How guard cell chloroplasts obtain energy

Date:
February 10, 2022
Source:
The University of Hong Kong
Summary:
Whether Guard Cells (GCs) carry out photosynthesis has been debated
for decades. Earlier studies suggested that guard cell chloroplasts
(GCCs) cannot fix CO2 but later studies argued otherwise. Until
recently, it has remained controversial whether GCCs and/or GC
photosynthesis play a direct role in stomatal movements. Researchers
have now discovered GCs' genuine source of fuel and untangled
the mystery.



FULL STORY ========================================================================== Whether Guard Cells (GCs) carry out photosynthesis has been debated for decades. Earlier studies suggested that guard cell chloroplasts (GCCs)
cannot fix CO2but later studies argued otherwise. Until recently,
it has remained controversial whether GCCs and/or GC photosynthesis
play a direct role in stomatal movements. Dr Boon Leong LIM, Associate Professor of the School of Biological Sciences of The University of Hong
Kong (HKU), in collaboration with Dr Diana SANTELIA from ETH Zu"rich, discovered GCs' genuine source of fuel and untangled the mystery. The
findings were recently published in journal Nature Communications.


==========================================================================
In the morning, sunlight triggers stomata, which are tiny pores on plant leaves, to open. This let CO2 in and O2 out to boost photosynthesis. The opening of stomata consumes a large amount of adenosine triphosphate
(ATP), the cellular energy currency, but the sources of ATP for stomata
opening remained obscure. Some studies suggested that GCCs carry
out photosynthesis and export ATP to the cytosol to energise stomata
opening. In mesophyll chloroplasts, ATP and NADPH (nicotinamide-adenine dinucleotide phosphate) are generated from photosystems, which are used
as fuel for fixing CO2.

By employing in planta fluorescence protein sensors, the team of Dr
Boon Leong Lim at HKU was able to visualise real-time production of ATP
and NADPH in the mesophyll cell chloroplasts (MCCs) of a model plant, Arabidopsis thaliana.

"However, we could not detect any ATP or NADPH production in GCCs during illumination. Puzzled by this unexpected observation, we contacted an
expert in guard cell metabolism, Dr Diana Santelia from ETH Zu"rich,
for a collaboration," Dr Lim said. Over the past decade, the Santelia lab provided deep and important insights into starch and sugar metabolism in
the guard cells (GCs) surrounding the stomatal pores on the leaf surface.

In joint efforts, the team shows that unlike mesophyll cells (MCs), GC photosynthesis is poorly active. Sugars synthesised and supplied by MCs
are imported into GCs and consumed by mitochondria to generate ATP for
stomatal opening. Unlike MCCs (Note 1), GCCs take up cytosolic ATP via
the nucleotide transporters (NTTs) on chloroplast membrane to energise
starch synthesis in daytime. At dawn, while MCs start to synthesise starch
and export sucrose, GCs degrade starch into sugars to supply energy and increase turgor pressure for stomatal opening. Hence, the function of GCCs
to serve as a store of starch is important for stomatal opening. While
MCs fix CO2 in chloroplasts via the Calvin-Benson-Bassham (CBB) cycle,
CO2 fixation in the cytosol is the main pathway of CO2 assimilation in
GCs, where the downstream product malate, is also an important solute
to increase turgor pressure for stomatal opening. In conclusion,
GCs behave more like a sink (receive sugars) than a source (provide
sugars) tissue. Their function is tightly correlated with that of MCs
to efficiently coordinate CO2 uptake via stomata and CO2 fixation in MCs.

"I was very excited when Dr Lim contacted me asking to collaborate
on this project," Dr Diana Santelia said. "We have been trying
to clarify these fundamental questions using molecular genetics
approaches. Combining our respective expertise has been a winning
strategy," she continued. Dr Sheyli LIM, the first author of the
article and a former PhD student of Lim's group remarked "The
in planta fluorescence protein sensors we developed are powerful
tools in visualising dynamic changes of the concentrations of energy
molecules in individual plant cells and organelles, which allow us to
solve some key questions in plant bioenergetics. I am happy to publish
our discoveries in Nature Communications using this novel technology." ========================================================================== 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, Sabrina Flu"tsch, Jinhong Liu, Luca Distefano, Diana
Santelia, Boon Leong Lim. Arabidopsis guard cell chloroplasts import
cytosolic ATP for starch turnover and stomatal opening. Nature
Communications, 2022; 13 (1) DOI: 10.1038/s41467-022-28263-2 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220210084945.htm

--- up 9 weeks, 5 days, 7 hours, 13 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)