Maria N. Somleva

Production of polyhydroxybutyrate in switchgrass, a value-added co-product in an important lignocellulosic biomass crop.

SUMMARY Polyhydroxyalkanoate bio-based plastics made from renewable resources can reduce petroleum consumption and decrease plastic waste disposal issues as they are inherently biodegradable in soil, compost and marine environments. In this paper, the successful engineering of the biomass crop switchgrass (Panicum virgatum L.) for the synthesis of polyhydroxybutyrate (PHB) is reported. Polymer production was monitored in more than 400 primary transformants grown under in vitro and glasshouse conditions. Plants containing up to 3.72% dry weight of PHB in leaf tissues and 1.23% dry weight of PHB in whole tillers were obtained. Results from the analysis of the polymer distribution at the cellular and whole plant levels are presented, and target areas for the improvement of PHB production are highlighted. Polymer accumulation was also analysed in the T(1) generation obtained from controlled crosses of transgenic plants. This study presents the first successful expression of a functional multigene pathway in switchgrass, and demonstrates that this high-yielding biomass crop is amenable to the complex metabolic engineering strategies necessary to produce high-value biomaterials with lignocellulose-derived biofuels.

PHA Bioplastics, Biochemicals, and Energy from Crops

Large scale production of polyhydroxyalkanoates (PHAs) in plants can provide a sustainable supply of bioplastics, biochemicals, and energy from sunlight and atmospheric CO(2). PHAs are a class of polymers with various chain lengths that are naturally produced by some microorganisms as storage materials. The properties of these polyesters make them functionally equivalent to many of the petroleum-based plastics that are currently in the market place. However, unlike most petroleum-derived plastics, PHAs can be produced from renewable feedstocks and easily degrade in most biologically active environments. This review highlights research efforts over the last 20 years to engineer the production of PHAs in plants with a focus on polyhydroxybutryrate (PHB) production in bioenergy crops with C(4) photosynthesis. PHB has the potential to be a high volume commercial product with uses not only in the plastics and materials markets, but also in renewable chemicals and feed. The major challenges of improving product yield and plant fitness in high biomass yielding C(4) crops are discussed in detail.

Enhanced polyhydroxybutyrate production in transgenic sugarcane

Polyhydroxybutyrate (PHB) is a bacterial polyester that has properties similar to some petrochemically produced plastics. Plant-based production has the potential to make this biorenewable plastic highly competitive with petrochemical-based plastics. We previously reported that transgenic sugarcane produced PHB at levels as high as 1.8% leaf dry weight without penalty to biomass accumulation, suggesting scope for improving PHB production in this species. In this study, we used different plant and viral promoters, in combination with multigene or single-gene constructs to increase PHB levels. Promoters tested included the maize and rice polyubiquitin promoters, the maize chlorophyll A/B-binding protein promoter and a Cavendish banana streak badnavirus promoter. At the seedling stage, the highest levels of polymer were produced in sugarcane plants when the Cavendish banana streak badnavirus promoter was used. However, in all cases, this promoter underwent silencing as the plants matured. The rice Ubi promoter enabled the production of PHB at levels similar to the maize Ubi promoter. The maize chlorophyll A/B-binding protein promoter enabled the production of PHB to levels as high as 4.8% of the leaf dry weight, which is approximately 2.5 times higher than previously reported levels in sugarcane. This is the first time that this promoter has been tested in sugarcane. The highest PHB-producing lines showed phenotypic differences to the wild-type parent, including reduced biomass and slight chlorosis.

Transgene autoexcision in switchgrass pollen mediated by the Bxb1 recombinase.

BackgroundSwitchgrass (Panicum virgatum L.) has a great potential as a platform for the production of biobased plastics, chemicals and energy mainly because of its high biomass yield on marginal land and low agricultural inputs. During the last decade, there has been increased interest in the genetic improvement of this crop through transgenic approaches. Since switchgrass, like most perennial grasses, is exclusively cross pollinating and poorly domesticated, preventing the dispersal of transgenic pollen into the environment is a critical requisite for the commercial deployment of this important biomass crop. In this study, the feasibility of controlling pollen-mediated gene flow in transgenic switchgrass using the large serine site-specific recombinase Bxb1 has been investigated.ResultsA novel approach utilizing co-transformation of two separate vectors was used to test the functionality of the Bxb1/att recombination system in switchgrass. In addition, two promoters with high pollen-specific activity were identified and thoroughly characterized prior to their introduction into a test vector explicitly designed for both autoexcision and quantitative analyses of recombination events. Our strategy for developmentally programmed precise excision of the recombinase and marker genes in switchgrass pollen resulted in the generation of transgene-excised progeny. The autoexcision efficiencies were in the range of 22-42% depending on the transformation event and assay used.ConclusionThe results presented here mark an important milestone towards the establishment of a reliable biocontainment system for switchgrass which will facilitate the development of this crop as a biorefinery feedstock through advanced biotechnological approaches.

Novel transcription factors PvBMY1 and PvBMY3 increase biomass yield in greenhouse-grown switchgrass (Panicum virgatum L.)

Increasing crop yield requires the coordination of multiple metabolic pathways spanning photosynthetic carbon fixation, central carbon metabolism, and finally targeted carbon deposition to end product. In this study, we used a transcriptome-based gene regulatory association network to search for transcription factor genes that could play a role in increasing carbon flow through pathways associated with these processes to increase biomass yield in switchgrass. Two novel switchgrass transcription factors, PvBMY1 (BioMass Yield 1, belonging to the APETALA2/Ethylene Response Factor family of transcription factors) and PvBMY3 (BioMass Yield 3, a member of the Nuclear-Factor Y family of transcription factors), with predicted roles in the regulation of photosynthesis and related metabolism were identified. These genes were overexpressed in switchgrass to determine their impact on biomass yield. A significant increase in both aboveground and root biomass was observed in transgenic greenhouse grown plants compared to wild-type control plants with the best line producing 160% more aboveground biomass than controls. Transgenic lines with elevated electron transport rate of photosystems I and II as well as increased levels of starch and soluble sugars were identified.