MICROTUBULES AND THE CONTROL OF CELL ELONGATION IN ARABIDOPSIS ROOTS

摘要

A role for cortical microtubules, cellulose microfibrils and cell wall enzymes in the control of plant cell expansion has been suggested for several decades, but has not yet been proven in any system. Arabidopsis root epidermis is an ideal tissue to test this hypothesis, and consequently is the subject of many recent reports. Microtubules of epidermal cells are transverse to the root axis in the zone of fast elongation, but when cells enter the differentiation zone microtubules switch to a slightly oblique orientation in atrichoblasts and to a disordered distribution in trichoblasts. Factors that severely affect root elongation do so by acting on the process of fast elongation. Ethylene, auxin, water stress as well as reduced cellulose synthesis induce a similar response. Within minutes a new and much shorter final cell size is defined, and cells do not grow beyond this length. Eventually, fast cell elongation is totally inhibited. In cells that are longer than this new final cell size, cortical microtubules change their orientation in the time frame of 10-15 min and adopt orientations typical for the differentiation zone, while the orientation of cellulose fibrils is not changed in the walls of the affected cells. Microtubules, however, are not the causal agents: as fast elongation and its inhibition are insensitive to microtubule disrupting or stabilizing drugs. Control of cell size seems to be located at the level of cell wall metabolism. The profile of xyloglucan endotransglucosylase action changes only slowly, but a significant callose deposition in the walls of the underlying cortex cells accompanies the inhibition of elongation.