A Genome-Scale Analysis of the PIN Gene Family Reveals Its Functions in Cotton Fiber Development

The PIN-FORMED (PIN) protein, the most important polar auxin transporter, plays a critical role in the distribution of auxin and controls multiple biological processes. However, characterizations and functions of this gene family have not been identified in cotton. Here, we identified the <i>PIN</i> family in <i>Gossypium hirsutum</i>, <i>Gossypium arboreum</i>, and <i>Gossypium raimondii</i>. This gene family was divided into seven subgroups. A chromosomal distribution analysis showed that <i>GhPIN</i> genes were evenly distributed in eight chromosomes and that the whole genome and dispersed duplications were the main duplication events for <i>GhPIN</i> expansion. qRT-PCR analysis showed a tissue-specific expression pattern for <i>GhPIN</i>. Likely due to the <i>cis</i>-element variations in their promoters, transcripts of <i>PIN6</i> and <i>PIN8</i> genes from the At (tetraploid genome orginated from <i>G. arboreum</i>) subgenome and <i>PIN1a</i> from the Dt (tetraploid genome orginated from <i>G. raimondii</i>) subgenome in <i>G. hirsutum</i> was significantly increased compared to the transcripts in the diploids. The differential regulation of these <i>PIN</i> genes after the polyploidization may be conducive to fiber initiation and elongation. Exogenously applied auxin polar transport inhibitor significantly suppressed fiber growth, which is consistent with the essential function of these <i>PIN</i> genes for regulating cotton fiber development. Furthermore, the overexpression of <i>GhPIN1a_Dt</i>, <i>GhPIN6_At</i>, and <i>GhPIN8_At</i> in <i>Arabidopsis</i> promoted the density and length of trichomes in leaves.