Improving Vegetative Biomass Yield and Digestibility in Alfalfa for Enhanced Livestock Production

Background

Alfalfa is the most extensively grown perennial forage crop in Canada, with an estimated global cropping area of approximately 3.8 million hectares. Improving biomass yield of alfalfa has historically been one of the main objectives of alfalfa breeding programs but enhancement of this trait in recent years has been rather limited.

Objectives

To determine how the down-regulation of several candidate genes improves the vegetative productivity and digestibility of alfalfa.

What They Did

Researchers down-regulated/disrupted two genes (MsFTb1 and MsSOC1a) in alfalfa that are known to promote flowering in other plant species, which would allow plants to stay productive longer and therefore increase total biomass produced and potentially also digestibility. They also looked at a gene with potential involvement in pectin synthesis (MsGAUT4a), which was found previously to alter the composition and crosslinking of pectin in plant cell walls and improve the extractability of cell wall sugars in other plant species when down-regulated. Alfalfa plants with down-regulated MsSOC1a and MsGAUT4a were evaluated for changes in growth characteristics, metabolism, degradability, and in the case of MsGAUT4a knockdown plants, carbohydrate composition and linkage. To gain further information about the function of these genes in alfalfa, the development of full knock-out plants with disruptions in MsGAUT4a, MsSOC1a, and MsFTb1 was also initiated.

What They Learned

Researchers identified 3 genes (MsGAUT4a, MsSOC1a, and MsFTb1) in alfalfa that could potentially function as negative regulators of biomass productivity and/or digestibility based on previous research in other plant species. To determine whether these genes have similar functions in alfalfa, plants with reduced expression of MsGAUT4a and MsSOC1a were produced, and numerous characteristics were assessed. MsGAUT4a knock-down plants showed no difference in aboveground or belowground growth or morphology. However, they did display reductions in galacturonic acid, which is one of the main components of the cell wall carbohydrate, pectin. There were also significant changes in the accumulation of certain metabolites and increases in total nitrogen content.

One possible reason why the MsGAUT4a knock-down plants did not show substantial improvements in growth characteristics may be that the extent to which the gene was down-regulated was not substantial enough to elicit these changes. As such, the researchers also successfully generated alfalfa plants with mutations in all four copies of the MsGAUT4a genes so that its function would be knocked out completely, and these plants will be further assessed in the near future.

In contrast to MsGAUT4a, MsSOC1a knock-down plants consistently flowered later, were slightly smaller, had narrower stems, and had shorter distances between leaves. In addition, they also displayed increased primary branching and leaf-to-stem ratios compared to wild-type alfalfa. Furthermore, these plants also exhibited increases in fibre degradability, which suggests that they may have improved digestibility. As such, the generation of soc1a alfalfa knock-out plants has been initiated for further evaluation.

Finally, the researchers also successfully utilized CRISPR/Cas9 technology to produce mutations in all four FTb1 alleles in the first generation. Research is ongoing to further assess the effects of these genes on various traits.

What It Means

Although the down-regulation of MsGAUT4a and MsSOC1a, respectively, did not improve biomass yield in alfalfa, these results suggest that the down-regulation of MsSOC1a could lead to improvements in digestibility. The research team have also initiated the development of alfalfa plants in which MsGAUT4a, MsSOC1a, and MsFTb1, respectively, are fully knocked out to see if they can elicit more substantial changes in biomass production and/or digestibility. These plants can be used downstream for further research in the form of field and feeding trials and have the potential to serve as germplasm for the downstream development of non-transgenic alfalfa with improved degradability.