Breakthrough in plant science: gene discovery provides new tool to develop drought-tolerant crops

Research groups of the Department of Biological and Environmental Sciences of the University of Helsinki and the University of California in San Diego, together with a team from Japan, have made an important plant research breakthrough: they discovered a key gene centrally involved in the regulation of carbon dioxide uptake for photosynthesis and water evaporation in plants. The discovery can aid the development of drought-tolerant crops or plants that store more CO2 and could play a significant role in the emerging field of bioenergy and biofuels. The breakthrough is reported in two articles published online ahead of print in Nature.

Stomata are tiny pores on the plant leaf surface, through which the leaves absorb carbon dioxide necessary for photosynthesis and release moisture into the air. The plasma membranes of the guard cells that surround the stomatal pore contain several types of ion channels which control the opening and closing of the circular guard cells when the plant encounters a stressful situation, such as increased ozone in the air or drought.

The regulation of stomata is an intensively-studied topic and several ion channel types that control their activity have been discovered earlier. However, an anion channel, which is of central importance in the regulation of stomatal activity, was identified only recently by Finnish and American scientists. A measuring device developed at the University of Tartu, Estonia, was of great help in the process.

Professor Jaakko Kangasjärvi and his research group from the University of Helsinki identified the anion channel using an ozone-sensitive mutation of Arabidopsis thaliana commonly known as thale cress. The mutant, called slac1, does not react by closing its stomata as a response to high ozone or carbon dioxide concentration in the air like a healthy plant does. Scientist at the University of California were then able to demonstrate with electrophysiological measurements that the gene identified effectively encodes an anion channel involved in the regulation of stomatal activities.

When the mutant plant is exposed to ozone, the leaves lose their dark green color and eventually become white. This is because the stomatal pores in the leaves stay open even in the presence of high ozone and are unable to protect the plant. - Professor Jaakko Kangasjärvi, lead author

The scientists named the gene which mediates CO2 sensitivity in the regulation of plant gas exchange SLAC1 (SLOW ANION CHANNEL-1). The SLAC1 protein is a distant homologue of bacterial and fungal C4-dicarboxylate transporters, and is localized specifically to the plasma membrane of guard cells.

SLAC1 is of central importance for the mechanisms of stomatal regulation. Unlike the ion channels detected previously, this newly discovered anion channel takes part in the regulation of all the main stomatal activities:
energy :: sustainability :: biomass :: bioenergy :: biofuels :: plant science :: carbon dioxide :: photosynthesis :: respiration :: transpiration :: climate change :: drought-tolerance :: biotechnology ::

Climate change makes it all the more important to know about the mechanisms involved in stomata regulation. Aridity is on the increase across the globe, as is the world population. Increasingly dry areas should be taken into cultivation to ensure food and fuel production. When developing crops that thrive in dry areas, it is important to know well the mechanisms that regulate stomata, through which plants evaporate moisture.

The effects of climate change, which increases atmospheric ozone and carbon dioxide concentrations, cause a new challenge for plants. Plants protect themselves against high ozone by closing the stomata on their leaves. While this protection mechanism minimises damage to the plant, it also reduces carbon dioxide uptake for photosynthesis and thus could reduce the sequestering of the excess atmospheric carbon in plant material.

Plants under drought stress will lose 95 percent of their water through evaporation through stomatal pores, and the anion channel is a central control mechanism that mediates stomatal closing, which reduces plant water loss. - Professor Julian Schroeder, biological sciences UC San Diego, author

A different kind of plant, however, could grow better in the new conditions. This discovery will provide a new tool for geneticists in the development of climate resilient plants.

Droughts, elevated ozone levels and other environmental stresses can impact crop yields. This work gives us a clearer picture of how plants respond to these kinds of stresses and could lead to new ways to increase their resistance. - Jean Chin, membrane protein grants overseer at the National Institute of General Medical Sciences

Because the opening and closing of stomatal pores also regulates water loss from plants, understanding the genetic and biochemical mechanisms that control the guard cells during closing of the stomatal pores in response to stress can have important applications for agricultural scientists seeking to genetically engineer crops and other plants capable of withstanding severe droughts.

We now finally have genetic evidence [...] and the gene to work with. - Professor Schroeder

The study was financed by grants from the National Science Foundation and the National Institute of General Medical Sciences.


Image 1: confocal picture of an Arabidopsis stoma showing two guard cells exhibiting green fluorescent protein and native chloroplast (red) fluorescence.

Image 2: Colored guard cells surround a stomatal pore. Credit: UC San Diego.

References:
Triin Vahisalu, Hannes Kollist, Yong-Fei Wang, Noriyuki Nishimura, Wai-Yin Chan, Gabriel Valerio, Airi Lamminmäki, Mikael Brosché, Heino Moldau, Radhika Desikan, Julian I. Schroeder & Jaakko Kangasjärvi, "SLAC1 is required for plant guard cell S-type anion channel function in stomatal signalling", Nature advance online publication 27 February 2008 | doi:10.1038/nature06608

Juntaro Negi, Osamu Matsuda, Takashi Nagasawa, Yasuhiro Oba, Hideyuki Takahashi, Maki Kawai-Yamada, Hirofumi Uchimiya, Mimi Hashimoto & Koh Iba, "CO2 regulator SLAC1 and its homologues are essential for anion homeostasis in plant cells", Nature advance online publication 27 February 2008 | doi:10.1038/nature06720

Eurekalert: Breakthrough in plant research - February 27, 2008.

UC San Diego: Gene That Controls Ozone Resistance of Plants Could Lead to Drought-Resistant Crops - February 27, 2008.