Leaves turn yellow naturally, in autumn, when they get old or when the plant is exposed to stresses like darkness or drought. Daria Chrobok compared in her PhD thesis different scenarios of leaf yellowing and analysed what happens on the metabolic level. She showed that mitochondria, the respiratory power stations of the cell, are crucial for a coordinated adjustment of metabolism during leaf yellowing. Mitochondria stay active until the last stages of leaf yellowing to provide the energy that is needed for recycling nutrients from the dying leaf. Daria Chrobok successfully defended her thesis on the 8th of June.
The yellowing of a leaf, also called senescence, occurs naturally in for example deciduous trees in autumn or when annual plants get old and produce seeds. However, also stresses like a lack of nutrients, drought or pathogens can induce senescence. Daria Chrobok compared naturally aging plants with plants where senescence was induced by darkening a single leaf. She showed that in both cases the mitochondria remain intact until the last stages of leaf senescence to provide the energy needed for the mobilisation and transport of nutrients.
In addition, she showed that especially the amino acid glutamate, that can be easily transported within the plant, accumulates during leaf senescence. She hypothesized that this accumulation of glutamate in the mitochondria and its conversion to glutamine in the cytosol are essential steps for the reallocation of nitrogen rich compounds to other parts of the plant.
The export of amino acids with high nitrogen content into developing parts of the plant, e.g. seeds, is of high importance to ensure that those seeds contain enough nitrogen and the survival of the next generation is guaranteed. Nitrogen is often a limiting factor for plant growth and development and therefore the reallocation of nitrogen during senescence is important for plants.
Without light, plants cannot perform photosynthesis and produce energy-rich carbon compounds like sugars. If only one leaf is darkened, the covered leaf will rapidly turn yellow. In contrast, when the whole plant is darkened, the leaves are repressing this induction of senescence, i.e. they stay green. The plant keeps all components needed for photosynthesis alive and intact so that upon sudden light exposure, the plants are ready to start photosynthesis and continue growing.
Daria Chrobok and her colleagues analysed how plants adjust their metabolism to those two darkening conditions and they compared these results with the light-dependent “stay-green” mutant. When one leaf of a stay-green plant is darkened, it stays green, whereas the same treatment in a wild type plant leads to the yellowing of the darkened leaf. This darkened stay-green leaf, as well as the whole darkened plant accumulate amino acids, especially those with high nitrogen and low carbon content.
The understanding of how “stay-green” plants manage to stay green is interesting for the food industry to keep vegetables green for longer time and for agriculture, to ensure proper grain and nutrient filling as well as other improved traits for crop plants.
The public defence took place in Lilla hörsalen at KBC, Umeå University, on Friday, 8th of June 2018. Faculty opponent was David Macherel, IRHS-MitoStress, University of Angers, France. Supervisors were Olivier Keech and Per Gardeström.
Title of Daria Chrobok’s thesis: “To “leaf” or not to “leaf” - Understanding the metabolic adjustments associated with leaf senescence”
Link to the doctoral thesis: urn:nbn:se:umu:diva-147700
Are you interested to read more? Have a look on the comic strip made by Neil E. Robbins II. He illustrated the results from the article Law et al., 2018 (Plant Physiology) that is included in Daria Chrobok’s thesis. The comic explains very nicely the metabolic adjustments during the different dark treatments:
Simon R Law, Daria Chrobok, Marta Juvany, Nicolas Delhomme, Pernilla Lindén, Bastiaan Brouwer, Abdul Ahad, Thomas Moritz, Stefan Jansson, Per Gardestrom, Olivier Keech (Plant Physiology) 2018; DOI: https://doi.org/10.1104/pp.18.00062
Title: Darkened leaves use different metabolic strategies for senescence and survival