Mapping sorghum's genome to create robust biomass crops

The University of Georgia's Center for Agribusiness and Economic Development is part of a growing effort in the U.S. to study sorghum as a biomass crop for second-generation cellulosic biofuels (previous post).

One of the goals is to turn the tropical crop into a perennial. Currently, sorghums are produced from seeds, which means the plants have to be harvested each season and then sown again. Making the crop a semi-perennial would make it a more suitable biomass crop for the production of cellulosic biofuels. Coming back year after year, perennials aren't as prone as annuals to causing erosion - a growing problem in the U.S. Moreover, perennials simply grow by themselves after harvest, thus cutting out the (relatively expensive) sowing process.

This transition from annual to perennial has already happened with sorghum's cousin, sugarcane. The great success of this crop is partly due to the fact that it can be grown and regrown sustainably in the same soils for decades. In some cases, sugarcane has been replanted for over 150 years at the same place without a decline in yields.

There are numerous widely-used species of sorghum, which is in fact a genus of grass species: grain sorghums are cultivated for their seeds and can readily replace corn for the production of 'first generation' ethanol. Compared to corn, sorghum needs about half as much water. Other sorghums are grown for silage, whereas sweet sorghums yield sugar-rich stalks, which are crushed, like sugarcane, to obtain a juice which can be fermented into ethanol. Finally, there are (drought-tolerant) hybrids that deliver both grains (for food), sweet stalks (for sugar and/or ethanol), and biomass for fodder. These hybrids are the focus of pro-poor biofuels initiatives in the developing world (earlier post).

Sorghum's genome
The photosynthetic efficiency of sorghum and its drought-tolerance has made the crop an interesting biofuel candidate. As a model for the tropical grasses sorghum is representative of the highly efficient "C4" photosynthesis process, using a complex combination of biochemical and morphological specializations resulting in more efficient carbon assimilation at high temperatures.

According to University of Georgia scientist Andrew Paterson, the efficient crop can easily make the transition from seed-based to whole-plant-based biofuels. The scientist is a distinguished research professor in the UGA College of Agricultural and Environmental Sciences' crop and soil science department, and director of the UGA Plant Genome Mapping Laboratory.

Paterson has spent 15 years studying sorghum's genetic blueprint. He's hoping now to find answers: why is the plant more drought-resistant than corn? How did it get its genetic makeup? What genes give certain plants height and others disease resistance?

Recently, the U.S. Department of Energy completed the sequencing of sorghum at its Joint Genome Institute (JGI), collaborating with Paterson's lab and several others (amongst them the ICRISAT, which developed the hybrid we have often discussed). The JGI is actively screening a large number of potentially interesting biofuel crops - from grass species and trees, to tropical crops like cassava and now sorghum (more here). Mapping the sorghum genome happened much more quickly than expected:
biofuels :: energy :: sustainability :: biomass :: bioenergy :: ethanol :: cellulose :: genomics :: sorghum ::

The sorghum sequence will be a valuable reference for assembling and analyzing the fourfold larger genome of maize (corn), a tropical grass that is the leading U.S. fuel ethanol crop (sorghum is second). Sorghum is an even closer relative of sugarcane, arguably the most important biomass/biofuels crop worldwide with annual production of about 140 million metric tons and a net value of about $30 billion. Sorghum and sugarcane are thought to have shared a common ancestor about 5 million years ago. The two have retained largely common gene order, and some genotypes can still be intercrossed.

The sorghum genus is also noteworthy in that it includes one of the world’s most noxious weeds. The same features that make "Johnson grass" (Sorghum halepense) such a troublesome weed are actually desirable in many forage, turf, and biomass crops that are genetically complex. Therefore, sorghum offers novel learning opportunities relevant to weed biology as well as to improvement of a wide range of other forage, turf, and biomass crops.

Thanks to a fellowship from the John Simon Guggenheim Memorial Foundation, Paterson can now examine sorghum's 740 million bases more thoroughly. At 740 million letters of DNA, sorghum has a genetic code roughly a quarter the size of the human genome.

Through computer models, the researcher will be able to deduce what the sorghum gene set looks like. And he can build hypotheses on why sorghum has certain traits such as height, flowering and disease resistance that can be tested in the field.

Sorghum is important now as a promising biomass crop, Paterson says. As the demand for biofuels increases, understanding the plant's building blocks grows in importance, especially as scientists look at moving from seed-based to plant-based biofuels. Several cooperation agreements have been signed between research organisations and the private sector with the aim of studying sorghum for this future role (earlier post).

In Europe, the crop is being analysed as a potential feedstock for the production of biogas. In Germany, scientists have created and planted a collection of 160 sorghum varieties from Asia and Africa to see whether the drought tolerant crop can be cultivated in the arid regions of the country during the dry season (earlier post and here). The Northsea Bioenergy Partnership is developing Sudan grass and sorghums (and their hybrids) for the same purpose (see here).

Image: Sorghums from the U.S. Department of Agriculture's Agricultural Research Service National Sorghum Germplasm Collection. Credit: Peggy Greb/USDA

References:
University of Georgia, Center for Agribusiness and Economic Development: UGA scientist sleuthing secrets of sorghum traits - June 7.

U.S. Department of Energy Joint Genome Institute: Why Sequence Sorghum?