Modifying crops to increase cell wall digestibility

Authors: Jung, Hans Joachim; Samac, Deborah - Debby; Sarath, Gautam

Submitted to: Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2011
Publication Date: 4/1/2012
Citation: Jung, H.G., Samac, D.A., Sarath, G. 2012. Modifying crops to increase cell wall digestibility. Plant Science. 185-186:65-77.

Interpretive Summary: Forages and annual crop residues are roughage feeds used in cattle production worldwide. Because these feeds have high concentrations of poorly digested fiber, genetic improvement of these crops is of great importance. Increasingly, biotechnological approaches are being used in crop improvement. The scientific literature was reviewed for reported progress to date and to identify new targets for genetically improving the fiber digestibility of roughages using biotechnology. The concentration of lignin, an inhibitor of fiber digestion, has been successfully reduced in several legume and grass crops; however, the greatest improvements in fiber digestibility were associated with poor plant fitness. Modifying carbohydrate composition of roughages has been less investigated and no positive impacts on fiber digestion were found. Increasing the percentage in stems of plant tissues that contain little lignin or limiting reductions in lignin content to specific stages of plant development offer possible improvements in fiber digestibility that may avoid the negative impacts of less lignin on plant viability. This review will be of benefit to plant biotechnologists in choosing targets for genetic modification to improve the fiber digestibility of roughages by livestock.

Technical Abstract: Improving digestibility of roughage cell walls will improve ruminant animal performance and reduce loss of nutrients to the environment. The main digestibility impediment for dicotyledonous plants are highly lignified secondary cell walls, notably in stem secondary xylem, which become almost non-digestible, whereas in grasses digestibility is slowed severely by lignification of most tissues, but these cell walls remain largely digestible. Cell wall lignification creates an access barrier to potentially digestible wall material by rumen bacteria if cells have not been physically ruptured. Traditional breeding has focused on increasing total dry matter digestibility rather than cell wall digestibility, which has resulted in minimal reductions in cell wall lignification. Transgenic approaches down-regulating genes in monolignol synthesis have produced plants with reduced lignin content and improved cell wall digestibility; however, major reductions in lignin concentration have been associated with poor plant fitness. Additional targets for genetic modification to enhance digestibility and improve roughages for use as biofuel feedstocks are discussed including novel lignin structures, reduced lignin/polysaccharide cross-linking, smaller lignin polymers, enhanced development of non-lignified tissues, and targeting specific cell types for lignin down-regulation. Greater tissue specificity of transgene expression will be needed to maximize benefits while avoiding negative impacts on plant fitness.