainable nematicides. A previous study by our research team showed that the combination of two nematicidal secondary metabolites (SMs) derived from bacteria, -cinnamic acid (-CA), and (4)-5-phenylpent-4-enoic acid (PPA) have a synergistic effect against RKNs in vitro. In this study, we considered in planta assays to assess the effects of this SM mixture on the virulence and reproductive fitness of the RKN in a cowpea. Factorial combinations of five -CA + PPA concentrations (0, 9.0, 22.9, 57.8, and 91.0 μg/ml) and two nematode inoculation conditions (presence or absence) were evaluated in 6-week growth chamber experiments. Results from this study showed that a single root application of the -CA + PPA mixture significantly reduced the penetration of infective juveniles (J2s) into the cowpea roots. The potential toxicity of -CA + PPA on RKN-susceptible cowpea seedlings was also investigated. The effect of -CA + PPA × nematode inoculation interactions and the -CA + PPA mixture did not show significant phytotoxic effects, nor did it adversely affect plant growth parameters or alter leaf chlorophyll content. Total leaf chlorophyll and chlorophyll content were significantly reduced (by 15 and 22%, respectively) only by the nematode inoculum and not by any of the SM treatments. Our results suggest that a single root application of a mixture of -CA and PPA reduces J2's ability to infect the roots without impairing plant growth or chlorophyll content.