Analysis of plant growth from the inside out - Involvement of cell walls, cortical microtubules and cell-cell relationships in guiding directionality of elongating cells

Plants are sessile organisms in need to acclimate to the changes in the surrounding environment by changing their metabolism and growth behavior. Responses to external stimuli such as gravity, light, or touch can result in tightly coordinated growth responses, regulating the final shape and size of an organ. The coordination of organ growth has to occur on several levels. Plant cells are surrounded by a rigid cell wall, which governs final cell shape and cell size. These walls allow for expansion by deposition and loosening of cell wall material; and simultaneously for directionality through alignment of the load-bearing cellulose fibrils. At the same time, these cell walls generate a connecting matrix that links adjacent cells and requires for these cells to grow in units, so called symplastic growth. In plants with helical aligned cell files along axial organs, this connection through the cell wall is thought to cause cells to rotate in-phase around the organ axis, with a decrease of cell file length and angle towards organ axis from the outer cell layers inwards. At the same time, if cell anisotropy is not altered, the organ diameter increases dependent on the angle described by the cell files. In addition to these biomechanical constraints imposed on cells within one organ, cell directionality also affects organ growth behavior. If elongating cells are oriented helically around an axial organ, their growth axis is not aligned with the organs growth axis. The cell growth vector can be divided into its components parallel and perpendicular to the organ growth axis, the first one drives organ growth, the second a rotation of the organ tip, both in an angle dependent manner. Organ growth is decreased, most likely due to the parallel vector component being shorter than the cell growth vector. Cell directionality is regulated by several cellular factors. Cortical microtubules are positioned along the plasma membrane of the cell. In elongating cells they are organized in parallel arrays, approximately perpendicular to the growth axis of the cell, forming hoop-like structures around the cell. They are thought to guide cellulose producing enzyme complexes into and within the plasma membrane, resulting in cellulose fibrils being aligned in a similar manner. Cellulose fibril alignment in turn guides turgor pressure driven expansion, and thus directionality of cell expansion. In this work, the mutant twd1, defective in a FKBP-like immunophilin that displays a helical cell file orientation, was used to elucidate relationships between cell-cell and cell-organ growth, as well as to investigate cortical microtubules and cell wall components in regard to the altered cell file orientation