Sorghum [Sorghum bicolor (L.) Moench] is the fifth most important cereal crop globally by harvested area and production. Its drought and heat tolerance allow high yields with minimal input. It is a promising biomass crop for the production of biofuels and bioproducts. In addition, as an annual diploid with a relatively small genome compared with other C4 grasses, and excellent germplasm diversity, sorghum is an excellent research species for other C4 crops such as maize. As a result, an increasing number of researchers are looking to test the transferability of findings from other organisms such as Arabidopsis thaliana and Brachypodium distachyon to sorghum, as well as to engineer new biomass sorghum varieties. Here, we provide an overview of sorghum as a multipurpose feedstock crop which can support the growing bioeconomy, and as a monocot research model system. We review what makes sorghum such a successful crop and identify some key traits for future improvement. We assess recent progress in sorghum transformation and highlight how transformation limitations still restrict its widespread adoption. Finally, we summarize available sorghum genetic, genomic, and bioinformatics resources. This review is intended for researchers new to sorghum research, as well as those wishing to include non-food and forage applications in their research.

Biomass sorghum is a promising feedstock for cellulosic biorefineries because of its high yield and drought tolerance. However, the difficulty of effectively drying sorghum in some regions means that it may require different handling than previously studied grassy feedstocks. This study compares the delivered cost and life-cycle greenhouse gas (GHG) footprint of field drying and baling, module storage (wrapped, densely packed biomass), pelletizing, and ensiling. Ensiling has not been widely considered for use in bioenergy production. For farms within 66 km of the biorefinery, ensiled biomass is of the lowest cost and GHG strategy despite additional cost and energy demand for hauling wet biomass. Field drying and baling, if feasible, is the most cost-effective option for sorghum between 66 km and 104 km, beyond which pellets are preferable. A 2000 bone-dry-metric ton (bdt)/day biorefinery can source sorghum with 18 bdt/ha yield cultivated on 5% of surrounding land at costs ranging from $122 (silage) to $167 (pellets)/bdt and a life-cycle GHG footprint of 111 (silage) to 179 kg CO2e/bdt (pellets). With 28 bdt/ha biomass yield, 10% cultivation of surrounding land, and low fertilizer application, costs can range from $66 (silage) to $85 (pellets)/ bdt and GHG footprint of 43 (silage) to 96 kg CO2e (pellets)/bdt.

Forage sorghum is a promising feedstock for the production of biofuels and bioproducts because it is drought tolerant, high-yielding, and familiar to farmers across the world. However, sorghum spans a diverse range of phenotypes, and it is unclear which are most desirable as bioenergy feedstocks. This paper explores four forage sorghum types, including brown-midrib (bmr), non-bmr, photoperiod sensitive (PS), and photoperiod insensitive (non-PS), from the perspective of their impact on minimum bioethanol selling price (MESP) at an ionic liquid pretreatment-based biorefinery. Among these types, there are tradeoffs between biomass yield, lignin content, and starch and sugar contents. High biomass-yielding PS varieties have previously been considered preferable for bioenergy production, but, if most starch and sugars from the panicle are retained during storage, use of non-PS sorghum may result in lower-cost biofuels (MESP of $1.26/L-gasoline equivalent). If advances in lignin utilization increase its value such that it can be dried and sold for $0.50/kg, the MESP for each scenario is lowered and non-bmr varieties become the most attractive option (MESP of $1.08/L-gasoline equivalent). While bmr varieties have lower lignin content, their comparatively lower biomass yield results in higher transportation costs that negate its fuel-yield advantage.

Pretreatment is an essential step to enable the efficient conversion of lignocellulosic biomass to biofuels and bioproducts. The most effective pretreatment methods currently in use are based on severe thermochemical approaches, which are costly and energy-intensive. Here we explored whether the common practice of ensiling grassy biomass, such as sorghum, could be used as a pre-processing step to increase the conversion efficiency under milder pretreatment conditions. We determined the impact of replacing dry sorghum biomass with ensiled sorghum biomass on the deconstruction efficiency, process economics, and carbon footprint of a lignocellulosic biorefinery that employs a separation-free ionic liquid pretreatment coupled to enzymatic saccharification and microbial conversion. Our results indicate that the use of ensiled biomass allowed for a 50% reduction in both the amount of ionic liquid (from 5 to 2.5% (w/w) as measured by initial pretreatment loading) and the time required for enzymatic saccharification (from 72 h to 24 h) without sacrificing efficiency. We show that the resulting hydrolysate can be used to cultivate an engineered strain ofRhodosporidium toruloidesto convert >90% of the monomeric sugars into bisabolene, a promising intermediate to biofuels and bioproducts. Overall, we estimate that the replacement of field-dried biomass sorghum with ensiled sorghum in combination with an ionic liquid-based deconstruction process could reduce the minimum selling price and carbon footprint of biofuel production in a biorefinery by at least 13% and 8.2%, respectively.

Sorghum is an attractive feedstock for biobased fuel and chemical production because it is familiar to farmers, naturally drought tolerant, and versatile as a food, feed, and fuel crop. Although sorghum is a promising feedstock, particularly in regions that experience drought stress, little is known about how drought conditions impact the ease of conversion of sorghum to fuels and products. This study combines agronomic field trials with a high-throughput experimental pipeline to explore the field performance and liquid biofuel (bisabolene) yields resulting from three sorghum types (photosensitive forage sorghum, optimized grain sorghum, and drought-resistant grain sorghum) grown under pre- and postflowering water limitations in two different California locations. Multiple drought treatments are compared to the control, as the timing (preflowering versus postflowering) of drought stress elicits different survival strategies and corresponding impacts on yield and composition. Forage-type sorghum maintained the highest biomass yields across all irrigation conditions and locations. Glucose and xylose yields resulting from ionic liquid pretreatment and enzymatic saccharification were not significantly impacted by irrigation treatments but differed by location and genotype. However, Rhodosporidium toruloides grown on the resulting plant hydrolysates unexpectedly produced higher titers of bisabolene for drought-stressed sorghum samples regardless of genotype.

Histones belong to a family of highly conserved proteins in eukaryotes. They pack DNA into nucleosomes as functional units of chromatin. Post-translational modifications (PTMs) of histones, which are highly dynamic and can be added or removed by enzymes, play critical roles in regulating gene expression. In plants, epigenetic factors, including histone PTMs, are related to their adaptive responses to the environment. Understanding the molecular mechanisms of epigenetic control can bring unprecedented opportunities for innovative bioengineering solutions. Herein, we describe a protocol to isolate the nuclei and purify histones from sorghum leaf tissue. The extracted histones can be analyzed in their intact forms by top-down mass spectrometry (MS) coupled with online reversed-phase (RP) liquid chromatography (LC). Combinations and stoichiometry of multiple PTMs on the same histone proteoform can be readily identified. In addition, histone tail clipping can be detected using the top-down LC-MS workflow, thus, yielding the global PTM profile of core histones (H4, H2A, H2B, H3). We have applied this protocol previously to profile histone PTMs from sorghum leaf tissue collected from a large-scale field study, aimed at identifying epigenetic markers of drought resistance. The protocol could potentially be adapted and optimized for chromatin immunoprecipitation-sequencing (ChIP-seq), or for studying histone PTMs in similar plants.