Abstract
Cotton (Gossypium hirsutum) provides the world's dominant renewable textile fiber, and cotton fiber is valued as a research model because of its extensive elongation and secondary wall thickening. Previously, it was assumed that fibers elongated as individual cells. In contrast, observation by cryo-field emission-scanning electron microscopy of cotton fibers developing in situ within the boll demonstrated that fibers elongate within tissue-like bundles. These bundles were entrained by twisting fiber tips and consolidated by adhesion of a cotton fiber middle lamella (CFML). The fiber bundles consolidated via the CFML ultimately formed a packet of fiber around each seed, which helps explain how thousands of cotton fibers achieve their great length within a confined space. The cell wall nature of the CFML was characterized using transmission electron microscopy, including polymer epitope labeling. Toward the end of elongation, up-regulation occurred in gene expression and enzyme activities related to cell wall hydrolysis, and targeted breakdown of the CFML restored fiber individuality. At the same time, losses occurred in certain cell wall polymer epitopes (as revealed by comprehensive microarray polymer profiling) and sugars within noncellulosic matrix components (as revealed by gas chromatography-mass spectrometry analysis of derivatized neutral and acidic glycosyl residues). Broadly, these data show that adhesion modulated by an outer layer of the primary wall can coordinate the extensive growth of a large group of cells and illustrate dynamic changes in primary wall structure and composition occurring during the differentiation of one cell type that spends only part of its life as a tissue.
Highlights
Cotton (Gossypium hirsutum) provides the world’s dominant renewable textile fiber, and cotton fiber is valued as a research model because of its extensive elongation and secondary wall thickening
This paper reports the surprising discovery that a specialized cell wall layer, which we called the cotton fiber middle lamella (CFML), fuses elongating cotton fibers into tissue-like bundles during elongation
LM5 did not label CFML in situ in transmission electron microscopy (TEM) immunolabeling experiments, perhaps because the LM5 epitope was rendered inaccessible by interaction with other polymers. These data show that an outer layer of the cotton fiber primary wall, the CFML, functions as an adhesive middle lamella, bundling fibers together and aiding their organized, extensive elongation within the confined locule space
Summary
Cotton (Gossypium hirsutum) provides the world’s dominant renewable textile fiber, and cotton fiber is valued as a research model because of its extensive elongation and secondary wall thickening. Losses occurred in certain cell wall polymer epitopes (as revealed by comprehensive microarray polymer profiling) and sugars within noncellulosic matrix components (as revealed by gas chromatography-mass spectrometry analysis of derivatized neutral and acidic glycosyl residues). Fiber from cotton (Gossypium hirsutum) is the world’s dominant natural textile fiber (Chen et al, 2007) This unique single cell is a valuable research model because of its extreme length ($2.25 cm) and thick (3–6 mm) cellulosic secondary wall (Kim and Triplett, 2001). Similar to other expanding plant cells, the primary walls of 10-DPA cotton fibers contain 23% cellulose fibrils and 22% protein as well as noncrystalline matrix polysaccharides including xyloglucan (XG) and pectin (Meinert and Delmer, 1977). The boll opens and the fiber and seed dry at approximately 45 to 60 DPA (Basra, 1999)
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