Genome of sorghum plant sequenced

Published in The Hindu on February 5, 2009

Sequencing has identified the location of the genes responsible for efficient photosynthesis. The difference between rice and sorghum genome is not much but their functions have changed. — Photo: Wikimedia Commons

With the genome of sorghum sequenced, scientists may soon be able to tell which genes or family of genes make sorghum drought-tolerant. Sorghum is the second plant after rice in the grass family to have its genome sequenced.

The results are published in the latest issue of journal Nature.

There are three main reasons why scientists started sequencing sorghum. First, the genome of sorghum is much smaller compared with other grass plants such as sugarcane, maize, wheat etc. Secondly, sorghum is well known for its drought tolerance. Thirdly, the high gene flow to weedy relatives has been a big problem for transgenic (genetic engineering) approaches. So knowing the “intrinsic genetic potential” becomes all the more important. Finally, sorghum is a good candidate for extracting biofuels.

But will the possibility of extracting biofuels from sorghum not lead to diversion of the food crop for biofuel production? “There will be no diversion,” said Dr. Tom Hash.

Dr. Hash is a Principal Scientist at the Hyderabad based International Crops Research Institute for the Semi-Arid Tropics (ICRISAT). The ICRISAT is one of the institutions involved in sorghum genome sequencing and data analysis work.

According to him, the grains of sweet sorghum would be first extracted. The stem would then be crushed and the sweet juice would go to produce a molasses like product. Biofuel would be produced from the molasses.

“The crushed stem can be used as a livestock feed. So producing biofuel is only a by-product of livestock feed production,” Dr. Hash said.

The findings

The total number of genes in rice and sorghum is about the same at 30,000. The sorghum genes occupy only 15-20 per cent of the genome sequence.

“The difference between rice and sorghum genome is not much,” he said.

Yet, while sorghum is a drought tolerant plant, rice needs water-logged conditions to grow. “The function of genes in sorghum and rice has changed. The probable reason could be that sorghum evolved in more stressful drought conditions. This may have contributed to what forms of genes were favoured,” said Dr. Hash.

Another noticeable difference is the vulnerability of the two plants to rust disease. While sorghum and other grass plants suffer from rust disease, rice does not. “Somehow rice has lost the gene that makes other plants vulnerable to rust disease,” he said. “So the loss of this gene has actually helped rice.”

While the genes responsible for more efficient photosynthesis system — fixing more carbon even at high temperature and low water availability — are already known, the genome sequencing has helped the scientists to actually identify the location of these genes.

Scientists in the U.S. and Philippines are already trying to introduce such genes into rice. If successful, it will make rice capable of growing at high temperature and low water availability conditions, which will be prevalent in the future as a result of global warming.

“It is theoretically possible to introduce the genes that are responsible for drought tolerance into rice if we are able to identify them,” Dr. Hash said.

For instance, in the case of castor oil producing plant, the genes and pathways by which (high temperature) fatty acid that does not breakdown at high temperature have been identified. Scientists are now trying to introduce these into other oil producing plants to get that particular trait. The research is still at the laboratory stage, though.

Identifying markers

With the genome sequenced, one of the first priorities for the scientists will be to look out for markers — unique pieces in the DNA sequence — that will help identify desirable genes.

They will actually be looking for unique pieces in the DNA that will mark the beginning of a sequence that contains a desirable gene and another that marks the end.

So if a desirable gene has to be introduced into another plant, scientists would look out for the sequence that contains the two markers signalling the beginning and the end.