by Mirjam Marti, Checkbiotech

The world's population is growing rapidly and is estimated to reach 8.9 billions by 2025. But alone today there are approximately 852 million undernourished people. So one of the most important goals for society is to provide enough food for all. By 2025 the global crop yield needs to increase by 25 percent.

Cereals are an important nutrition source for humans and livestock. The three main cereals are rice (23 percent), wheat (17 percent) and maize (10 percent).

However, rice is not only of great global importance, but it also is a model organism for cereals. It has the smallest genome of the main cereals, it shares many similar genomic regions with other cereals and it can be easily transformed. Therefore there are many genetic markers known and different mutants available.

In 2002, the rice genome was completely mapped. This makes rice an interesting object for research and resulted in the further development of products such as Golden Rice, a rice species that is genetically modified to produce vitamin A.

A group of Japanese and Chinese researchers, headed by Dr. Motoyuki Ashikari from the Bioscience and Biotechnology Center of Nagoya University and Dr. Hitoshi Sakakibara of the Plant Science Center in Yokohama, searched for means to increase the yield of rice.

The research group also included scientists from the Honda Research Institute and the China National Rice Research Institute. They recently published their results in Science under the title "Cytokinin Oxidase Regulates Rice Grain Production."

Agriculturally important traits such as growth height or grain number are often ruled by a number of genes located on quantitative trait loci (QTLs). A bigger yield can be achieved by increasing number of the grains or by producing taller plants. Taller plants, however, are more sensitive to weather. Therefore economically desirable plants are small and have many grains.

The group led by Dr. Ashikari and Dr. Sakakibara focused on the QTLs for plant growth and grain number. To run a QTL analysis, the researchers used two rice varieties. One was short with many grains and one was tall with few grains. By crossing those two varieties they managed to identify five QTLs concerning grain number (Gn) and four concerning plant height (Ph).

Next, the most effective QTLs - Gn1 and Ph1 - were chosen for further research. From their work, the group succeeded in identifying the two main genes of these QTLs, a gene called semi-dwarf 1 (sd1) and another called OxCKX2.

When inactivated sd1 decreases the plant height about 20 percent. OsCKX2 encodes the enzyme Cytokinin Oxidase. If this enzyme loses its function the grain yield is increased by about 44 percent. Comparisons with today's rice varieties helped to verify those discoveries. If both genes are shut down, the rice variety produces 23 percent more grains than a normal plant. The increase of grain yield caused by the inactivation of OsCKX2 compensates for the loss of yield due to a smaller plant from the inactivation of sd1.

Dr. Ashikari's laboratory hopes the results of their research will contribute to breeding. Their study helps to understand the function of some important rice genes, while also shedding light on some basic mechanisms of rice metabolism. Other researchers will be able to use this information.

Dr. Ashikari told Checkbiotech, "This time, we are trying to clarify the mechanism of grain yielding. Thanks to progress of genomics with rice, many important genes will appear soon. We hope our results apply to other cereals as well."

At the moment, the group is cloning many other important agricultural traits. They are specially focusing on yield traits, such as grain number and panicle length. They are also checking the field traits including taste or negative side effects. This will take some time, however.

Now the scientists and their sponsor (Honda) are breeding rice using these results. "We are thinking both, traditional breeding and a genetic engineering approach are necessary, because Golden Rice could not have been produced by traditional approach," Dr. Ashikari told Checkbiotech about their breeding project.

"We are not concerned about GMO [genetically modified organisms]. It will be definitely necessary in the future. But scientists have to explain that it is safe to use."

Cytokinin Oxidase Regulates Rice Grain Production
Motoyuki Ashikari et al.
Science, Vol. 309, 29 July 2005

Mirjam Marti is a Biology student at the University of Zurich and a Science Writer for Checkbiotech.