Plants take in carbon dioxide (CO2) from the environment for photosynthesis through microscopic breathing pores called stomata. Also, water vapor is released into the atmosphere through these pores in a process called transpiration. Stomata can tighten to save water when CO2 is abundant, but researchers didn't know how that worked until now. Julian Schroeder and colleagues from the University of California in San Diego pinpointed the enzymes responsible for stomatal response to CO2. Enzymes that react with CO2 cause cells surrounding the opening of the pores to close down, the team reports in the current issue of Nature Cell Biology.

Schroeder and his team identified a pair of proteins that are required for the CO2 response in Arabidopsis. The proteins, enzymes called carbonic anhydrases, split CO2 into bicarbonate and protons. Arabidopsis mutants lacking the carbonic anhydrase genes βCA1 and βCA4 showed impaired response to increased CO2 concentrations in the atmosphere. On the other hand, plants that overexpress these genes showed enhanced water use efficiency.

Several types of plant cells contain carbonic anhydrases, including mesophyll cells which are responsible for photosynthesis. Schroeder and colleagues were able to show that the enzymes work directly within a pair of cells, called guard cells, that control stomatal opening. Introduction of carbonic anhydrase genes designed to work only in guard cells restored the CO2-triggered pore-tightening response in mutant plants.

The researchers believe that the enzymes could be targeted to make crops "water-wise" and more responsive to CO2. "It could be that with these enzymes, you can improve how efficiently plants use water, while taking in CO2 for photosynthesis. Our data in the lab suggest that the CO2 response can be cranked up," Schroeder said.

Read the original story http://ucsdnews.ucsd.edu/newsrel/science/12-09Stomata.asp The complete paper published by Nature Cell Biology is available to subscribers at http://dx.doi.org/10.1038/ncb2009