Cotton, the world's most important 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.

Taken together, our results demonstrate that constitutive co-expression of CrBKT and HpBHY derived from the constitutive promoter CmpL can generate a new upland cotton germplasm with valuable to high-efficient biosynthesis of astaxanthin. Endowing cotton with the ability to synthesize astaxanthin offers numerous benefits. It can enhance the quality of cottonseed oil, thereby boosting its market value as an edible oil. Simultaneously, leaves and cottonseeds accumulating high levels of astaxanthin could serve as a valuable source of livestock feed. This approach not only provides innovative strategies for maximizing the comprehensive utilization of cotton by-products but also lays a foundation for promoting green and sustainable development within the cotton industry.