Engineering more digestible grass could reduce livestock pastures and give us cheaper biofuels

When you look at livestock through the lens of environmental sustainability, one of the biggest drawbacks is the sheer volume of grass these animals eat before finding their way onto our dinner plates.

In a world with unlimited grazing land, this might not be such a bad thing. But on a planet with finite land resources, this need for grass creates a stumbling block in our efforts to meet an ever-growing demand for meat.

But what if we could figure out a way to help cows and other animals get more out of the grass they eat? Recent research suggests we may be headed that direction, with scientists discovering a gene which can be turned down to make grass more digestible – and more efficient.

Grasses owe their evolutionary success partly to sturdy cell walls that make them difficult to digest. This deters many herbivores, and poses problems even for those animals which are adapted to eat it. Unlike humans, cows and sheep have stomachs that allow them to graze on grass. Still, they can’t release all of the grass’s energy. Much of a plant’s calorific value is tied up in a robust cell wall, making it inaccessible to livestock.

More digestible grass would make it possible to raise cows on smaller pastures, reducing the pressure put on the land by beef and dairy production. It also has the potential to boost bioenergy efforts. Crops such as maize, sugarcane and rice have those same robust cell walls, and this makes them difficult to process into biofuels.

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Trailers filled with sugarcane sit at Biosev SA’s Santa Elisa mill in Sertaozinho, Brazil, in 2015, ready to be converted to ethanol. | Paulo Fridman/Bloomberg

The parts of the sugarcane plant which aren’t made into sugar are a perfect source of fuel, yet it takes large amounts of energy to turn them into bioethanol. They already are used in Brazil, where they’re given a high energy “pre-treatment” followed by treatment with enzymes to release sugars from the cell walls. These sugars are then fermented to make bioethanol. In theory, more digestible biomass should lower the amount of energy and enzymes needed, making bioethanol production both cheaper and more environmentally friendly.

Previous efforts to find more digestible grasses have generally had little success. Sometimes the methods have been quite simple, such as screening plants to identify those with high digestibility. Others have worked on suppressing genes. One crop has been brought to market, brown-midrib maize, in which a natural mutation decreases lignin production. It attracts a premium in the US as an animal feed, but the mutation has the side-effect of reduced yield.

New research findings

The new research, released recently in New Phytologist, identifies a key gene involved in the stiffening of grass cell walls, and demonstrates that suppressing it can increase the release of sugars, making the grass more digestible.

Scientists have known for a long time that ferulic acid contributes to strong grass cell walls. This small molecule creates cross-links in the cell wall, making them stronger. It has been hard to identify the gene responsible for adding ferulic acid to the cell walls, which encodes an enzyme, although a collaboration between researchers in the UK, Brazil and the US has now had success. The project was led by Dr. Rowan Mitchell from Rothamsted Research in the UK and Dr. Hugo Molinari at the Laboratory of Genetics and Biotechnology at Embrapa Agroenergy, part of the Brazilian Agricultural Research Corporation (Embrapa).

Mitchell first identified a possible gene in 2007. He compared the fully sequenced genomes of rice, a grass and Arabidopsis, a non-grass commonly used as a model organism in the lab. By looking for a gene that was highly expressed in grasses but not in non-grasses, and judging that it was the right type of gene based on the sequence of the protein it encodes, he came up with a likely candidate.

Demonstrating its role in the lab, however, has proved difficult. In the 10 years since the discovery, the team has tried various approaches, and were finally successful using RNA interference (RNAi). Using genetic modification techniques, they added a gene which suppresses the target gene to around 20 percent of normal activity.

This increased the release of sugars by up to 60 percent and makes it a promising target for improving grass crops for both bioenergy and animal feed.

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Steps towards commercialization

The new results come from a model grass species that’s easy to work with in the lab. The next step is transferring the gene to crops which could be used for feed or biofuel. The possible targets include sugarcane, maize, rice and various pasture grasses. There are some scientific challenges to overcome.

In the current study, the suppression had no obvious effect on the plants’ biomass production or appearance. There were, however, some unwanted changes. For example, the seeds were smaller. This isn't surprising given the importance of ferulic acid in the cell wall, but side effects such as these need to be overcome. Seed size may not be so important for pasture grass, but for crops where the seeds become food it would be unacceptable to growers. The problem of small seeds could possibly be overcome by ensuring the transgene is only expressed in the leaves and stems. This is more complex, particularly if non-GM approaches are tried.

Challenges will also come if more genes are involved in the process in other species. The current study’s results came from Setaria, a grass which is used as a model organism in the laboratory. The work was also done in a second model species, with less striking results, possibly because there are other genes with the same function which haven’t been supressed.

The Brazilian team at Embrapa Agroenergy already has transferred the gene to sugarcane, and they anticipate the first product to reach the market from this project will be a GM sugarcane variety. However, the difficulties of using genetic modification techniques on certain species means that GM approaches won’t be suitable for all crops. Combined with the negative reaction to GM in many countries, this has prompted the team to look for alternative approaches.

They plan to search for natural variants of the gene in tropical forage grasses by looking at gene sequences. This would allow them to introduce these genes by cross-breeding with the tropical grasses with pasture grasses. However, it is likely that a more complex solution will be needed. There next step is to modify the gene using CRISPR gene editing technology, although we don’t yet know whether or not this will be regulated as GM in some parts of the world, particularly the EU.

The journey towards commercialisation may be a long one. The researchers will need to confirm the crops are more digestible and overcome any negative effects on the plant. If it is successful though, there is a significant potential market for the economic and environmental benefit the crops could bring.

Rebecca Nesbit is the author of Is that Fish in your Tomato? a popular science book covering the fact and fiction of genetically modified food. You can follow her on Twitter as @RebeccaNesbit.


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