Application of plant defense elicitors fails to enhance herbivore resistance or mitigate phytoplasma infection in cranberries

Authors: RODRIGUEZ-SAONA, CESAR - Rutgers University; Polashock, James; KYRYCZENKO-ROTH, VERA - Rutgers University; HOLDCRAFT, ROBERT - Rutgers University; JIMENEZ-GONZALEZ, GIOVANNA - Universidad Nacional Abierta Y A Distancia; DE MORAES, CONSUELO - Universidad Nacional Abierta Y A Distancia; MESCHER, MARK - Swiss Federal Institute Of Technology Zurich

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/29/2021
Publication Date: 8/12/2021
Citation: Rodriguez-Saona, C., Polashock, J.J., Kyryczenko-Roth, V., Holdcraft, R., Jimenez-Gonzalez, G., De Moraes, C.M., Mescher, M.C. 2021. Application of plant defense elicitors fails to enhance herbivore resistance or mitigate phytoplasma infection in cranberries. Frontiers in Plant Science. 12:Article 700242. https://doi.org/10.3389/fpls.2021.700242.
DOI: https://doi.org/10.3389/fpls.2021.700242

Interpretive Summary: Diseases in plants can affect their growth, reproduction, and environmental interactions. These responses can be mediated by plant hormones by 'turning on' plant defenses. Here we used synthetic mimics (elicitors) of the hormones salicylic acid (SA) and jasmonic acid (JA) to 'turn on' plant defenses in cranberry plants infected by the bacterial disease, false blossom. Our intent was to determine the complex responses in the infected plants to two insect herbivores, one of which transmits false blossom as well as affects on bacterial pathogen gene expression and plant nutrient content. We found that bacterial infection and elicitor treatment generally improved insect herbivore growth compared to uninfected, untreated controls. Bacterial infection reduced plant size, increased nitrogen content and SA concentrations, and lowered carbon/nitrogen ratios compared to uninfected plants, irrespective of elicitor treatment. Together, our findings indicate positive effects of both bacterial infection and elicitor treatment on the performance of the insect herbivores in cranberries, likely through improved plant nutritional content. We found no evidence that the tested elicitors of plant defenses increased resistance to insect herbivores or reduced disease incidence and do not recommend their use in cranberry for false blossom or insect control. These data will be useful to research scientists and extension agents as we continue to explore sustainable methods for pest control in cranberries and related crops.

Technical Abstract: Synthetic elicitors of the salicylic acid (SA) and jasmonic acid (JA) plant defense pathways can be used to increase crop protection against herbivores and pathogens. Here, we hypothesized that elicitors of plant defenses interact with pathogen infection to influence crop resistance against vector and nonvector herbivores. To test this hypothesis, we used a trophic system consisting of cranberries (Vaccinium macrocarpon), the phytoplasma that causes false blossom disease, and two herbivores—the blunt-nosed leafhopper Limotettix vaccinii (the vector of false blossom disease) and the gypsy moth Lymantria dispar (a nonvector). We tested four commercial elicitors—three that activate mainly SA-related defenses (called Actigard, LifeGard, and Regalia) and one activator of JA-related defenses (called Blush) in plants. In a greenhouse experiment, potted phytoplasma-infected and uninfected plants were either treated once every months from April through June with the elicitors, following label instructions, or remained untreated. We then measured the performance of L. vaccinii nymphs and L. dispar larvae as well as phytoplasma titer and the size, mass, carbon (C) and nitrogen (N) levels, and SA and JA levels in cranberry plants. Generally, phytoplasma infection and elicitor treatment individually improved L. vaccinii and L. dispar mass compared to uninfected, untreated controls; however, SA-based elicitor treatments reduced L. vaccinii mass on infected plants. Regalia also improved L. vaccinii survival. Phytoplasma infection reduced plant size and mass, increased N and SA concentrations, and lowered C/N ratios compared to uninfected plants, irrespective of elicitor treatment. Although none of the elicitor treatments affected the transcript levels of a phytoplasma-specific marker gene, all of them increased N and reduced C/N levels, and the three SA activators also reduced JA levels. Together, our findings indicate positive effects of both phytoplasma infection and elicitor treatment on the performance of the vector L. vaccinii and nonvector L. dispar in cranberries, likely through improved plant nutritional content and changes in phytohormonal composition such as increased SA and decreased JA levels, respectively. Therefore, we found no evidence that the tested elicitors of plant defenses increase resistance to insect herbivores or reduce disease incidence in cranberries.