Plant cell walls regulate plant cell development and function. The major structural elements are cellulose microfibrils, whose orientation in the expanding cell wall dictates the direction of expansion and hence cell shape, contributing largely to the shape of the plant organ assembled from these cells.
Cotton fibres are remarkable single-celled trichomes that develop about from about 30% of cotton seed coat epidermal cells. The fibres emerge as small balloons at the day of flowering, and once the flowers open, begin rapid elongation to a final length of up to 4 cm. Transversely-oriented cytoplasmic microtubules ensure that the cellulose microfibrils are also transversely-oriented during this period of rapid expansion. Once the fibres cease elongating, about 15-18 days after flower opening, there is considerable additional deposition of cellulose to form a thick cellulosic secondary cell wall, and the predominant microfibril orientation becomes more oblique to the long axis of the fibre.
We are interested in new methods to examine cellulose microfibril orientation during fibre development. Here, we used second harmonic imaging from a multiphoton confocal to reveal the orientation of crystalline structures within the walls of mature cotton fibres.
Strong second harmonic signals were obtained from mature cotton fibres, in both the forward and reverse directions. The subtle differences between these two signals may indicate details of inter-microfibril organisation, similar to the interpretation of signals from collagen assemblages in animal cells. Varieties of cotton producing different types of fibre also generated different second harmonic signals. However, the nature of the crystalline structures generating these signals is unclear, and we are currently exploring additional structural analyses of fibre walls, as well as following the development of second harmonic signals over time. These and additional data will be discussed with reference to the use and interpretation of second harmonic signals from plant cell walls.