Genome-Wide Study of the GATL Gene Family in Gossypium hirsutum L. Reveals that GhGATL Genes Act on Pectin Synthesis to Regulate Plant Growth and Fiber Elongation

Pectin is a major polysaccharide component that promotes plant growth and fiber elongation in cotton. In previous studies, the galacturonosyltransferase-like (GATL) gene family has been shown to be involved in pectin synthesis. However, few studies have been performed on cotton GATL genes. Here, a total of 33, 17, and 16 GATL genes were respectively identified in Gossypium hirsutum, Gossypium raimondii, and Gossypium arboreum. In multiple plant species, phylogenetic analysis divided GATL genes into five groups named GATL-a to GATL-e, and the number of groups was found to gradually change over evolution. Whole genome duplication (WGD) and segmental duplication played a significant role in the expansion of the GATL gene family in G. hirsutum. Selection pressure analyses revealed that GATL-a and GATL-b groups underwent a great positive selection pressure during evolution. Moreover, the expression patterns revealed that most of highly expressed GhGATL genes belong to GATL-a and GATL-b groups, which have more segmental duplications and larger positive selection value, suggesting that these genes may play an important role in the evolution of cotton plants. We overexpressed GhGATL2, GhGATL9, GhGATL12, and GhGATL15 in Arabidopsis and silenced the GhGATL15 gene in cotton through a virus induced gene silencing assay (VIGS). The transgenic and VIGS lines showed significant differences in stem diameter, epidermal hair length, stamen length, seed size, and fiber length than the control plant. In addition, the pectin content test proved that the pectin was significantly increased in the transgenic lines and reduced in VIGS plants, demonstrating that GhGATL genes have similar functions and act on the pectin synthesis to regulate plant growth and fiber elongation. In summary, we performed a comprehensive analysis of GhGATL genes in G. hirsutum including evolution, structure and function, in order to better understand GhGATL genes in cotton for further studies.