Protein expression during early cotton fiber development

The total protein composition of developing cotton (Gossypium hirsutum L.) ovules and fibers was examined at 6, 3 and 1 day preanthesis, anthesis and 2, 6, 10, 16 and 20 days postanthesis (-6, -3, -1, 0, 2, 6, 10, 16 and 20 dpa) using two-dimensional (20) gel electrophoresis. Ovule and fiber proteins from three fiber mutants were also examined. Fiber-specific and fiber-enriched proteins were identified by probing western blots of 20 gels with a fiber-specific serum. In addition, in vitro ovule culture was used to test the ability of preanthesis ovules to grow fibers.

Results indicate that cotton ovules express an early group of proteins from -3 through 2 dpa and a late group from 6 or 10 dpa through 20 dpa. The transition from early to late protein expression occurs between 2 and 10 dpa. Also, a constitutively expressed group of proteins is present.

Using the fiber-specific serum, 35 proteins were detected on western blots of total proteins from -6 to 20 dpa, 9 are fiber-specific, occurring only in the fibers and not in the ovules, and 9 are fiberenriched, occurring at a higher concentration or for a longer time in fibers versus ovules. The others are common to both fibers and ovules. Immunodetected proteins show early and late expression patterns similar to the expression trends seen with total proteins. Expression of early proteins coincides with initiation and elongation, and the expression of late proteins coincides with secondary cell wall synthesis.

Based on early protein expression patterns, the "initiation plateau hypothesis" was proposed. According to the hypothesis, epidermal cells are potentiated to fiber cells as early as -3 dpa; initiation of prefiber cells is triggered by phytohormonally; and initiation can occur any time following potentiation. By manipulating the hormonal environment in vitro, preanthesis ovules were shown to acquire the ability to produce fibers between -3 and -2 dpa. Preanthesis ovules (-2 or -1 dpa) maintain the ability to produce fibers in response to phytohormones after culture in hormone-free medium through the in vivo scheduled day of anthesis but not beyond. These data indicate the preprogramming of fiber cells between -3 and -2 dpa and a link between the preprogramming event and anthesis.