Pigment Glands as a Developmental and Metabolic Hub in Cotton: Gossypol Biosynthesis, Biotic Stress Defence, and Breeding Strategies.

The pigment gland is a specialised lysigenous cavity structure that stores gossypol and its derivatives in cotton (Gossypium spp.). These secondary metabolites provide an intense colour to the gland cavity, appearing as dark dots on the surfaces of various cotton organs or tissues. Gossypol and its derivatives serve as phytoalexins, contributing to plant defence against pests and diseases. However, the direct roles of pigment glands in responses to biotic stresses still need to be clarified. Recent advances in multi-omics and molecular biology have enabled us to elucidate the mechanisms underlying pigment gland formation and development, gossypol biosynthesis, and the role of gossypol in biotic stress responses in cotton. Morphologically, the pigment gland is composed of a layer of secretory cells surrounded by two to three layers of flattened sheath cells, formed via programmed cell death of gland primordia. Pigment gland formation is governed by numerous glandular genes, which constitute a regulatory network that precisely controls gland morphogenesis, gossypol biosynthesis, and enhanced stress resistance. Rational utilisation of pigment glands could improve the value of cotton fibre and cottonseeds. The multifunctionality of pigment gland formation-related genes, encompassing growth and development, stress responses, and cottonseed edibility, provides a theoretical basis for genetic engineering and future research directions.