GENES INVOLVED IN OSMOREGULATION DURING TURGOR-DRIVEN CELL EXPANSION OF DEVELOPING COTTON FIBERS ARE DIFFERENTIALLY REGULATED

Cotton (Gossypium hirsutum L.) fibers are single-celled trichomes that synchronously undergo a phase of rapid cell expansion, then a phase i ncluding secondary cell wall deposition, and finally maturation. To de termine if there is coordinated regulation of gene expression during f iber expansion, we analyzed the expression of components involved in t urgor regulation and a cytoskeletal protein by measuring levels of mRN A and protein accumulation and enzyme activity. Fragments of the genes for the plasma membrane proton-translocating ATPase, vacuole-ATPase, proton-translocating pyrophosphatase (PPase), phosphoenolpyruvate carb oxylase, major intrinsic protein, and cu-tubulin were amplified by pol ymerase chain reaction and used as probes in ribonuclease protection a ssays of RNA from a fiber developmental series, revealing two discrete patterns of mRNA accumulation. Transcripts of all but the PPase accum ulated to highest levels during the period of peak expansion (+12-15 d postanthesis [dpa]), then declined with the onset of secondary cell w all synthesis. The PPase was constitutively expressed through fiber de velopment. Activity of the two proton-translocating-ATPases peaked at +15 dpa, whereas PPase activity peaked at +20 dpa, suggesting that all are involved in the process of cell expansion but with varying roles. Patterns of protein accumulation and enzyme activity for some of the proteins examined suggest posttranslational regulation through fiber d evelopment.