Transcriptome Analysis of Arabidopsis Wild-Type and gl3-sst sim Trichomes Identifies Four Additional Genes Required for Trichome Development

Transcriptome analyses have been performed on mature trichomes isolated from wild-type Arabidopsis leaves and on leaf trichomes isolated from the g13-sst sim double mutant, which exhibit many attributes of immature trichomes. The mature trichome profile contained many highly expressed genes involved in cell wall synthesis, protein turnover, and abiotic stress response. The most highly expressed genes in the g13-sst sim profile encoded ribosomal proteins and other proteins involved in translation. Comparative analyses showed that all but one of the genes encoding transcription factors previously found to be important for trichome formation, and many other trichome-important genes, were preferentially expressed in gl3-sst sim trichomes. The analysis of genes preferentially expressed in gl3-sst sim led to the identification of four additional genes required for normal trichome development. One of these was the HDG2 gene, which is a member of the HD-ZIP IV transcription factor gene family. Mutations in this gene did not alter trichome expansion, but did alter mature trichome cell walls. Mutations in BIT resulted in a loss of trichome branch formation. The relationship between bit and the phenotypically identical mutant, sti, was explored. Mutations in PEL3, which was previously shown to be required for development of the leaf cuticle, resulted in the occasional tangling of expanding trichomes. Mutations in another gene encoding a protein with an unknown function altered trichome branch formation.

Phylogenetically Distinct Cellulose Synthase Genes Support Secondary Wall Thickening in Arabidopsis Shoot Trichomes and Cotton Fiber

Through exploring potential analogies between cotton seed trichomes （or cotton fiber） and arabidopsis shoot trichomes we discovered that CesAs from either the primary or secondary wall phylogenetic clades can support secondary wall thickening. CesA genes that typically support primary wall synthesis, AtCesA 1,2,3,5, and 6, underpin expansion and secondary wall thickening of arabidopsis shoot trichomes. In contrast, apparent orthologs of CesA genes that support secondary wall synthesis in arabidopsis xylem, AtCesA4,7, and 8, are up-regulated for cotton fiber secondary wall deposition. These conclusions arose from： （a） analyzing the expression of CesA genes in arabidopsis shoot trichomes; （b） observing birefringent secondary walls in arabidopsis shoot trichomes with mutations in AtCesA4, 7, or 8; （c） assaying up-regulated genes during different stages of cotton fiber development; and （d） comparing genes that were co.expressed with primary or secondary wall CesAs in arabidopsis with genes up- regulated in arabidopsis trichomes, arabidopsis secondary xylem, or cotton fiber during primary or secondary wall deposition. Cumulatively, the data show that： （a） the xylem of arabidopsis provides the best model for secondary wall cellulose synthesis in cotton fiber; and （b） CesA genes within a ＂cell wall toolbox＂ are used in diverse ways for the construction of particular specialized cell walls.