Identification and expression analysis of Tubulin gene family in upland cotton

Background Cotton fibers are single-celled extensions of the seed epidermis, a model tissue for studying cytoskeleton. Tubulin genes play a critical role in synthesizing the microtubules (MT) as a core element of the cytoskeleton. However, there is a lack of studies concerning the systematic characterization of the tubulin gene family in cotton. Therefore, the identification and portrayal of G. hirsutum tubulin genes can provide key targets for molecular manipulation in cotton breeding. Result In this study, we investigated all tubulin genes from different plant species and identified 98 tubulin genes in G. hirsutum. Phylogenetic analysis showed that tubulin family genes were classified into three subfamilies. The protein motifs and gene structure of α-, β-tubulin genes are more conserved compared with γ-tubulin genes. Most tubulin genes are located at the proximate ends of the chromosomes. Spatiotemporal expression pattern by transcriptome and qRT-PCR analysis revealed that 12 α-tubulin and 7 β-tubulin genes are specifically expressed during different fiber development stages. However, Gh.A03G027200, Gh.D03G169300, and Gh.A11G258900 had differential expression patterns at distinct stages of fiber development in varieties J02508 and ZRI015. Conclusion In this study, the evolutionary analysis showed that the tubulin genes were divided into three clades. The genetic structures and molecular functions were highly conserved in different plants. Three candidate genes, Gh.A03G027200, Gh.D03G169300, and Gh.A11G258900 may play a key role during fiber development complementing fiber length and strength.

ROS accumulation in cotton ovule epidermal cells is necessary for fiber initiation

Cotton (Gossypium hirsutum) fiber, an extremely elongated and thickened single cell of the seed epidermis, is the world’s most important natural and economical textile fiber. Unlike Arabidopsis leaf trichomes, fiber initials are randomly developed and frequently form in adjacent seed epidermal cells and follow no apparent pattern. Numerous publications suggested cotton fiber development shares a similar mechanism with Arabidopsis leaf trichome development. Here we show that H2O2 accumulation in cotton ovule epidermal cells by NBT staining ovules at different development stages between TM1 and N1n2, a lintless-fuzzless doubled mutant originated from TM1. In contrast, Arabidopsis and cotton leaf trichomes do not show H2O2 content. By adding DPI (H2O2 inhibitor) and SHAM (H2O2 activator) in vitro ovule cultures, we show fiber initiation directly involves with H2O2 accumulation. We propose that the directional accumulation of H2O2 in cotton ovule epidermal cell is the drive for fiber initiation, elongation.

The transcription factor AtDOF4.2 regulates shoot branching and seed coat formation in Arabidopsis

Plant-specific DOF (DNA-binding with one finger)-type transcription factors regulate various biological processes. In the present study we characterized a silique-abundant gene AtDOF (Arabidopsis thaliana DOF) 4.2 for its functions in Arabidopsis. AtDOF4.2 is localized in the nuclear region and has transcriptional activation activity in both yeast and plant protoplast assays. The T-M-D motif in AtDOF4.2 is essential for its activation. AtDOF4.2-overexpressing plants exhibit an increased branching phenotype and mutation of the T-M-D motif in AtDOF4.2 significantly reduces branching in transgenic plants. AtDOF4.2 may achieve this function through the up-regulation of three branching-related genes, AtSTM (A. thaliana SHOOT MERISTEMLESS), AtTFL1 (A. thaliana TERMINAL FLOWER1) and AtCYP83B1 (A. thaliana CYTOCHROME P450 83B1). The seeds of an AtDOF4.2-overexpressing plant show a collapse-like morphology in the epidermal cells of the seed coat. The mucilage contents and the concentration and composition of mucilage monosaccharides are significantly changed in the seed coat of transgenic plants. AtDOF4.2 may exert its effects on the seed epidermis through the direct binding and activation of the cell wall loosening-related gene AtEXPA9 (A. thaliana EXPANSIN-A9). The dof4.2 mutant did not exhibit changes in branching or its seed coat; however, the silique length and seed yield were increased. AtDOF4.4, which is a close homologue of AtDOF4.2, also promotes shoot branching and affects silique size and seed yield. Manipulation of these genes should have a practical use in the improvement of agronomic traits in important crops.

Development of the strophiole in seeds of white clover (Trifolium repens L.)

The seed coat of Trifolium repens L. was studied with special emphasis on the development of the strophiole, which is the site for water entry during imbibition in leguminous seeds. The epidermal cells of the strophiole are longer than the cells in the remainder of the seed epidermis in the mature ovule. During seed development the median cells of the strophiolar epidermis divide periclinally into an outer layer of palisade cells and an inner layer of isodiametric cells. Prior to maturity a fissure is formed between some of the palisade cells in the centre of the strophiole. It is suggested that tension develops between the palisade cells and the iso-diametrical cells during later maturation stages causing the formation of the fissure which it is believed functions in water uptake. It is indicated that the ‘light line’ is caused by alteration of cellulose microfibrillar orientation in palisade cell walls. It is confirmed that removal of the epicuticular wax from hard seeds by rinsing in absolute alcohol or hexane does not induce water imbibition. Only when seed coats are mechanically abraded do hard seeds germinate.

Development of helicoidal texture in the prerelease mucilage of quince (Cydonia oblonga) seed epidermis

Quince (Cydonia oblonga Mill.) seed epidermis was examined cytologically during its development. Three developmental phases were delimited: immaturity, transition to maturity, and maturity. These cytological phases corresponded with phases of competence to release hydrated mucilage on wetting, immature tissue being completely incompetent and mature tissue fully competent. Growing cells of immature tissue were vacuolate and thin walled. By contrast, protoplasts of nongrowing mature epidermal cells had contracted to a remnant and been replaced by periplasmic deposits. Within these deposits, surrounded by amorphous material, were massive arrays of widely spaced microfibrils arranged helicoidally. In the oldest sample examined, periplasmic material appeared to be spewing through the broken outer walls of some cells. The periplasmic material is interpreted to be prerelease mucilage, which progressively fills the periplasm during a brief transitional phase. It seems that amorphous periplasmic material is deposited initially and microfibrils later intermingle with it. At some stage during filling of the periplasm, the microfibrils begin to organize, ultimately becoming helicoidal. Orderliness seems to begin in the central region of the periplasmic pool, not at its edges. It is proposed that nucleation of liquid crystalline helicoidal arrays occurs in the periplasm and that these arrays remain fluid until their disintegration during release as a result of hydration.