Navigational Signals for Insect and Slug Parasitic Nematodes: The Role of Ascorbate-Glutathione System and Volatiles Released by Insect-Damaged Sweet Pepper Roots

Abstract

This study of underground multitrophic communication, involving plant roots, insects, and parasitic nematodes, is an emerging field with significant implications for understanding plant-insect-nematode interactions. Our research investigated the impact of wireworm (Agriotes lineatus L. [Coleoptera: Elateridae]) infestations on the ascorbate-glutathione system in sweet pepper (Capsicum annuum L.) plants in order to study the potential role in root-exudate-mediated nematode chemotaxis. We observed that an A. lineatus infestation led to a decrease in leaf ascorbate levels and an increase in root ascorbate, with corresponding increases in the glutathione content in both roots and leaves. Additionally, a pigment analysis revealed increased carotenoid and chlorophyll levels and a shift towards a de-epoxidized state in the xanthophyll cycle. These changes suggest an individual and integrated regulatory function of photosynthetic pigments accompanied with redox modifications of the ascorbate-glutathione system that enhance plant defense. We also noted changes in the root volatile organic compound (VOC). Limonene, methyl salicylate, and benzyl salicylate decreased, whereas hexanal, neoisopulegol, nonanal, phenylethyl alcohol, m-di-tert-butylbenzene, and trans-β-ionone increased in the roots of attacked plants compared to the control group. Most notably, the VOC hexanal and amino acid exudate cysteine were tested for the chemotaxis assay. Nematode responses to chemoattractants were found to be species-specific, influenced by environmental conditions such as temperature. This study highlights the complexity of nematode chemotaxis and suggests that VOC-based biological control strategies must consider nematode foraging strategies and environmental factors. Future research should further explore these dynamics to optimize nematode management in agricultural systems.

Keywords: Agriotes lineatus; Capsicum annuum; ascorbate–glutathione system; entomopathogenic nematodes; root volatile compounds; slug parasitic nematodes.

Figure 1

To start the experiment, three 1 cm diameter marks were made on the bottom of each Petri dish: one in the center and two 1.5 cm from the edge on the right and left. Then, 0.03 μg/mL tested substance was applied to the right side of the agar, while the left side was treated with 10 μL of distilled water/96% ethanol as the control. VOCs were applied just before introducing 100 IJs of nematodes, in a 50 μL droplet, to the center of the agar. The control setup also used distilled water/96% ethanol for both sides and placed a 50 μL droplet of 100 IJs in the center.

Figure 2

Concentrations of total ascorbate (nmol/g DW) and dehydroascorbate (% of total) in the roots and leaves of non-attacked C. annuum plants and C. annuum plants attacked by A. lineatus. Asterisks * indicate statistical differences (p < 0.05) tested with an independent t-test between roots and leaves from attacked and non-attacked plants.

Figure 3

(A) Concentrations of cysteine (nmol/g DW) and cystine (% of total) and (B) total glutathione and GSSG (% of total) in the roots and leaves of non-attacked C. annuum plants and C. annuum plants attacked by A. lineatus. Asterisks * indicate statistical differences (p < 0.05) tested with an independent sample t-test between roots and leaves from attacked and non-attacked plants.

Figure 4

The chart displays the percentage of different nematode IJs (infective juveniles) in the outer circles after 24 h, depending on the temperature used in the experiment. The error bars represent the standard error. Capital letters denote statistically significant differences (p < 0.05) among the various nematode species at the same temperature, while lowercase letters indicate statistically significant differences (p < 0.05) among the different temperatures within the same nematode species. The nematode species used in the experiment are as follows: Hb = Heterorhabditis bacteriophora; Sc = Steinernema carpocapsae; Sf = Steinernema feltiae; Om = Oscheius myriophilus; Pp = Phasmarhabditis papillosa.