Molecular analysis of caffeoyl residues related to pigmentation in green cotton fibers

The pigment components in green cotton fibers were isolated and identified as 22-O-caffeoyl-22-hydroxymonodocosanoin and 22-O-caffeoyl-22-hydroxydocosanoic acid. The concentration of 22-O-caffeoyl-22-hydroxymonodocosanoin correlated positively with the degree of colour in the green fibers, indicating a role for caffeoyl derivatives in the pigmentation of green cotton fibers. Upland cotton (Gossypium hirsutum L.) contains four genes, Gh4CL1-Gh4CL4, encoding 4-coumarate:CoA ligases (4CLs), key enzymes in the phenylpropanoid biosynthesis pathway. In 15-24-day post-anthesis fibers, the expression level of Gh4CL1 was very low, Gh4CL3 had a similar expression level in both white and green cottons, Gh4CL2 had a significantly higher expression level in green fibers than in white fibers, while Gh4CL4 had a higher expression level in white fibers than in green fibers. According to enzyme kinetics analysis, Gh4CL1 displayed a preference for 4-coumarate, Gh4CL3 and Gh4CL4 exhibited a somewhat low but still prominent activity towards ferulate, while Gh4CL2 had a strong preference for caffeate and ferulate. These results suggest that Gh4CL2 might be involved in the metabolism of caffeoyl residues and related to pigment biosynthesis in green cotton fibers. Our findings provide insights for understanding the biochemical and molecular mechanisms of pigmentation in green cotton fibers.