The role of isoprene in regulating senescence and leaf abscission in velvet bean (Mucuna pruriens)

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Alyka Zahnd, Amie Haddock, Devon Lindstrom, Lauren Tinoco, Jessica Poush, Mehreen Tai, Kaitlyn Atkins, Bryan Jenkins, Rachael Prawitz, Edward Miles, Mackenzie Helsel and Csengele Barta, Biology, Missouri Western State University, Saint Joseph, MO

Background/Question/Methods
Several plant species synthesize and emit isoprene, the most abundant source of non-anthropogenic volatile carbon released into the atmosphere. Plant isoprene emissions play key roles in determining air quality and reinforcing global warming. In planta isoprene is pivotal for plant defence against environmental stressors, maintaining membrane integrity and acting as an antioxidant. Isoprene is synthesized in chloroplasts in a light- and temperature dependent manner and is sensitive to variations in other environmental factors.
Our team found in a long-term field study a strong correlation between the isoprene emission capacity and the timing of the onset of senescence in oaks. The current work focused on understanding whether variations in isoprene emission rates, as modulated by the environment, affect plant development in a controlled environment.
Potted velvet bean (Mucuna pruriens) was grown in greenhouse chambers at very high (42°C; HT1), high (36°C; HT2), intermediate (30°C; IT) and low (23°C; LT) temperatures, exposed to either 1100 (high light; HL) or 400 (low light; LL) µmolm-2s-1 irradiance, to modulate isoprene emission rates throughout the plants’ developmental cycle. The onset of senescence was monitored in periodically sampled leaf segments, for biochemical markers, pigment degradation, phytohormone levels and photosynthesis. Isoprene fluxes were quantified by gas chromatography-mass spectrometry.
Results/Conclusions
Plants with the highest isoprene emission rates (grown under HT1 and HT2/1100 µmolm-2s-1 irradiance) exhibited a substantial delay in the onset of senescence (indicated by the absence of detectable levels of lipid peroxidation products, reactive oxygen species accumulation, pigment degradation and maintained high rates of carbon assimilation), and abscisic acid accumulation, as compared to plants grown under IT or LT and lower irradiances, whose emission rates were lower or completely suppressed. Plants with the highest isoprene emission rates also accumulated H2O2 at higher rates throughout their lifecycle than plants under the other tested conditions, not associated with membrane degradation, exhibited upregulation in their antioxidant network and retained 10 days longer their senescent leaves than low- or non-emitter controls.
We attribute the delay in the onset of senescence and abscission in plants emitting isoprene at high rates to the possible antioxidant action of isoprene, neutralizing reactive oxygen species triggering senescence, and the “priming” effect of the sustained physiologically higher H2O2 levels, serving to upregulate the plants’ endogenous antioxidant network, and potentially isoprene emission. Extensions in the vegetation’s life cycle may significantly alter litter formation, nutrient recycling and plant-biotic environment interactions, effects to be considered in reforestation and landscaping applications.