4-hydroxyphenethyl alcohol, a phenolic metabolite synthesized by GhADH1, positively regulates cotton fiber elongation

Cotton is the dominant natural fiber for global textiles, with fiber length being a critical determinant of quality and agronomic yield. While phytohormones and primary metabolism are known to affect fiber growth, secondary metabolism's regulatory roles remain poorly characterized. Here, we identify two medium-chain dehydrogenase/reductase (MDR) homeolog genes, GhADH1-A and GhADH1-D, highly expressed during the fiber elongation stage in Gossypium hirsutum (G. hirsutum). Functional analyses showed that GhADH1 knockout plants produced significantly shorter fibers and smaller bolls, whereas GhADH1 overexpression enhanced both traits. Metabolomic profiling revealed reduced levels of the phenolic alcohol 4-hydroxyphenethyl alcohol (4-HPEA) in fibers of GhADH1 knockout lines while overexpression lines contained enhanced 4-HPEA levels. LC-MS and in vitro enzymatic assays showed that GhADH1 catalyzes the conversion of 4-hydroxyphenylacetaldehyde (4-HPAA) to 4-HPEA. In vitro ovule culture assays showed that both 4-HPAA and 4-HPEA promote fiber elongation in a concentration-dependent manner, with 4-HPAA effect requiring GhADH1 activity. Transcriptomic profiling showed that GhADH1 overexpression and 4-HPEA treatment trigger consistent responses, with auxin signaling as the dominant regulatory hub. GhADH1 and 4-HPEA increased endogenous auxin levels in fibers, while auxin inhibitor treatments abrogated their elongation-promoting effects, confirming the pathway's dependence on auxin homeostasis. Together, these findings uncover a GhADH1-4-HPEA metabolic pathway positively regulating cotton fiber elongation by modulation of auxin homeostasis, revealing a new role for secondary metabolism in fiber development.