In plant biology and agronomy, straw is defined as the collective term for the stem, leaf, and (in some cases) spike portions of mature crops, representing the residual biomass left after the harvest of seeds or other economic parts. It primarily originates from crops such as rice, wheat, maize, potatoes, rapeseed, cotton, and sugarcane, belonging to families like Poaceae and Fabaceae. Straw is an inevitable byproduct of agricultural production systems, embodying significant ecological functions and economic value. During photosynthesis, approximately 40–60% of a crop's assimilated products are allocated to vegetative organs like straw. Globally, around XX billion tons of crop straw are produced annually, with China contributing nearly XX tons each year—a figure that continues to rise with increasing agricultural yields. When scientifically utilized, this vast biomass resource can serve as a cornerstone for sustainable agriculture; however, improper management may transform it into a source of environmental pollution. The production of straw is intricately linked to the growth and developmental processes of crops. During the reproductive growth phase, edible organs such as grains or tubers mature and accumulate dry matter, while vegetative organs like stems and leaves, having fulfilled their photosynthetic roles, gradually senesce, forming the straw biomass at harvest. The morphology and yield of straw vary significantly across different crop species. From a biochemical perspective, straw is a complex organic composite comprising structural carbohydrates and non-structural components.

Biochemical Composition of Straw From a biochemical perspective, straw is a complex organic composite consisting of structural carbohydrates and non-structural components. Organic Component Composition Cellulose (XX%): The primary component of cell walls. Hemicellulose (XX%): Composed of xylans, arabinans, and other polysaccharides, readily degradable by microorganisms. Lignin (XX%): A complex phenolic polymer forming a barrier against biodegradation, providing structural support to plants. Crude Protein (XX%): Primarily found in cytoplasmic residues. Soluble Sugars (XX%): Including low-molecular-weight carbohydrates such as sucrose and glucose.

Globally, approximately 5 billion tons of straw are produced annually, yet a significant portion remains underutilized, often burned or discarded, leading to environmental pollution and resource waste. According to literature, straw return is the most common utilization method, with 50% of straw recycled between 2014 and 2017. However, excessive straw return can lead to elevated soil organic carbon (SOC) and nitrous oxide (N₂O) levels, reducing soil productivity (Optimizing straw return to enhance grain production and approach carbon neutrality in intensive cropping systems). Additionally, due to the low economic value of straw, many farmers opt for burning, with approximately 81.1 million tons incinerated in 2015. In China, only 3.4% of straw is utilized for material applications such as straw-based panels or degradation technologies (https://www.agriir.cn/resources/detail/1/B0F96F2D-5016-4E6D-90FA-176CD5F7B945.html?projectId=4063f80a-2997-11e7-b5f5-3440b5b17484). Straw, primarily composed of cellulose, hemicellulose, and lignin, holds significant potential for resource utilization: Cellulose (approximately 30–38%): The primary component of cell walls, cellulose can be hydrolyzed into glucose and processed via acid or enzymatic treatments to produce clean fuels such as bioethanol and butanol. It can also be dissolved in ionic liquids (e.g., [Emim][OAc]) for spinning into packaging films (Simultaneous isolation of cellulose and lignin from wheat straw and catalytic conversion to valuable chemical products). Hemicellulose (approximately 20–32%): Composed of xylose, arabinose, and other monosaccharides, hemicellulose is widely used in biofermentation, food additives, and biobased packaging materials. Through chemical modification or compounding with other biopolymers, it can be used to create biodegradable films and packaging materials, leveraging its hydrophilicity for applications such as wound dressings (Study of Purified Cellulosic Pulp and Lignin Produced by Wheat Straw Biorefinery). Lignin (approximately 7–25%): A complex aromatic polymer, lignin can be converted into biochar, adhesives, UV-resistant coatings, and novel functional materials. Due to its role in providing structural support to plants, lignin-based coatings are more durable than traditional coatings (Lignin-based organic coatings and their applications: A review). Through efficient pretreatment and separation technologies in molecular synthesis, the three major components of straw can be individually developed for high-value applications, providing a solid foundation for building a circular bioeconomy.

The lignocellulosic complex, formed by the intricate anti-degradation structure of lignin, cellulose, and hemicellulose, poses significant challenges to the separation and complete degradation of lignin. Lignin's chemical inertness and hemicellulose's susceptibility to hydrolysis result in conventional separation and degradation methods often damaging hemicellulose's structure while leaving residual lignin, significantly limiting their applications (Control of conversion and separation of lignocellulose components by phase-separation conditions). Additionally, straw's low bulk density, high moisture content, seasonal concentration, and large storage space requirements contribute to high processing costs. Straw has a bulk density of only 60–150 kg/m³, roughly one-tenth that of coal, leading to substantial transportation costs. To prevent mold growth, straw must be dried to a moisture content below 15%, with hot air drying costing approximately 80–120 CNY per ton or 100 kWh per ton. Due to the short 20–30 day harvest period, straw is often stored until the next harvest, resulting in losses exceeding 30% (Impacts of regional governmental incentives on the straw power industry in China: A game-theoretic analysis). High processing and handling costs, coupled with limited government subsidies, result in low added value for straw-derived products, making them less cost-competitive than alternatives. For instance, polylactic acid (PLA) plastics derived from straw cellulose are 2.3 times more expensive to produce than petroleum-based polypropylene (PP) plastics. As a fuel, cellulose ethanol from straw costs 1.8 times more than corn ethanol. As a composite material, straw-based panels have lower strength but higher prices compared to bamboo-wood OSB boards (China's Straw Recycling Policy Averts Nearly 19,000 Premature Deaths a Year, Study Finds). Consequently, farmers often prefer on-site straw burning over storage or producing derived products. In Heilongjiang's Sanjiang Plain, rice straw burning rates exceed 40%. In northeastern Thailand, rice straw burning contributes 38% to Bangkok's winter PM2.5 levels. In Indonesia, palm straw burning on Sumatra Island causes Singapore's annual Pollution Standards Index (PSI) to exceed safe levels.

To increase the added value of straw, we propose utilizing its cellulose as a novel textile raw material. The global textile market is valued at USD 1,837.27 billion. Cotton, wool, and Lyocell dominate the market, with these high-end fibers commanding relatively high prices.

The primary bottleneck in utilizing straw cellulose for textile production is residual lignin. Lignin causes instability in the viscosity of spinning solutions and frequent yarn breakage, resulting in fibers with significantly lower tensile strength and elongation compared to cotton or Lyocell. While conventional chemical methods can remove lignin and enhance cellulose purity, they often damage the cellulose chain and are costly. Biological enzymatic methods, although capable of gently degrading lignin while preserving quality, require large enzyme quantities, long reaction cycles, and incur high costs. Therefore, to achieve widespread application of straw cellulose in the textile industry, it is essential to develop a novel process that is efficient, low-energy, preserves cellulose structure, and is cost-effective.

To address the issue of residual lignin, we employ synthetic biology techniques by introducing multiple lignin-degrading enzymes to efficiently degrade lignin in straw, thereby enhancing cellulose purity and optimizing its spinning performance. We selected laccase (Lac) and versatile peroxidase (VP) from fungal sources, along with polysaccharide monooxygenase (LPMO) assisted by multicopper oxidases. Pichia pastoris was chosen as the heterologous expression system due to its eukaryotic processing capabilities and high expression efficiency, particularly suitable for enzymes like Lac, VP, and LPMO derived from fungi and actinomycetes. These three enzymes are mechanistically complementary: Lac oxidizes phenolic structures on the lignin surface, VP cleaves non-phenolic linkages, and LPMO disrupts the cellulose-lignin interface, facilitating penetration and accelerating degradation by other enzymes. Through their synergistic action, the lignin structural barrier is effectively weakened, significantly improving cellulose accessibility and subsequent processing performance.

To enhance the synergistic action of laccase (Lac), versatile peroxidase (VP), and polysaccharide monooxygenase (LPMO), we designed a multi-enzyme complex system. By fusing the three enzymes with specific dockerin tags and combining them with scaffold proteins containing distinct cohesin modules, we achieved spatial clustering of the enzymes, forming a "lignin minicellulosome" that mimics the structure of natural cellulosome complexes. This structure effectively enhances the proximity effect and electron transfer efficiency among enzymes, thereby improving lignin degradation capacity. To stably anchor this complex to the cell surface and enable efficient lignin degradation in the extracellular environment, we further introduced the AGA1/AGA2 α-agglutinin system from Saccharomyces cerevisiae. By fusing the main scaffold protein with AGA2 and expressing it in S. cerevisiae, the complex binds to the cell wall via AGA1, enabling surface display of the "lignin minicellulosome." This strategy not only improves enzyme stability and degradation efficiency but also provides an effective enzymatic platform for the subsequent bioconversion of straw feedstock.

Pure cellulose spinning materials exhibit significant limitations in mechanical performance. Their tensile strength typically ranges from 200–400 MPa, substantially lower than high-performance synthetic fibers, such as polyvinyl alcohol (PVA)-based composite fibers, which can achieve 1.5–2.0 GPa. This strength disparity primarily arises from the reliance of cellulose on hydrogen bonds for intermolecular interactions, lacking the robust covalent crosslinking networks common in synthetic fibers. Additionally, cellulose materials display high brittleness, with an elongation at break of only 3–10%, compared to 10–30% for synthetic fibers. This low ductility stems from the rigid pyranose ring structure of cellulose molecular chains, which restricts chain slippage and elastic deformation. The rigidity of the molecular chains prevents cellulose fibers from dissipating stress through plastic deformation, leading to brittle fracture under stress. These mechanical deficiencies significantly hinder the application of pure cellulose spinning materials in textiles, particularly in scenarios requiring high strength and durability. To address these shortcomings, cellulose materials often require modification, such as compounding with other polymers or chemical crosslinking, to enhance their mechanical strength and toughness. Works Cited "天然纤维素抗拉强度和断裂伸长率." Engineering Mechanics, https://www.engineeringmechanics.cn/cn/supplement/4bfc97f7-b9f5-41a2-9b96-33d19aaae068. Accessed 25 June 2025. Li, Jian, et al. "Research on Preparation and Properties of High Strength and High Modulus PVA/CNF/GO Composite Fiber." Chinese Journal of Solid Mechanics, 2010, https://pubs.cstam.org.cn/data/article/cjsm/preview/pdf/gtlxxb201006004.pdf. Accessed 25 June 2025. "合成纤维伸长率." Yjsyi, https://www.yjsyi.com/fenxijiance/20614.html. Accessed 25 June 2025. "天然纤维素在生物降解过程中超分子结构的变化." National Natural Science Foundation of China, 1998, https://www.nsfc.gov.cn/csc/20345/20348/pdf/1998/%E5%A4%A9%E7%84%B6%E7%BA%A4%E7%BB%B4%E7%B4%A0%E5%9C%A8%E7%94%9F%E7%89%A9%E9%99%8D%E8%A7%A3%E8%BF%87%E7%A8%8B%E4%B8%AD%E8%B6%85%E5%88%86%E5%AD%90%E7%BB%93%E6%9E%84%E7%9A%84%E5%8F%98%E5%8C%96.pdf. Accessed 25 June 2025.

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