The GhTT2_A07 gene is linked to the brown colour and natural flame retardancy phenotypes of Lc1 cotton (Gossypium hirsutum L.) fibres

Some naturally coloured brown cotton fibres from accessions of Gossypium hirsutum L. can be used to make textiles with enhanced flame retardancy (FR). Several independent brown fibre loci have been identified and mapped to chromosomes, but the underlying genes have not yet been identified, and the mechanism of lint fibre FR is not yet fully understood. In this study, we show that both the brown colour and enhanced FR of the Lc1 lint colour locus are linked to a 1.4Mb inversion on chromosome A07 that is immediately upstream of a gene with similarity to Arabidopsis TRANSPARENT TESTA 2 (TT2). As a result of the alternative upstream sequence, the transcription factor GhTT2_A07 is highly up-regulated in developing fibres. In turn, genes in the phenylpropanoid metabolic pathway are activated, leading to biosynthesis of proanthocyanidins and accumulation of inorganic elements. We show that enhanced FR and anthocyanin precursors appear in developing brown fibres well before the brown colour is detectible, demonstrating for the first time that the polymerized proanthocyanidins that constitute the brown colour are not the source of enhanced FR. Identifying the particular colourless metabolite that provides Lc1 cotton with enhanced FR could help minimize the use of synthetic chemical flame retardant additives in textiles.