Fiber quality is significantly influenced by the rate of fiber elongation and fiber length. These two critical parameters of fiber development are determined by the internal turgor pressure of the growing cotton fiber. Thus, investigating the genes and gene products involved in regulating turgor pressure of fiber cells is important to elucidate the molecular determination of desirable fiber properties.

A key enzyme involved in generating turgor pressure and thus determining the rate of fiber elongation and fiber length is the vacuolar proton ATPase (H⁺-ATPase). The vacuolar H⁺-ATPase is a multimeric enzyme composed of a 70 kDa catalytic subunit, a 60 kDa subunit and a 16 kDa integral membrane proteolipid. This enzyme complex functions as a proton pump and consequently generates the driving force for solute transport across the vacuolar membrane. The transport and accumulation of osmoregulatory solutes to vacuoles is the mechanism by which the cell regulates internal turgor pressure. Therefore, the process of fiber elongation and determination of fiber length is dependent upon the activity of vacuolar H⁺-ATPases.

A family of cDNA clones have been isolated from developing cotton ovules using a cDNA clone encoding the 70 kDa subunit of the H⁺ATPase from carrot. Northern blot analysis of total RNA isolated from ovules collected at anthesis demonstrated that several cotton cDNA clones hybridized to the same size 2.5 kb mRNA species as the heterologous probe from carrot. These results suggest that the cotton c DNA clones encode the vacuolar H⁺-ATPase.

The expression of the vacuolar H⁺-ATPase in developing cotton ovules was examined at the RNA level by Northern blot and RNA slot blot analysis. Results indicate that the vacuolar ATPase is differentially expressed during fiber development. More importantly, however, is the temporal-specific accumulation of ATPase mRNAs to peak levels coincident with fiber initiation and with the onset of maximum fiber elongation. These results provide an important link in establishing a role of the vacuolar ATPases in fiber initiation and elongation.