Plant defence system in multiple parallel biotic stresses

Nikoleta Rubil

Doctoral thesis

Abstract:

Dual and multiple plant interactions with fungi and insect pests frequently occur in nature. Plants respond to such biotic stresses by activating distinct defence mechanisms; however, little is known about how plants cope with multiple stresses, and how symbiotic associations affect the subsequent fungal infections. The goal of such phytopathological studies is to better understand plant defence strategies and unravel sustainable and eco-friendly alternatives to chemical pesticides. Another way is usage of plant elicitors which can boost plant defence and make plants resistant to subsequent attackers. Firstly, I developed a choice test system to study the preference of insect pests to different leaf treatments, as described in Chapter 5.1. In Chapter 5.2, I hypothesised that the combined interaction of fungal infection caused by Leptosphaeria maculans could alter oilseed rape palatability to Plutella xylostella caterpillars or Brevicoryne brassice aphids. Feeding preference tests were complemented by analyses of defence gene transcription, and levels of glucosinolates (GLSs) and volatile organic compounds (VOCs) in inoculated and non-inoculated leaves to determine the possible cause of insect choice. I confirmed that caterpillars and aphids prefer true leaves of oilseed rape to cotyledons; therefore true leaves were used for all further experiments involving oilseed rape. Leaves inoculated with L. maculans were more palatable to P. xylostella caterpillars in the early stage of disease development (3 days post inoculation, dpi), whereas the preference disappeared in the later stage (7 dpi). On the other hand, there was no preference of B. brassicae aphids at 3 dpi; however, L. maculans leaves were less palatable to aphids at 7 dpi. These differences in insect preference could be caused by the underlying mechanisms leading to changes in metabolic composition. In Chapter 5.3, I hypothesized that specific resistance inducers can alter oilseed rape palatability to P. xylostella caterpillars. Indeed, leaves directly treated with benzothiadiazole (BTH) and ulvan were less palatable to caterpillars. Further studies need to be done to unravel this cause of larval choice. In Chapter 5.4, I focused on the activation of salicylic acid (SA) and jasmonic acid (JA) signaling in Arabidopsis leaves during B. brassicae infestation in high spatio-temporal resolution. Despite the increasing understanding of transcriptomic changes upon infestation, discrepancies in results frequently arise due to variations in sampling methodologies. To address this issue, I employed genetically encoded fluorescent biosensors, histochemistry, and quantitative reverse transcription PCR (qRT-PCR) techniques to accurately trace the activation patterns of distinct branches of phytohormonal signaling. I observed a rapid induction of SA and JA signaling in cells surrounding stylet puncture, co-localizing with callose deposition. For both PR1 and JAZ10 activation was detected at 24 hpi, increasing and spreading along the veins until 72 hpi and, to a lesser extent, within the epidermal pavement cells. The SA signaling wave appeared in parallel with JA-associated, and continued to increase in time. These results first show a local activation of SA- and JA-related responses after stylet penetration of Arabidopsis leaves and bring a detailed insight into the spatio-temporal complexity of plant defence activation during specialist aphid attack. In Chapter 5.5, I hypothesized that root association with a plant-beneficial fungus Metarhizium robertsii will prime the plant for defence against subsequent L. maculans infection. However, I observed that symbiotic association by M. robertsii supported the infection compared to control group; additionally, plants were susceptible when treated with 0.05 % Triton X compared to water treatment. This might be due to observed changes in phytohormonal signaling responses, as M. robertsii and 0.05 % Triton X activated JA signaling and suppre

Abstract:

ssed the activation of ET and oxidative stress responses.

Keywords

elicitors, glucosinolates, herbivores, pathogens, phytohormones, plant interactions, symbionts, volatiles

Language used: English

Date on which the thesis was submitted / produced: 2. 6. 2024

Thesis defence

• Supervisor: prof. Ing. Pavel Ryšánek, CSc.
• Reader: Jaroslav Salava, Ing., CSc., externi, Michaela Sedlářová, Ing., externi, Radovan Pokorný, externi

Czech University of Life Sciences Prague

Faculty of Agrobiology, Food and Natural Resources

Doctoral programme / field:
Agricultural Specialization / Agricultural and Forestry Phytopathology and Plant Protection