Increased carbon dioxide channels in plant membranes have little effect on photosynthesis.

In the face of climate change and other environmental issues, modifying photosynthesis has become a research target to boost crop yields and feed a growing worldwide population. A team from the Australian National University (ANU) explored the consequences of increasing the number of carbon dioxide channels in plant membranes in a recent study published in the Journal of Experimental Botany, but found no influence on photosynthesis in model tobacco plants.

Carbon dioxide (CO2) is transported to the chloroplasts within leaf cells, where it is converted into sugars by the enzyme Rubisco. CO2 must diffuse into the leaf and through the leaf mesophyll cells to reach the chloroplast, crossing barriers such as cell walls and membranes. Increased CO2 diffusion from mesophyll cells into the chloroplast (also known as mesophyll conductance) can promote photosynthesis-boosting yields in crops while simultaneously reducing water usage.

"We wanted to see if we could improve CO2 transfer efficiency by adding new channels for CO2 diffusion into cell membranes," said RIPE researcher Dr Tory Clarke, who conducted the research at ANU.

The ANU team raised the number of aquaporin proteins in the plasma membranes of test tobacco plants to target CO2 transfer across plant cell membranes.

Dr. Michael Groszmann, a senior author, noted, "Aquaporins are membrane channels that allow molecules like water and gases to pass through them more easily. The channels are found in the leaf cell plasma membrane, according to our findings."

Previous research has shown that a subset of plant aquaporins called Plasma-membrane Intrinsic Proteins (PIPs) may transfer CO2 in test settings, but there have been conflicting findings about their significance in plant mesophyll conductance. "In our investigation, we were able to inject extra PIP aquaporin channels into the membrane of the mesophyll cell," Clarke added. "However, there was no discernible influence on CO2 conductance through the mesophyll cell or photosynthetic rates."

"Plant growth and environmental conditions may have a substantial influence in aquaporins' ability to change mesophyll conductance," said Susanne von Caemmerer, an Australian National University Research School of Biology Professor of Molecular Plant Physiology who co-led the work alongside Groszmann. "In addition, we used computer modelling to estimate how variations in CO2 permeability in the membrane might affect overall mesophyll conductance. We discovered that increasing the quantity of CO2 that must pass the plant cell membrane by 20% would boost overall mesophyll conductance by 20% "twice."

While improved photosynthesis was not observed in this study, it did contribute to a better knowledge of CO2 transport from the atmosphere to the chloroplast.

"With what we've discovered in this study, we can now concentrate our efforts on learning more about aquaporin function and how we may increase mesophyll conductance and photosynthesis," Groszmann said.