Auxin Regulates Cotton Fiber Initiation via GhPIN-Mediated Auxin Transport

Cotton fibers are seed trichomes that make cotton unique compared with other plants. At anthesis, IAA, a major auxin in plants, accumulates in the fiber cell to promote cell initiation. However, many important aspects of this process are not clear. Here, auxin distribution patterns indicated by auxin-dependent DR5::GUS (beta-glucuronidase) expression in cotton ovules were studied during fiber cell differentiation and cell initiation [-2 to 2 DPA (days post-anthesis)]. The nucellus and fiber cell were two major sites where auxin accumulates. The accumulation in the nucellus started from -1 DPA, and that in fiber cells from 0 DPA. Immunolocalization analysis further suggests that the IAA accumulation in fiber initials began before flower opening. Furthermore, we demonstrate that accumulated IAA in fiber initials was mainly from efflux transport and not from in situ synthesis. Eleven auxin efflux carrier (GhPIN) genes were identified, and their expression during ovule and fiber development was investigated. Ovule-specific suppression of multiple GhPIN genes in transgenic cotton inhibited both fiber initiation and elongation. In 0 DPA ovules, GhPIN3a, unlike other GhPIN genes, showed additional localization of the transcript in the outer integument. Collectively, these results demonstrate the important role of GhPIN-mediated auxin transport in fiber-specific auxin accumulation for fiber initiation.