Genetic Improvement Of Switchgrass By Transgenic Approaches

Switchgrass (Panicum virgatum L.) has been developed into a dedicated herbaceous bioenergy crop. We have developed a highly efficient transformation system for the widely used switchgrass cultivar Alamo by identifying tissue culture responsive genotypes and optimizing transformation parameters. The transformation efficiency of this system (number of independent transgenic plants/number of calli used for infection) exceeds 90%. We take transgenic approaches to genetically modify the quality or quantity of switchgrass biomass suitable for biofuel production. Recalcitrance to saccharification is one of the major limitations for conversion of lignocellulosic biomass into ethanol. Major lignin biosynthetic genes were isolated from switchgrass and transgenic plants carrying RNAi constructs were produced. Reduced enzyme activity, decreased lignin content or alerted ratios of S/G were found in different transgenic lines. The down-regulation of lignin biosynthesis with certain transgenes resulted in improved sugar recovery and ethanol yield. Biomass yield is a major target trait for genetic improvement of switchgrass. A miR156b precursor was overexpressed in switchgrass. miR156 controls apical dominance and floral transition in switchgrass by suppressing its target SPL genes. Relatively low levels of miR156 overexpression were sufficient to increase biomass yield while producing phenotypically normal plants. Moderate levels of miR156 led to improved biomass but the plants were non-flowering. These two groups of plants produced 58-101% more biomass yield compared with the control. A standardized protocol for sample collection for cell wall and biomass analyses was developed to maintain consistency in switchgrass evaluation methods and to enable comparisons of data obtained from different studies.