Abstract：

natural fibre crop, has been extensively studied, with research predominantly focusing on fibre quality and yield. Cottonseeds contain 17%–22% oil and 22%–24% protein (Yan et al., 2025). However, fibres account for only one-fifth of the cotton total biomass (Cai et al., 2024). The

ratio of cotton straw to fibre is approximately 5:1, with leaves and bolls accounting for about 24.9% and 10.7% of the biomass, respectively (Zhong et al., 2025). Therefore, cotton by-products are the essential source for edible oil and livestock feed. Despite this, the potential of cotton by-products, particularly straw, remains largely untapped, leading to significant waste of resources. These results may be caused by the lack of

specialization of cotton by-products. Thus, to address this issue, leveraging synthetic biology tools to develop innovative strategies for the fullest utilization of by-products of cotton is an effective way to enhance the comprehensive use of this crop. Astaxanthin is a lipid-soluble high-value carotenoid with superior antioxidant activity and serves as a natural food and feed colouring agent. It has been widely used in foods, feed, pharmaceuticals, and cosmetics. Due to the limited yields of natural astaxanthin from microalgae and the lower bioactivity of chemically synthesized astaxanthin, plant metabolic engineering has been used to biosynthesize astaxanthin. For instance, introducing b-carotene hydroxylase (CrtZ) and b-carotene ketolase (CrtW) into tobacco chloroplasts boosted the astaxanthin accumulation to 5.44 mg/g dry weight leaves (Hasunuma et al., 2008), while co-expressing b-carotene ketolase gene from Chlamydomonas reinhardtii (CrBKT ) and b-carotene hydroxylase gene from Haematococcus pluvialis (HpBHY ) in tomato fruits promoted the accumulation of astaxanthin up to 16.1 mg/g DW (Huang et al., 2013). In staple crops, endosperm-specific expression of the key genes phytoene desaturase gene (CrtI ), phytoene synthase gene (PSY1), BKT, and BHY generated an astaxanthin-rich rice variety named aSTARice, with an astaxanthin production of 16.23 mg/kg DW (Zhu et al., 2018). The astaxanthin biosynthesis pathway in maize seeds, using the similar genes, promoted the accumulation of astaxanthin higher than 100 mg/kg DW, attributed to an adequate amount of precursors in yellow maize (Liu et al., 2021). However, research on the synthesis of astaxanthin in cotton remains limited. In this study, we reconstructed the astaxanthin biosynthesis pathway in cotton by leveraging synthetic biology strategies and metabolic engineering techniques that have been successfully applied in other organisms.

Key Words: 

Astaxanthin, cotton, livestock feed.