This image is an extreme closeup of a stomate (singular, the plural form is stomata). These two cells, called guard cells, control the plant’s respiration: how much carbon dioxide gets in and how much oxygen and water vapor gets out. The control isn’t very good, though. Most plants just have their stomata open all day every day so they can pull in lots of CO2 to use during photosynthesis to make sugar. And that means a lot of water, painstakingly pulled up from the soil, through the roots, gets lost. If stomata could be more selective, only opening when more CO2 was needed for photosynthesis, then water could be conserved.
An enzyme called carbonic anhydrase raises the levels of CO2 in chloroplasts so the plant can make plenty of sugar. It does this by converting CO2 from its storage form carbonic acid back to it’s useable form: CO2 + H2O ⇌ H2CO3.
Carbonic anhydrase also appears in the guard cells, where it controls the opening and closing of stomata.
Julian Schroeder, Professor of Biology at UC, San Diego hypothesized that more carbonic anhydrase in the guard cells would place tighter control over opening and closing. His group tried shutting off the carbonic anhydrase gene in the stomata of a little plant called Arabidopsis. Those plants were unable to respond to increased CO2 concentrations in the air, remaining open all day. They also tried expressing additional copies of the carbonic anhydrase gene in the stomata. Those plants closed their stomata when water was scarce. This makes sense – carbonic anhdrase needs water to function, so it can’t function when water’s not around.
Honghong Hu, a postdoctoral research working on the project, said in the press release Newly Identified Enzymes Help Plants Sense and Respond to Elevated Carbon Dioxide and Could Lead to Water-wise Crops: “The guard cells respond to CO2 more vigorously. For every molecule of CO2 they take in, they lose 44 percent less water.”
This research, Carbonic anhydrases are upstream regulators of CO2-controlled stomatal movements in guard cells, published in January 2010, indicates that increasing the number of carbonic anhydrase genes in the stomata could potentially decrease the water lost through stomata in crops. The implications for drought prone regions are obvious. Plants could need less water and could hold on to the water they have longer. It won’t be plug and play, though. As stated in the press release, water that evaporates from stomata cools the plants just like water evaporating from our pores cools us. Increased expression of carbonic anhydrase will have to be tested to determine its effects on plants in high temperature environments.
Hu H, Boisson-Dernier A, Israelsson-Nordström M, Böhmer M, Xue S, Ries A, Godoski J, Kuhn JM, & Schroeder JI (2010). Carbonic anhydrases are upstream regulators of CO2-controlled stomatal movements in guard cells. Nature cell biology, 12 (1) PMID: 20010812
Thanks to @RivenCactus for bringing this research to my attention by Tweeting a link to the TreeHugger article Newly Discovered Enzyme Could Create Crops That Thrive in Dry, High CO2 Conditions.
If a gene like this was used to make crops more drought tolerant, could it spread to weeds and make weeds weedier?
Yes and no.
If there was a sexually compatible wild relative or weed species growing nearby the drought tolerant crop, it is possible that weed/crop hybrids could include the gene. Sexual compatibility means that the weed not only has to be a fairly close relative to the crop but also means that they have to be pollinated by the same method, have pollen shed at the same time, not have any incompatibility genes, etc. In the United States, there are few weed species that are sexually compatible with crop species, but there are some. In these cases, farmers can use the same sort of strategies to reduce gene flow that they would use to avoid spread of a conventionally bred trait.
If gene flow does happen, the gene will only be present in the weed population at low levels, unless the gene makes the weeds that have it able to outcompete weeds that don’t have it. See Escape! Crop-Specific Gene Flow to Wild Relatives and Those naughty plants! on Biofortified for more discussion of gene flow.