Flavonoid biosynthetic and starch and sucrose metabolic pathways are involved in the pigmentation of naturally brown-colored cotton fibers

Abstract The pigment of naturally brown-colored Upland cotton fibers is caused by condensed tannin formed by the oxidation of proanthocyanidins (PAs). However, the specific roles of PA biosynthesis and glucose metabolism in brown pigmentation during fiber development are unclear. Here, proteomes and metabolomes of ovules collected at 0 days post anthesis (DPA) and fibers collected at 10, 20, and 30 DPA from the brown-colored cotton ‘Zong 1-61’ (Z161) and white-colored cotton ‘RT’ were integrated. Compared with in RT, the proteins in the flavonoid biosynthetic pathway of Z161 were highly accumulated at 10 DPA and 20 DPA. The metabolome data showed that epi-flavan-3-ols were the major components in the brown fibers of Z161 from 10 to 30 DPA. Significantly lower levels of the catalytic enzymes of the starch and sucrose metabolic pathway, which affects cellulose biosynthesis, were found in Z161 compared with in RT, except endoglucanase and uridine-5′-diphosphate (UDP)- d -glucose pyrophosphorylase. However, the UDP- d -glucose content was significantly higher in Z161 than in RT because UDP- d -glucose pyrophosphorylase catalyzes the formation of glucose-1-phosphate, rather than sucrose, and it may be a precursor for brown cotton fiber pigment biosynthesis and glycosylation. The final biosynthetic pathway involved in the formation of brown cotton fiber pigmentation is the generation of PAs, which then form condensed tannins through the actions of a condensation enzyme and polyphenol oxidase, rather than the anthocyanin pathway. These findings provide insights into the roles of glucose metabolism in the development of brown cotton fibers and the mechanisms underlying their pigmentation.