Survival strategy of starving spruce trees: The critical role of reserves

Credit: Pixabay/CC0 public domainClimate extremes are threatening trees and whole forests around the world. Trees are sessile organisms and cannot escape the effects of changing environmental conditions. They must adapt their metabolism to cope with these threats. Photosynthesis is crucial for the production of carbohydrate-rich sugar molecules (energy-rich sugar molecules) in plants. These compounds are both energy sources as well as basic building blocks for all metabolism processes. Photosynthesis becomes less efficient in extreme climates like drought and heat. Trees also produce less carbohydrates as CO2 uptake decreases and water is scarce. Plants must rely on their stored reserves in order to meet the demand for energy-rich sugars. Plants can become susceptible to diseases and insect attacks if their reserves are depleted.It was thought that they were important, but it wasn't clear until now. Soluble sugars, starch, and fats can only be formed when the photosynthetic environment is favorable and their production rate exceeds that of other functions, such as growth. This is not logical evolutionary thinking. Trees must survive for many decades before they can reproduce. A reliable source of reserves is crucial for trees to survive frequent unfavorable times," Dr. Henrik Hartmann, group head at the Max Planck Institute for Biogeochemistry, Jena, says. Why should a tree invest on growth when it can ensure its survival and future by accumulating more reserves?Dr. Jianbei, a postdoctoral researcher in this research group, and the first author of the current study, gave young spruce tree trees a starvation treatment. He kept them at low CO2 levels for several weeks to see if storage is important for their survival. He was able to simulate the reduced photosynthetic rate that occurs during extreme climates by growing the plants at very low CO 2 levels for several weeks. As they were needed for metabolism, the readily available storage compounds initially decreased as expected. They could not be replenished with reduced CO 2. Surprisingly though, as CO2 starvation progressed the availability of storage compounds decreased as expected. Trees stopped growing and their levels stabilized. Huang concludes that "when photosynthetic output was too low to supply sufficient carbon for all functions," the trees decreased their growth and diverted resources to storage.Researchers also studied the genetic activity and expression of genes involved in the metabolism of plant cells three to five weeks after CO2 starvation. Huang states that they found the first time that prolonged starvation led to an increase in the production of enzymes involved in fast-access storage compounds. However, gene expression of growth enzymes, such as cellulose or lignin, was significantly reduced. This confirms the trade-off between storage versus growth at the molecular level.It was even more surprising that metabolic pathways for alternative energy production were enhanced, as a result of increased production enzymes for the conversion complex fat molecules into energy rich carbohydrates. Hartmann says that plants seem to prefer to lose expendable structures, and even to digest themselves, than to abandon storage formation. "So, the strategy for energy storage and production, while shutting off unnecessary energy consumption for growing, is always implemented during CO 2 starvation."This strategy could extend the life span of trees and may also help them to survive extreme climate conditions. It is important that researchers continue their research on this topic. The surprising discovery that carbon-starved spruce tree build up reserves offers hope that forests can recover from climate stress.Studies on plant storage strategies were previously limited to short-lived herbaceous plants like Arabidopsis. These studies covered only a few hours to a few weeks. Findings on Arabidopsis might not be relevant for long-lived plants, which can take decades to reproduce, and are constantly exposed to extreme climate conditions and changing seasons. Huang says that trees must have a storage strategy that allows them more time than biennial herbs to thrive. "How else can they survive on Earth for almost 400 million years?""Our joint findings that even under extreme stresscarbon depletionspruce tree still can store reserves are novel and unanticipated," said Prof. Nicole van Dam from FSU and iDiv. These findings give hope that severely stressed forest tree may be able to use these reserves for recovery.Many ecosystems in the Northern Hemisphere are dominated by conifers such as the spruce and pine trees. They also have important ecological functions, including absorbing and storing carbon dioxide. Many conifer species aren't adapted to the changing climate and are therefore at risk. In vegetation models, their survival and overall forest growth are simulated. These models are based on older ideas that growth is driven by photosynthesis, and ignore the storage and reserve of energy-rich sugars. Hartmann says that based on new research, these models can be now designed more realistically. Hartmann also emphasizes that "more reliable model outcomes are extremely important for predicting the future of forests, particularly under climate change."Learn more Why trees grow at nightMore information: Huang, J., et al. The storage of carbon reserves in spruce forests is preferred to growth in the face carbon limitation. PNAS (2021). Information from the Journal: Proceedings of the National Academy of Sciences Huang J., and co-authors. (2021). DOI: 10.1073/pnas.2023297118German Centre for Integrative Biodiversity Research