Understanding Plant Defence Under Multi-Herbivore Attack: Successful PhD Defence by Jessil Ann Pajar

12.11.2025

Supervisor N. van Dam with J. A. Pajar after her successful defense. Photo: IGZ/S. Panthee.

On 22 September 2025, Jessil Ann Pajar successfully defended her PhD thesis at Friedrich Schiller University Jena. Conducted within the IGZ Research Group ‘Plant Biotic Interactions’ and supervised by Prof. Dr Nicole van Dam, her work examined how plants respond to simultaneous herbivore attacks above- and belowground, with a particular focus on improving plant defence against the specialist cabbage aphid Brevicoryne brassicae (L.). Her findings provide new insights into the metabolic mechanisms that mediate plant resistance under complex biotic stress.

Plants in natural and agricultural systems are frequently exposed to multiple herbivores at the same time. Yet most research focuses on plant responses to single attackers. This gap limits the development of strategies to enhance plant resilience against realistic, multi-species pressures. Pajar’s doctoral research addressed this knowledge gap by investigating how the black mustard, Brassica nigra, responds when attacked aboveground by cabbage aphids and belowground by nematodes. Understanding these interactions is important for predicting pest dynamics and designing sustainable pest management strategies.

The chemical ecologist used targeted and untargeted metabolomics combined with gene expression analyses to explore how different belowground nematode infections influence plant defence chemistry, and how these changes subsequently affect aphid performance. She showed that infections by the root-knot nematode (Meloidogyne incognita) and the root-lesion nematode (Pratylenchus penetrans) each induced distinct defence-related metabolites in Brassica nigra roots, particularly glucosinolates. However, when both nematodes infected the plant simultaneously, glucosinolate levels decreased, indicating altered defence investment during co-infection.

Her research further demonstrated that nematode infections in the roots caused changes in leaf and phloem metabolites that influenced the performance of the specialist cabbage aphid Brevicoryne brassicae. Early infection by P. penetrans changed the levels of various defensive metabolites in leaves and in phloem sap, which correlated with reduced aphid survival.

Finally, Pajar investigated the role of vitamin C as a defence-related metabolite. She observed that vitamin C levels were significantly reduced in root-lesion nematode-infected black mustard. Using a vitamin C-deficient version of the model and relative plant Arabidopsis thaliana, she confirmed reduced aphid survival in association with altered primary and secondary metabolic profiles. These included increased sugars and amino acids, potential mediators of osmoregulatory stress for aphids, as well as phenylpropanoids.

Pajar’s findings generate new hypotheses about how B. nigra allocates metabolic resources when challenged by multiple herbivores and how belowground interactions shape aboveground resistance traits. Her work provides a deeper understanding of how defence pathways are coordinated within plants and highlights the importance of studying herbivore interactions in combination rather than in isolation.

Jessil Ann Pajar conducted her PhD research under the supervision of Prof. Dr Nicole van Dam. She began her PhD within the International Max Planck Research School (IMPRS) at the Max Planck Institute for Chemical Ecology in Jena, completed part of her research at the German Centre for Integrative Biodiversity Research (iDiv) in Leipzig, and before joining the IGZ in January 2023. She is currently preparing applications to obtain independent research funding abroad and aims to continue her work on the molecular biology underlying nematode effectors excretion.

Prof. van Dam emphasises her strong scientific potential: “She has the whole package: a fine set of brains, a talent for doing scientific research and particularly a passion for understanding biological interactions. In a short time, she has established herself as an internationally recognized expert in the chemical and molecular ecology of nematode-plant interactions. I wish Jessil all the best for her career and personal life, and hope that our paths cross again in the near future!”

We warmly congratulate Dr Jessil Ann Pajar on the successful completion of her PhD and wish her continued success in her research career.

Selected Publications

Pajar, J.A., Leonar, A.L., Otto, P., Döll, S., Hanschen, F.S., & van Dam, N.M. (2025). Dual and single-species nematode infections distinctly modulate defense metabolism in Brassica nigra roots. Journal of Chemical Ecology, 51(5): 90. doi:10.1007/s10886-025-01637-8
Pajar, J.A., Otto, P., Leonar, A.L., Döll, S. & van Dam, N.M. (2024). Dual nematode infection in Brassica nigra affects shoot metabolome and aphid survival in contrast to single-species infection. Journal of Experimental Botany, 75(22), 7317–7336. https://doi.org/10.1093/jxb/erae364