The function of a lily pollen tube is to elongate down the style of a lily flower and deliver its sperm cells to the ovule to complete the process of double fertilization. This highly polarized growth process relies on the arrangement and dynamic activity of the actin cytoskeleton. This dissertation will focus on the structure of the actin cytoskeleton in lily pollen tubes, pH dynamics and their effect on growth, pH effects on actin, and actin activity as gleaned from the analysis of organelle movements. Chapter 2 reveals that lily pollen tubes posses a cortical actin fringe in their apical domain, composed of parallel actin bundles located only in the cortex about 1-5 μm from the apex. The actin fringe is fragile and only present during growth. Chapter 3 focuses on pH microdomains and oscillations during pollen tube growth. Using cross-correlation analysis, a rise in alkalinity was found to lead the growth process, identifying pH as a potential prime regulator of growth. Interestingly, the alkaline band in the tip of pollen tubes coincides with the cortical actin fringe. In addition, two actin binding proteins, actin depolymerizing factor (ADF) and actin interacting protein (AIP), the former being pH sensitive, bind in the same vicinity as the cortical actin fringe. Since cytoplasmic acidification leads to a fragmented actin cytoskeleton and a rearrangement of ADF and AIP distribution, it is likely these proteins play a role in pollen tube growth. Finally, in Chapter 4, the actin dependent motion of the endoplasmic reticulum (ER), mitochondria, and the vacuole is characterized. ER motion and distribution oscillates, alternating between an inverted cone shape and a solid configuration close to the vesicle rich apex. These changes occur with the same periodicity as growth oscillations; however, phase analysis reveals that the ER platform arises 4 seconds before maximal growth rates, and could be indicative of actomyosin activity leading the growth process. The ER and mitochondria move similarly, but the vacuole is quite different from both of these organelles, implying differential myosin activity. Future studies should focus on deciphering mechanisms of how actin turnover contributes to pollen tube growth.*
*This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Adobe Acrobat; Microsoft Office; Windows MediaPlayer or RealPlayer.
