Location: Wheat, Sorghum and Forage Research2018 Annual Report
Objective 1: Identify and manipulate the genetic and biochemical mechanisms controlling lignin deposition to develop improved sorghum germplasm for bioenergy and forage uses. Subobjective 1A: Characterize the effects six recently identified brown midrib mutants (bmr) on phenylpropanoid metabolism and lignin deposition. Subobjective 1B: Evaluate ways to increase lignin deposition and alter phenolic composition of biomass through overexpression of monolignol genes. Objective 2: Identify and manipulate the genetic and biochemical mechanisms controlling starch and phosphorus composition of grain to develop novel sorghum traits for food, biofuel and livestock production. Subobjective 2A: Identify and characterize mutants that alter phosphorus composition and reduce phytate in grain. Subobjective 2B: Develop germplasm with altered starch composition and content in grain. Objective 3: Identify resistance to fungal pathogens in lignin modified sorghum germplasm for development of stalk rot-resistant grain, bioenergy, and forage sorghums. Subobjective 3A: Determine the responses of sorghum lines with six recently identified bmr mutants to stalk pathogens. Subobjective 3B: Assess impact of the stalk pathogen Fusarium thapsinum on sorghum with altered the monolignol synthesis. Subobjective 3C: Determine the response of sorghum stalk moisture phenotypes on stalk rot pathogens. Subobjective 3D: Determine whether beneficial microorganisms increase protection of bmr mutants against root and stalk pathogens. Objective 4: Identify resistance interactions between sorghum grain with novel composition and fungal pathogens for food, fuel, and feed uses. Subobjective 4A: Determine whether pericarp pigments provide protection against grain pathogens. Subobjective 4B: Determine whether grain tannins prevent fungal infection.
Sorghum (Sorghum bicolor) is a climate resilient crop, which is capable of providing grain and forage (biomass) to both the existing agricultural markets and the emerging bioenergy markets in the United States. To compete in these markets, compositional improvements to both sorghum grain and forage are needed and an understanding how these changes affect the fungal pathogens of sorghum. The objectives of this project will focus on the genetic, biochemical, and physiological mechanisms affecting the composition of sorghum biomass and grain. Efforts will result in sorghum with altered lignin content and/or composition of biomass, and increased starch content and reduced phytate content of grain for improved bioenergy conversion, livestock utilization and human nutrition. The impacts of fungal pathogens on sorghum with compositionally modified biomass and grain will be determined. Sorghum germplasm with desirable traits enhancing sorghum biomass and grain utilization will be developed, fully characterized, released and deposited into USDA–ARS National Plant Germplasm System (NPGS) for use by public and private sector plant breeders for developing improved hybrids and cultivars. The project consists of three integrated components: germplasm development, molecular biology, and plant pathology. Molecular and conventional methodologies will be utilized, and the project scale will range from gene-level to field-level. The project also has extensive formal and informal collaborations enhancing our ability to conduct this research. Anticipated products include improved sorghum germplasm for the sorghum seed industry with value-added traits and biotic stress tolerance, and tools to assess these biological pathways and fungal pathogen responses of sorghum.
Lignin is a component of the plant cell wall, and its presence affects the use of sorghum as livestock forage or bioenergy feedstock. In Subobjective 1A, next-generation sequencing technology was used to identify a candidate gene for the brown midrib 30 (bmr30) mutant. Experiments are being performed to verify that the mutation identified is responsible for the changes to lignin observed in bmr30 mutant plants. The identification of this bmr gene using this approach will validate the use of this technology for identifying candidate genes of bmr mutants. This will pave the way to use next-gen sequencing to discover 3 other bmr loci and aid in the development of rational strategies to combine bmr mutants to alter lignin content and composition for bioenergy and forage uses in sorghum and other grasses. Phytate is a major phosphorus storage compound in seeds, but it is also an antinutrient for both animals and humans and causes phosphorus management problems in their waste. In Subobjective 2A, several sorghum mutants with reduced levels of phytate were identified. Tests to determine the number of genes affected in these mutants are being performed. The ability to develop reduced phytate sorghum would increase the use of sorghum in animal feed. Stalk rot pathogens are destructive to sorghum, but sorghum with altered ability to produce lignin appears to be a source of resistance to stalk rot. There are also beneficial microorganisms in the soil that may prevent stalk rot pathogens from infecting sorghum. In Subobjective 3D, potentially beneficial microorganisms were applied to sorghum seeds, and then germination and seedling size were measured in two types of sterilized field soil. The seeds tested included brown midrib 6 (bmr6), bmr12, the double mutant and their normal counterparts, which were treated with three fluorescent Pseudomonas spp. bacterial strains, two of which showed promising results in previous test of biological controls for stalk rot. Soil type, seed type and bacterial strain did not affect germination. However, one bacterial strain caused a statistically significant reduction in the size of seedlings for most of the seed types and will therefore be eliminated from future experiments. The next set of experiments will investigate the effects of the stalk rot pathogen on sorghum when grown in the presence of the potentially beneficial microbes. This work is important because biological controls are emerging tools to control pests and pathogens in sorghum grain, forage and bioenergy production.
1. Identification of bmr lines with resistance to Fusarium stalk rot under field and drought conditions. Reducing lignin increases the conversion efficiency of biomass into sugars, but lignin is important for plant defenses against pathogens. Fusarium stalk rot of sorghum reduces grain and biomass yields, and the destructiveness of this disease increases under drought conditions. ARS scientists in Lincoln, Nebraska investigated how fungi that cause Fusarium stalk rot affected sorghum plants with two different mutations, bmr6 and bmr12, that impair lignin synthesis under field and controlled-drought conditions. The bmr mutant and normal plants were infected with Fusarium stalk rot at two field locations, one irrigated and the other dryland in Nebraska. None of the bmr lines had more disease than the normal sorghum lines, following infection at both locations. However, the stacked line, which contained both bmr6 and bmr12 mutations, was more resistant than the normal line under irrigated conditions. These results demonstrated that the bmr lines can withstand Fusarium stalk rot infections better than normal plants under dryland and irrigated conditions. A greenhouse test was also successfully developed to infect plants with Fusarium stalk rot under drought, which will allow ARS scientists to quickly identify sorghum lines resistant to Fusarium stalk rot under conditions where the disease can be particularly devastating.
2. Identification of a transcription factor that directs sorghum metabolism toward lignin synthesis. Lignin is the major structural component of plant cell walls (biomass), whose abundance and composition within plant cell walls affect livestock digestibility and conversion of biomass into biofuels and bioproducts. ARS scientists in Lincoln, Nebraska and Manhattan, Kansas investigated the effects of SbMyb60 on other biochemical pathways in order to better understand its role in controlling lignin synthesis, which had been previously discovered by ARS scientists. It was discovered that SbMyb60 not only affects lignin synthesis, but it also redirects plant metabolism towards lignin production. SbMyb60 increased the synthesis of the amino acid phenylalanine and other cofactors required for lignin synthesis. SbMyb60 affects biochemical pathways that also lead to lignin synthesis. Therefore, SbMyb60 is one of a few factors that can be used to manipulate the amount of lignin in sorghum and other bioenergy grasses to improve this crop for forage and bioenergy uses.