Investigation and characterization of the GhPPO gene family and its expression in response to fibre cell growth and biotic stress in Gossypium hirsutum L.

BACKGROUND: Cotton is an important crop that provides a natural fiber source for the textile industry. Polyphenol oxidase (PPO) is a type of ubiquitous metalloproteinase in plants, which play crucial roles in regulating plant growth and development, as well as plant resistance against biotic and abiotic stresses. Although the whole genome sequence of Gossypium hirsutum L. (G. hirsutum) has long been published, this gene family has not yet been well studied in cotton. RESULTS: In the present paper, we have identified the 51 PPO genes in cotton, and deeply analyzed 15 PPO genes in G. hirsutum, which are named GhPPOs. We found that three conserved domains - tyrosinase, DWL, and KFDV, were present in all 15 GhPPOs. We also analyzed the molecular characterization, phylogenetic relationships, gene structure, chromosome locations, 3D structure, and expression profiles of all the PPO genes identified. Based on the phylogenetic analysis, these PPO genes were classified into five groups (I, II, III, IV and V). The 15 GhPPO genes are distributed across eight chromosomes, and gene structure analysis showed that nine GhPPOs are devoid of introns. Analysis of cis-acting elements in GhPPOs promoters indicated their potential fundamental roles in response to plant growth, development, and stresses. Transcript profiling and RT-qPCR analysis revealed that GhPPO3/11 showed high expression levels during cotton fibre elongation, and GhPPO3/5 can be induced in response to cotton bollworm infestation. Moreover, the 3-dimensional structures of GhPPO3/ 11 and GhPPO3/ 5 were predicted by homolog modeling, and we found that all of them possess similarity spatial structures, with two independent domains, which could be the structural basis for performing biological functions. CONCLUSIONS: These findings not only provide valuable insights into the phylogenetic relationships, gene structure, cis-acting elements, and functional characteristics of upland cotton PPOs, but also shed light on their potential roles in cotton fibre elongation and response to biotic stresses.