UC Riverside

In the process of pollination, a pollen tube grows from a pollen grain that has fallen on the stigma of a flower. This tube grows towards the ovary of the flower where it will deliver male reproductive material. This growth process is of significant interest in plant cell biology as it provides an understanding of internal cell dynamics that affect observable structural characteristics like cell diameter, length, and growth rate. Knowledge of these dynamics will provide a basis for understanding more complex cells that exhibit similar growth behavior. Pollen tube growth is the result of two main systems: a mechanical system that deforms the cell wall, and a signaling system that brings vesicles to the growth site. The mechanical system is a balance of forces between internal turgor pressure pushing outwards and cell wall resistance that maintains cell shape. Cell deformation occurs when this balance is broken when vesicles fuse with the cell membrane. Fusing vesicles release substances that reduce wall resistance, but also material that is used to maintain cell wall thickness. This cycle of fusion-growth continues until the pollen tube reaches its destination.

This thesis presents two computation models for tip growth and a model for cell segmentation. The first computational growth model considers the tip region as a single entity that is deformed via an affine transformation. The values of the transformation matrix (scale and sheer) change depending on the extensibility of the cell wall, which in turn depends on the number of vesicles that have fused with the cell wall. The second model treats the tip region as a collection of line segments whose motion in the normal direction produces tip deformation. The change in length of each line segment depends on its extensibility and number of fused vesicles. The segmentation method combines fluorescence and brightfield images to produces a single segmentation of the cell. This process is especially advantageous when the segmentation of each modality shows an incomplete cell shape. Experiments with the above methods show their strengths and advantages in pollen tube growth over other methods that do not use experimental videos for validation.