Few of us would recognize a sorghum field in bloom and fewer still would be able to distinguish a corn seedling from a sorghum seedling. Yet, this old-world crop is slowly transforming our bioenergy economy.
Sorghum is a member of the grass family and a close relative of maize. It traces its origins to Africa, where it has been domesticated for over 10,000 years for food and fodder. Unlike maize, however, the domestication and breeding of sorghum has not been accompanied by the same intensive agricultural practices associated with the green revolution, namely intensive fertilization regimes and irrigation. As a consequence, sorghum is often planted on lands not suitable for growing maize that contain lower nitrogen levels and often in regions with limited rainfall. In the U.S. most sorghum is grown in the western region of the Great Plains (Kansas, Texas, Colorado, Oklahoma and South Dakota) with total acreage (approx. 5 million acres) that still lags far behind maize (approx. 30 million acres) for those same states. Nevertheless, the U.S. is the largest producer of grain sorghum in the world (with many applications for this versatile crop, including bioenergy.
Sorghum is a particularly attractive feedstock for bioenergy in part because of its versatility. There are four main varieties of sorghum—grain, forage, sweets and biomass. Although all of these types are interfertile, each has been bred to meet a specific demand. Grain sorghums produce seeds that are easily digestible for human and animal consumption or can be used for ethanol production in the same way as maize grain. Indeed, up to 30 percent of sorghum produced in the US is used for ethanol production, a similar percentage as maize. Forage sorghums are harvested as feed for livestock. These lines have been bred for digestibility in the ruminant gut and thus, have many of the same attributes that could facilitate their use in a lignocellulosic-based ethanol plant. Namely, the cell walls can be broken down by weak acid hydrolysis and contain less lignin that is typically difficult to break down by enzymatic hydrolysis.
Sweet sorghums accumulate high levels of sucrose in their stems and can be processed in the same mills that press sugarcane to release sucrose. The sucrose is then used directly in the yeast fermentation process, eliminating the need for an enzymatic (amylase) pretreatment that is necessary when starchy corn or sorghum grain is used as the feedstock. By using sugars directly, it is both cheaper and results in a more consistent feedstock to start the fermentation process.
Biomass or bioenergy sorghums are sorghum varieties that are photoperiod sensitive, that is, they will flower only when the days are short and thus, when grown in the summer in the U.S., will not flower until very late in the season. As a result, most of the photosynthate produced is directed toward the vegetative tissues and the plants can grow as high as 30 feet tall in a growing season. Although still expensive at this time, research is centered on developing cost-effective solutions to break down this cell wall (lignocellulosic) material and convert it into simple sugars that can be used to drive the fermentation process.
Importantly, unlike many potential bioenergy grasses (e.g. Miscanthus or switchgrass), there is a solid infrastructure supporting the breeding and distribution of sorghum to growers. As we begin the transition from grain-based (seeds) to lignocellulosic (stems and leaves) ethanol production, it will be important to have a crop with the versatility to be fed into multiple processing streams. For instance, in gulf coast states, sweet sorghums could be rotated with sugarcane plantings to produce high sugar content feedstocks that could be crushed at the sugarmills. As sorghum is an annual crop, breeding for higher yielding sweet varieties occurs at a much faster pace than does sugarcane, which contains a highly complex polyploid genome and is a perennial that is often grown in four to five year cycles, greatly slowing the breeding process. As more lignocellulosic-based ethanol plants come online, forage or biomass sorghums can be planted with the same equipment as grain sorghums, facilitating the transition from food crop to dedicated bioenergy crops with potentially much higher yields.
Recognizing the potential of sorghum as a driver for the new bioenergy economy, the Department of Energy has begun investing heavily in sorghum research. This includes investments by DOE in four major bioenergy centers that collectively will receive approximately $40 million in 2018 alone with plans for five additional years of funding. Additional DOE and DOE/USDA partnerships are continuing to advance research in sorghum-based biofuels to enhance pest/pathogen resistance and to develop feedstocks that use even less water and nitrogen fertilizers to deliver high yields. These critical investments in sorghum research will likely pay dividends in the years ahead, as nearly every climate prediction model is indicating warmer summers with less rain throughout the U.S.
Investing in sustainability
At the Enterprise Rent-a-Car Institute for Renewable Fuels—a research unit within the Danforth Center—scientists are exploring how to improve sorghum bioenergy yields. The ultimate goal is to develop sustainable bioenergy crops that won’t compete with land currently in use for food production, as the demand for both increases with growing global populations and economic development. Through recent investments in new high throughput imaging technologies, computational resources and a vast array of plant growth facilities and greenhouses, the Danforth Center has embarked on a new era of plant systems and synthetic biology. Scientists at the Enterprise Institute are now able to monitor plant growth daily and remotely through sensitive imaging tools that when combined with novel computational approaches track plant biomass accumulation, leaf temperature (a signature of drought stress), and photosynthesis on thousands of plants. Such precision measurements would have taken hundreds of scientists to obtain just a few years ago, and are now automated. These highly resolved phenotypes provide geneticists and breeders with the potential to identify genes that control traits such as photosynthetic capacity, nitrogen and water use efficiency. Knowing which genes controls these traits, immediately helps the breeding process where molecular markers can be designed to these gene and used to accelerate the breeding process. New genome editing tools such as CRISPR/Cas9 are now enabling molecular biologists to precisely engineer the sorghum genome to deliver optimized genes to enhance the productivity and water use efficiencies of sorghum. It is truly an exciting time in plant science— through the convergence of high throughput phenotyping, computational biology and genome editing, the next generation of bioenergy crops are being custom designed to produce higher yields under more sustainable agricultural practices, and sorghum will likely be one of the first crops to benefit.
Thomas P. Brutnell, PhD, is the Director of the Enterprise Institute for Renewable Fuels at the Donald Danforth Plant Science Center. The primary objective of his program is to expand the research portfolio to include the use of model plant systems to accelerate gene discovery and the development of secondgeneration lignocellulosic feedstocks.