Seeking comfort from the cold

Scientists at the University of Arizona have discovered a critical cold-tolerance gene in Arabidopsis. As published in the April 15th issue of Genes & Development, the identification of ICE1 by Dr. Jian-Kang Zhu and colleagues holds promising implications for the improvement of cold tolerance in agriculturally important crops.

Cold temperature is one of the major factors affecting crop yield in temperate climates, with the farming industry loosing billions of dollars each year to freezing temperatures. Much research has focused on ways to improve crops’ tolerance to cold and/or freezing temperatures, with the aim to both increase productivity and broaden geographical range.

In 1988, scientists identified the Arabidopsis CBF family of transcription factors. CBF proteins regulate the expression of cold-responsive genes in Arabidopsis, which enable the plant to acclimate to, and survive in, cold temperatures.

As reported in their current G&D paper, Dr. Zhu and colleagues have now discovered a key transcriptional regulator of CBF genes – a marked advance in the research effort to understand and ultimately improve cold tolerance in plants.

To identify genes act upon CBF genes and affect cold tolerance in plants, Dr. Zhu and colleagues carried out a genetic screen with Arabidopsis plants that were genetically engineered to glow in the cold. The researchers inserted a luciferase/CBF3 transgene (a recombinant DNA molecule containing the firefly luciferase gene under the control of the CBF3 gene regulatory region) into the Arabidopsis genome, in order to generate plants that bioluminesce under cold stress. These cold-responsive bioluminescent plants were mutagenized, and plants that no longer glowed in cold temperatures were selected.

One particularly striking mutant exhibited ten times less luminescence after 12 hours at 0ºC than the wild-type bioluminescent plants. Dr. Zhu and colleagues cloned the gene that had been mutated in this plant, and named it ICE1 (inducer of CBF expression). Further research by the group revealed that ICE1 is also a transcription factor: During periods of cold stress, ICE1 binds to and turns on the CBF3 gene, which, in turn, induces the expression of cold-responsive genes. Using microarray analysis, Dr. Zhu and colleagues demonstrated that in ICE1-mutant plants, over 70% of cold-responsive genes are misregulated, causing the plants to exhibit severely reduced cold tolerance.

Dr. Zhu and colleagues also demonstrated that the increased expression of ICE1 in Arabidopsis plants leads to increased cold tolerance. This result is expected to garner significant attention from the agricultural community, as the transgenic expression of ICE1 in domesticated, cold-sensitive crops — like soybeans, tomatoes, potatoes, rice and barley – may provide a new way to increase the ability of such plants to survive in the cold.

As Dr. Zhu explains, “The significance of our findings on ICE1 may be two-fold: It is likely useful for the genetic improvement of plant freezing tolerance, and the identification of ICE1 takes us one step closer to address the question how cold signals are sensed and transduced.”