How Guard Cell Chloroplasts Get Energy – ScienceDaily
Whether guard cells (GCs) carry out photosynthesis has been debated for decades. Previous studies have suggested that guard cell chloroplasts (GCCs) cannot fix CO2 but later studies supported the opposite. Until recently, it remained controversial whether GCCs and/or GC photosynthesis played a direct role in stomatal movements. Dr. Boon Leong LIM, Associate Professor at the School of Biological Sciences at the University of Hong Kong (HKU), in collaboration with Dr. Diana SANTELIA of ETH Zürich, discovered the true fuel source of GCs and broke through the mystery. The results were recently published in the journal Nature Communication.
In the morning, sunlight triggers the opening of stomata, which are tiny pores on plant leaves. This leaves CO2 in and O2 to boost photosynthesis. Stomatal opening consumes a large amount of adenosine triphosphate (ATP), the cellular energy currency, but the sources of ATP for stomatal opening have remained obscure. Some studies have suggested that GCCs perform photosynthesis and export ATP to the cytosol to energize stomatal opening. In mesophyll chloroplasts, ATP and NADPH (nicotinamide-adenine dinucleotide phosphate) are generated from photosystems, which are used as fuel to fix CO2.
Employing in the factory fluorescence protein sensors, Dr. Boon Leong Lim’s team at HKU was able to visualize in real time the production of ATP and NADPH in the mesophyll cell (MCC) chloroplasts of a model plant, Arabidopsis thaliana. “However, we could not detect any production of ATP or NADPH in the GCCs during illumination. Intrigued by this unexpected observation, we contacted an expert in guard cell metabolism, Dr Diana Santelia from the ETH Zürich, for a collaboration,” said Dr. Lim. Over the past decade, the Santelia lab has provided extensive and important insights into starch and sugar metabolism in guard cells (GCs) surrounding the stomatal pores on the surface of the leaves.
In joint efforts, the team shows that unlike mesophyll cells (MC), GC photosynthesis is not very active. Sugars synthesized 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 nucleotide transporters (NTTs) on the chloroplast membrane to boost starch synthesis during the day. At dawn, as MCs begin to synthesize starch and export sucrose, GCs degrade starch into sugars to provide energy and increase turgor pressure for stomata opening. Therefore, the function of GCCs to serve as a starch reserve is important for the opening of stomata. While the MCs fix the 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 malate is produced downstream, is also an important solute in increasing turgor pressure for stomatal opening. In conclusion, GCs behave more like a sink (receiving sugars) than a source tissue (providing sugars). Their function is closely correlated with that of MCs to effectively coordinate CO2 absorption via stomata and CO2 fixing in the MCs.
“I was very excited when Dr. Lim contacted me to ask me to collaborate on this project,” said Dr. Diana Santelia. “We tried to clarify these fundamental questions using molecular genetic approaches. Combining our respective expertise has been a winning strategy,” she continued. Dr. Sheyli LIM, the first author of the paper and former doctoral student in Lim’s group remarked “The in the factory The fluorescence protein sensors we have developed are powerful tools for visualizing dynamic changes in the concentrations of energy molecules in individual plant cells and organelles, allowing us to address some key questions in plant bioenergetics. I am happy to publish our findings in Nature Communication using this new technology.