DEBOTTLENECKING FIBER PRODUCTION: FIELD EVALUATION OF LOW-LIGNIN POPLAR VARIETIES FOR PULP INDUSTRIES

Our long-term goal is to create suitable and efficient agroforestry systems aimed at fostering a more sustainable wood fiber bioeconomy. However, the major bottleneck in efficient production of wood fiber is the presence of lignin, a polymer that makes wood recalcitrant to be deconstructed into cellulose and sugars (Himmel et al., 2007; Ball et al., 2023). Within the pulp industry, delignification processes are resource-intensive, both in terms of reagents and energy consumption, resulting in high chemical recovery costs (Chen et al., 2016). Hence, an ideal agroforestry for wood fiber production needs the elite trees producing the wood without the recalcitrance of lignin. Through the precise manipulation of lignin levels and composition using gene editing tools, we have successful developed the elite poplar varieties exhibiting enhanced traits for fiber pulping and reduced carbon emissions in our greenhouse trial (Sulis et al., 2023). The cultivation of these climate-smart elite trees offers a promising solution to a significant operational challenge faced by the paper and pulp industry (Figure 1). It holds the potential to introduce remarkable operational efficiencies, create new bioeconomic prospects, and yield substantial environmental benefits for both forestry landowners and paper and pulp mills.The United States is a significant player in the global paper and pulp industry. As of 2021, the US was both the world's largest producer of pulp for paper and the second-largest consumer of paper and paperboard, utilizing approximately 64 million metric tons (FAO, 2022). Around 10 million tons of lignin need to be removed per year as a by-product in the US paper and pulp industry (Demuner et al., 2021). The lignin removal process incurs significant energy expenses and releases chemicals into the environment causing pollution (Monte et al., 2009). Given that paper and pulp mills in southern US accounting for 74% of the national pulping capacity (Piva et al., 2014), evaluating our edited poplar varieties in southern US field trial is a crucial step in transforming fundamental knowledge generated from lab and greenhouse to the condition closer to industrial exploitation. To offer more hands-on guidance to pulpwood landowners, we have planned a field trial for our low-lignin elite variety in Alabama. This state is the largest producer of pulpwood among all southern states (Winn et al., 2023), making it an ideal location for practical testing and guidance. Although loblollypine(Pinus taeda L.) monoculture plantations with long-rotation management are most predominated agroforest inAlabama as the softwood pulpwood feedstock for local mills (Schultz, 1999; Kandhola et al., 2022), hardwood roundwood production still increased by 8% between 2020 and 2021, accounting for 20% of total pulpwood production (~ 22 million cord) in Alabama (Winn et al., 2023). Our poplar varieties offer a sustainable source of hardwood pulpwood with optimized properties for mill (Sulis et al., 2023). They can be managed in short rotations in the Southern US, allowing for harvesting every three to five years (Stanturf et al., 2015; Zalesny et al., 2019; Ile et al., 2022). The short rotation of poplar agroforestry could provide consistent revenue as pulpwood and bioenergy resource for landowners while minimizing soil disruption and promoting soil health restoration (Fritsche et al., 2017; Cowie et al., 2017). Consequently, the field trial in Alabama utilizing our lignin-modified elite poplar varieties presents a valuable opportunity for Southern US mills and landowners to produce pulpwood feedstock in a more cost-effective, reliable, and sustainable manner. Agroforestry in southern US is often subject to extreme weather conditions, such as drought, hurricanes, and ice storms (Bragg et al., 2003; Marengo et al., 2023; Peterson et al., 2012; Schoeneberger et al., 2017; Sharma et al., 2021). Here, field-grown trees experience annual seasonal growth and dormancy cycles while interacting with various biotic and abiotic environmental factors, including wind, drought, cold, and pathogens (Bishaw et al., 2021). These environmental factors can significantly alter the wood property, composition, and growth of trees (Harfouche et al., 2014; Cesarino et al., 2019; De Meester et al., 2023). Therefore, we should not only assess the growth and performance of edited trees from the field trial, but also measure the wood property and pulp yield of the wood harvested from trial plantation.Our planned field trial of these modified poplar varieties, along with associated physiological measurements, will be conducted at Auburn University (AU)'s granted plantation in Alabama. This choice is due to Alabama's prominent position as the leading pulpwood producer in the US. Wood chemistry and pulping tests for field-harvested wood will be carried out at North Carolina State University (NCSU). It's worth noting that these poplar varieties were originally created and characterized by Co-PI Wang at NCSU using a multiplex editing approach, and relevant platforms, such as nuclear magnetic resonance spectroscopy (NMR) and micro pulping, which were established to assess the efficiency of wood harvested for fiber pulping (Sulis et al., 2023). For practicality, we obtained 6-month-old edited poplar varieties from a greenhouse. Examining these trees revealed a remarkable reduction in lignin content, reaching up to 50% in certain varieties, as well as a 228% increase in the C-L ratio in others, while maintaining crucial wood growth and properties such as elasticity and density (Sulis et al., 2023). Furthermore, conducting pulp yield and carbon footprint assessments could enable mills to produce up to 40% more sustainable fibers while reducing greenhouse gas emissions associated with pulp production by up to 20% (Sulis et al., 2023). The large-scale cultivation of these elite poplar varieties represents a significant step toward a more sustainable fiber production system, aligning with the United Nations Sustainable Development Goals (Kümmerer et al., 2020). Additionally, the results from our field trial can be swiftly adopted by agroforestry landowners and mills to establish a novel fiber production system in the Southern US.