Specific expression of a fusion gene Lc-GhPAP1D results in colored cotton fibers with increased flavonoid and lignin synthesis, but impaired elongation and secondary cell wall deposition

Naturally colored cotton (NCC) represents a kind of eco-friendly and sustainable textile material. Limited colors and inferior yield and quality are the major obstacles to the wide application of NCCs. The present work aimed to generate new colored cotton by synthesizing and accumulating anthocyanins in fibers. Two anthocyanin regulatory genes Lc and GhPAP1D were fused and specifically expressed in fibers of the secondary cell wall (SCW) stage. The transgenic fibers exhibited pronounced purplish-red color at 20 to 30 DPA (days post anthesis), and reddish-brown color at maturation. Meanwhile, expressing Lc and GhPAP1D led to reduced elongation rate and impaired SCW deposition in fibers, finally decreased fiber strength and length, and low lint percentage at maturation. Metabolomic and transcriptomic analyses indicated that the whole flavonoid pathway was significantly up-regulated, and multiple flavonoids, including anthocyanins, proanthocyanidins and flavonols, were accumulated in developing and mature fibers. It was also found that lignin biosynthesis and accumulation were significantly increased in fibers of the SCW synthesis stage. Our results provided a feasible strategy to promote anthocyanin synthesis and accumulation in cotton fibers, and also its side effects on fiber coloration and development, which laid the foundation for future NCC color innovation.