Roche, Meghan C. (2007-08). A study of programmed cell death in cotton (gosypium hirsutum) fiber. Master's Thesis.
Cotton fiber has been postulated to undergo a process of programmed cell death (PCD) during the maturation phase of development. A parallel may exist between cotton fibers and xylem tracheary elements, which have periods of elongation, secondary cell wall deposition and death. Secondary wall formation and PCD are purported to be coupled events in tracheary elements. In this study, an attempt was made to observe the occurrence and timing of PCD in cotton fibers by TUNEL staining to detect DNA strand breaks, and also to monitor DNA content by PI staining. The staining patterns produced by PI and TUNEL left room for interpretation. TUNEL-positive and PI-stained areas were observed, but failure to observe nuclei of conventional appearance in my cytological preparations at any time-point, along with possible nonspecific staining or autofluorescence of cell wall and intracellular components, made it difficult to draw firm conclusions of significance. Thus, additional analyses will be needed to prove or disprove current PCD theories. Nevertheless, the differences in TUNEL and PI signals across fiber development stages indicate that the observed fluorescence patterns are marking discrete developmental phases. The PI signal is dispersed throughout the cell during the elongation phase (5-15 DPA) and appears to condense during secondary cell wall synthesis (25- 40 DPA). TUNEL-positive signal may be observed as early as 25 DPA, but the signal is not widespread until 45 DPA. At 50 DPA and beyond, PI staining is reduced. Visually detectable DNA can be extracted from cotton fiber nuclei between 5 and 40 DPA, although a laddering pattern was not visible at any time-point. The results, although inconclusive, point to the possibility that PCD may be a process leading to maturation in the cotton fiber, succeeding completion of secondary cell wall synthesis.