Fenlin Deng, Lili Tu, Daojun Yuan, Juan Hao, Xianlong Zhang*
National Key Laboratory of Crop Genetic Improvement, Huazhong Agricultural University

In order to find functional genes involved in fiber development in cotton, 10,979 high-quality expressed sequence tags (ESTs) were generated by Sanger sequencing of the normalized cDNA library. With ESTs from 454 sequencing of fiber mRNA together, 26,797 unique sequences（12,932 contigs and 13,865 singletons）were assembled. The gene expression profile differences of 5, 10, 15, 20, 25DPA fiber between upland cotton and sea-island cotton were analyzed by deep sequencing method (Solexa) with the assembled unigenes as reference sequences. In 5DPA, gene expression of upland cotton was more active than sea-island cotton, which was opposite in 20, 25 DPA. Late in the fiber elongation, redox homeostasis and flavonoid metabolism were more active in sea-island cotton, and the basic metabolisms had the same trend.
GbPDF1 (PROTODERMAL FACTOR 1) was found to be expressed preferentially during fiber initiation and early elongation and accumulated highest in 5-DPA fiber cells. GbPDF1 promoter:GUS constructs in transgenic cotton expressed predominantly in the epidermis of ovules and developing fibers. Progressive deletions of the GbPDF1 promoter showed that a 236-bp promoter fragment was sufficient for the basal transcription of GbPDF1 in cotton. Mutation of putative regulatory sequences showed that an element named HDZIP2ATATHB2 within the fragment was found to be essential for PGbPDF1-1 expression. The binding activity between the cis-element and nuclear extracts from 5-DPA fiber-bearing cotton ovules was specific. PDF1-silenced cotton caused retardation of fiber initiation and produced shorter fibers compared to wild type, indicating the gene was required for cotton fiber development. Consistently, ethylene synthesis related genes during fiber development decreased significantly in the transgenic cotton accompanying with high accumulation of H2O2. Three proteins interacting with GbPDF1 in yeast and in planta might involve in cellular signaling or metabolism. We supposed that GbPDF1 plays a critical role together with the interaction partners in H2O2 homeostasis, steady biosynthesis of ethylene and pectin during fiber development.
Another gene GbTCP is preferentially expressed in cotton fiber from 5 to 15 DPA, but a little level in stem and leaf. The GbTCP promoter mainly exhibited in developing cotton fibre cells and trichomes of Arabidopsis. Constitutive expression of GbTCP in cotton lead to abnormal phenotype including smaller plant, more branches, smaller flowers, leaves and buds, longer flower stalk and lower quality fiber. RNAi suppression of GbTCP gene had no effect on the fiber initiation, but it delayed fiber elongation and induced shorter and lower quality fiber.