Revisiting energy flow in photosynthetic plant cells

Date:
June 30, 2020
Source:
The University of Hong Kong
Summary:
By developing innovative methods to visualize energy changes in
subcellular compartments in live plants, researchers recently
solved a controversial question in photosynthesis: what is the
source of NADH (Reduced Nicotinamide adenine dinucleotide) for
mitochondria to generate ATP (Adenosine triphosphate)?


FULL STORY ==========================================================================
By developing innovative methods to visualize energy changes in
subcellular compartments in live plants, the team of Dr Boon Leong
LIM, Associate Professor of the School of Biological Sciences of The
University of Hong Kong, after showing how chloroplasts optimizes its
energy efficiency 2 years ago, recently solved a controversial question
in photosynthesis: what is the source of NADH (Reduced Nicotinamide
adenine dinucleotide) for mitochondria to generate ATP (Adenosine triphosphate)? The results were just published in the journal Nature Communications.


========================================================================== Photosynthesis utilizes light as an energy source for plant chloroplasts
to synthesize carbohydrates from water and CO2 molecules. ATPplays an
important role in this process, as it promotes plant growth and supply
energy for various cellular activities. It had been a general belief
that mature plant chloroplasts can import ATP from cytosol since 1969,
but it was shown to be untrue by Dr Lim and his team in 2018 (Note 1),
through introducing a novel ATP sensor in the subcellular compartments
of a C3 model plant, Arabidopsis thaliana. This finding has revised our understanding on chloroplast bioenergetics during daytime and nighttime
and how mature chloroplasts optimize energy efficiency.

Another unresolved problem in photo-energy is that the source of NADH as
a fuel for mitochondria (the major ATP synthesizing organelle in cells)
to produce ATP during photosynthesis is unclear. Some researchers
suggested that excess reducing equivalents carried by surplus NADPH
(Reduced Nicotinamide adenine dinucleotide phosphate) can be exported
to the cytosol in the form of malate, which can then enter mitochondria
through the malate-OAA shuttle, and converted into OAA and NADH in the mitochondrial matrix.

On the other hand, some researchers proposed that during photorespiration glycine decarboxylase generates a large amount of NADH in mitochondria
for ATP production and surplus reducing equivalents carried by NADH is
exported by the mitochondrial malate-OAA shuttle to the cytosol.

In the above two pathways, the directions of the malate-OAA shuttle
across the mitochondrial membrane during photosynthesis are opposite to
each other and therefore this issue had been a matter of debate.

To study this problem, Dr Lim's group introduced two novel sensors that
measure real-time dynamic changes in NADPH levels and NADH/NAD+ ratios
(this ratio reflects the reduction/oxidation status of the cellular compartments) in Arabidopsis thaliana. The conventional detection
methods require extraction and purification of plant metabolites and determination by chemical methods. These methods have a few drawbacks:
in planta measurement and real-time dynamic measurement not feasible;
incapable of measurement the energy molecules in different cell types
or different subcellular compartments.

"Our novel technique solves all of the problems above. By employing
these energy sensors, we found that photorespiration supplies a large
amount of NADH to mitochondria during photosynthesis, which exceeds
the NADH-dissipating capacity of the mitochondria. Consequently, the
surplus NADH must be exported from the mitochondria to the cytosol
through the mitochondrial malate-OAA shuttle," said Ms Sheyli Lim,
a PhD student and the first author of a manuscript published in
Nature Communications. "Solving this question allows us to understand
more about the energy flow between chloroplasts and mitochondria
during photosynthesis, which could help us to booth the efficiency of photosynthesis in the future." "We are the first group to introduce these three novel energy sensors in plants. They will have wide applications
in researches regarding plant bioenergetics. Now we are employing them to
study bioenergetics of guard cells, pollen tube growth and C4 plants with international collaborators," said Dr Lim. "It is a great satisfaction to revisit and clarify some general believes in my field. I wish our findings
can eventually help humans to boost agriculture production,' he added.


========================================================================== Story Source: Materials provided by The_University_of_Hong_Kong. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Shey-Li Lim, Chia Pao Voon, Xiaoqian Guan, Yi Yang, Per Gardestro"m,
Boon
Leong Lim. In planta study of photosynthesis and photorespiration
using NADPH and NADH/NAD fluorescent protein sensors. Nature
Communications, 2020; 11 (1) DOI: 10.1038/s41467-020-17056-0 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200630111447.htm

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