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

Date:
August 23, 2021
Source:
Max-Planck-Gesellschaft
Summary:
Trees continue to form reserves even during long periods of
starvation, study shows.



FULL STORY ========================================================================== During climate extremes, plants cannot produce sufficient energy-rich
carbon compounds through photosynthesis and they become dependent on
stored reserves.

According to current understanding, these reserves are only formed
when the supply of photosynthesis products exceeds demand to support
processes like growth. Scientists from the Max Planck Institute for Biogeochemistry demonstrate that trees continue to form reserves even
during long periods of starvation. To achieve this, trees stop growing and
even digest non-essential energy-rich components. This knowledge can be
used to improve predictions of how trees will respond to climate change.


========================================================================== Trees and entire forests worldwide are threatened by increasing climate extremes and ensuing insect infestations. As sessile organisms, trees
cannot escape threatening environmental conditions and must adapt their metabolic processes to confront the threats. Crucially important for
plants is the production of energy-rich sugar molecules (carbohydrates)
by photosynthesis.

These compounds serve as both energy sources and basic building blocks
for all metabolic processes. During climate extremes such as prolonged
drought or heat, photosynthesis is impaired and trees produce less carbohydrates, because CO2 uptake declines and water is scarce. The
demand for energy-rich sugars is then not met and plants have to rely
on stored reserves to maintain vital metabolic processes. When reserves
become depleted, plants may starve to death or become vulnerable to
disease and insect attacks as the defense system fails.

Despite their critical role, it was assumed until now that reserves
like soluble sugars, starch or fats are only formed when photosynthetic conditions are favorable and their rate of production exceeds the demand
from other functions such as growth. "From an evolutionary perspective
that doesn't make sense. Trees have to survive for decades before they
can reproduce, and a reliable source of quickly available reserves plays
a crucial role in surviving frequent unfavorable periods" underlines
Dr. Henrik Hartmann, group leader at the Max Planck Institute for Biogeochemistry (MPI-BGC) in Jena. "Why should a tree invest in growth
instead of ensuring immediate and future survival by accumulating more reserves?" To investigate the importance of storage for tree survival,
Dr. Jianbei Huang, postdoctoral researcher in the research group and
first author of the recent study published in Proc.Natl.Acad.Sci.USA,
subjected young spruce trees to a starvation treatment by growing them
at very low CO2 concentrations for several weeks. This allowed him
to simulate reduced photosynthetic rates that occur during climate
extremes when studying the plants' carbohydrates. Initially, readily
available storage compounds decreased as expected, since they were used
for metabolism and could not be replenished under reduced CO2 supply.

Surprisingly, as CO2 starvation progressed, the storage compounds
stabilized at a constant level and trees stopped growing. "When
photosynthetic output became too low to adequately supply carbon to
all functions, the trees reduced their growth and diverted available
resources to storage," Huang concludes.

Three to 5 weeks after the onset of CO2 starvation, the researchers also examined the genetic activity of plant cells, in particular expression
of genes that encode enzymes involved in metabolic processes. "We found
for the first time that after prolonged starvation the production of
enzymes responsible for fast-access storage compounds was increased"
Huang says. In contrast, gene expression of enzymes involved in growth processes, such as cellulose and lignin production, was greatly reduced,
thus confirming the trade-off between storage and growth at the molecular level.

Even more surprising, metabolic pathways for alternative energy
production were boosted, as found by an increased production of enzymes responsible for the conversion of complex fat molecules into energy-rich carbohydrates. "It seems that plants prefer to sacrifice expendable
structures and apparently even digest themselves, rather than to give
up on storage formation" says Hartmann.

"So the strategy for energy production and storage, while shutting down unnecessary energy consumption for growth, is consistently implemented
during CO2 starvation." How long trees may survive climate extremes using
this strategy and whether apparently healthy looking trees might already
be in the emergency mode of self-digestion, are follow-up questions
that research should urgently tackle. Altogether, the novel finding
that carbon-starved spruce trees build up reserves gives hope that this adaptation allows forests to recover from climate stress.

Previous studies on storage strategies in plants were limited to
short-lived herbaceous species like Arabidopsis, covering only hours to
a few days.

However, for long-lived plants that take decades to reproduce and that
are continuously exposed to changing seasons and sporadic climate
extremes, findings on Arabidopsis may be of little relevance. "Of
course, trees must follow a storage strategy that allows them to
survive longer than biennial herbs" says Huang, "how else have they
managed to persist on Earth for nearly 400 million years?" Conifers,
such as the spruce trees studied here, dominate many Northern Hemisphere ecosystems and have other very important ecological functions besides
absorbing and storing the greenhouse gas carbon dioxide. Yet, many
conifer species are not adapted to the warmer and drier conditions
imposed by climate change, and are thus particularly threatened. Their
survival and overall forest development is simulated in vegetation
models. However, these are based on the older ideas that photosynthesis
supply directly drives growth, ignoring allocation of energy-rich
sugars into storage and reserves. "Building on our new findings, such
models can now be designed more realistically," Hartmann emphasizes,
"and more reliable model outcomes are extremely important to predict
the future of our forests, especially under advancing climate change." ========================================================================== Story Source: Materials provided by Max-Planck-Gesellschaft. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Jianbei Huang, Almuth Hammerbacher, Jonathan Gershenzon, Nicole
M. van
Dam, Anna Sala, Nate G. McDowell, Somak Chowdhury, Gerd Gleixner,
Susan Trumbore, Henrik Hartmann. Storage of carbon reserves in
spruce trees is prioritized over growth in the face of carbon
limitation. Proceedings of the National Academy of Sciences, 2021;
118 (33): e2023297118 DOI: 10.1073/pnas.2023297118 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210823104317.htm

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