Abstract
Cotton fibers arise through highly anisotropic expansion of a single seed epidermal cell. We obtained evidence that apical cell wall synthesis occurs through examining the tips of young elongating Gossypium hirsutum (Gh) and G. barbadense (Gb) fibers. We characterized two tip types in Gh fiber (hemisphere and tapered), each with distinct apical diameter, central vacuole location, and distribution of cell wall components. The apex of Gh hemisphere tips was enriched in homogalacturonan epitopes, including a relatively high methyl-esterified form associated with cell wall pliability. Other wall components increased behind the apex including cellulose and the α-Fuc-(1,2)-β-Gal epitope predominantly found in xyloglucan. Gb fibers had only one narrow tip type featuring characters found in each Gh tip type. Pulse-labeling of cell wall glucans indicated wall synthesis at the apex of both Gh tip types and in distal zones. Living Gh hemisphere and Gb tips ruptured preferentially at the apex upon treatment with wall degrading enzymes, consistent with newly synthesized wall at the apex. Gh tapered tips ruptured either at the apex or distantly. Overall, the results reveal diverse cotton fiber tip morphologies and support primary wall synthesis occurring at the apex and discrete distal regions of the tip.
Highlights
Ovules were dissected at ages indicated for each data set
For fluorescent detection of cellulose by Pontamine Fast Scarlet 4B (S4B; Sigma-Aldrich, S479896), dissected ovules were laid on a 24 × 50 mm coverslip, which allowed greater working distance on the inverted microscope
We demonstrated that Tinopal did not passively diffuse away from cell walls in the cotton fiber tip during 5 h after staining in a control experiment in which ovules were handled the same way except for being killed after staining with a third wash containing 0.02% sodium azide
Summary
Deltapine 90 (DP90) and G. barbadense cv. PhytoGen 800 were grown as before[32] in a greenhouse of the NC State University Phytotron. G. hirsutum cv Coker[312] (C312) analyzed by cryo-FE-SEM was grown in a greenhouse as described previously[32]. Ovules were dissected at ages indicated for each data set. Fixation, when used, commenced immediately upon dissection of ovules from the bolls in the greenhouse and continued for 1 h (RT; gentle rocking; HistoChoice, Electron Microscopy Sciences, 64115-01)
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