The price of heat stress: functional and resource constraints to thermal tolerance in arid zone plants
- Publication Type:
- Thesis
- Issue Date:
- 2020
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Understanding how plants cope with extreme temperatures is key to determining species distribution under climate change. Plants possess an inherent ability to withstand high temperatures and acquire greater thermal tolerance seasonally. The membranes and photosynthetic apparatus in leaves are particularly susceptible to heat damage and likely to respond to different environmental cues. The question arises as to how these two systems differ in acquiring thermal tolerance and what roles proteins have in raising thresholds. As part of the stress response and to aid in thermal tolerance, heat shock proteins (HSP) are upregulated, but there are associated resource costs, of particular concern for natural populations. In extreme environments, like deserts, the additional stressors of water and nutrient limitation may affect how plants allocate resources to growth, reproduction and survival. My thesis is important in linking ecology, plant physiology and molecular biology over seasonal time scales in wild Australian desert plant species 𝘪𝘯 𝘴𝘪𝘵𝘶 in desert conditions. I estimated temperature thresholds of photosystem II (PSII, using chlorophyll a fluorescence) membrane stability (via electrolyte leakage) and fitness (via reproductive output) in response to heat stress across seasons. To determine how relative protein expression changes with conditions, I also quantified the complete proteome using shotgun proteomics with tandem mass spectrometry. Overall, species acquired higher thresholds of PSII and membranes and HSP expression was dependent upon season, with little sHSP detected in winter. Cost of three-hour heat stress was reduced in plants with access to additional nutrients, but unexpectedly, heat stress in spring was found to be less costly than in summer, likely due to more severe summer conditions making recovery hard. I show that changes to the proteome are complex, but consistent patterns emerged, with lipid metabolism, ROS homeostasis and HSPs meeting expectations of higher expression during summer. Also, regardless of species or heat-stress treatment, small HSPs were detected in greatest amounts in summer, emphasising the importance of small-HSPs for acquired thermal tolerance in desert species. Importantly, species differences were highlighted throughout the research. Across broad climatic zones, species have many modes for achieving the same outcome and microhabitat likely has an effect on driving adaptation. My work underscores the temporal dynamics of plant thermal tolerance in non-crop species in the environment and how this is achieved through proteome changes. However, my findings suggest that for species from harsh microhabitats, increasing heat stress in summer may have particularly severe consequences.
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