The control of single-celled cotton fiber elongation by developmentally reversible gating of plasmodesmata and coordinated expression of sucrose and K+ transporters and expansin

Each cotton fiber is a single cell that elongates to 2.5 to 3.0 cm from the seed coat epidermis within similar to 16 days after anthesis (DAA). To elu cidate the mechanisms controlling this rapid elongation, we studied the gat ing of fiber plasmodesmata and the expression of the cell wall-loosening ge ne expansin and plasma membrane transporters for sucrose and K+, the major osmotic solutes imported into fibers. Confocal imaging of the membrane-impe rmeant fluorescent solute carboxyfluorescein (CF) revealed that the fiber p lasmodesmata were initially permeable to CF (0 to 9 DAA), but closed at sim ilar to 10 DAA and re-opened at 16 DAA. A developmental switch from simple to branched plasmodesmata was also observed in fibers at 10 DAA. Coincident with the transient closure of the plasmodesmata, the sucrose and K+ transp orter genes were expressed maximally in fibers at 10 DAA with sucrose trans porter proteins predominately localized at the fiber base. Consequently, fi ber osmotic and turgor potentials were elevated, driving the rapid phase of elongation. The level of expansin mRNA, however, was high at the early pha se of elongation (6 to 8 DAA) and decreased rapidly afterwards. The fiber t urgor was similar to the underlying seed coat cells at 6 to 10 DAA and afte r 16 DAA. These results suggest that fiber elongation is initially achieved largely by cell wall loosening and finally terminated by increased wall ri gidity and loss of higher turgor. To our knowledge, this study provides an unprecedented demonstration that the gating of plasmodesmata in a given cel l is developmentally reversible and is coordinated with the expression of s olute transporters and the cell wall-loosening gene. This integration of pl asmodesmatal gating and gene expression appears to control fiber cell elong ation.