Configuration of the microtubule cytoskeleton in elongating fibers of flax ( Linum usitatissimum L.)

There are three basic types of plant cell growth: isodiametric, unidirectional diffuse, and tip growth. During plant cell growth, microtubules are present in the cell cortex, appressed against the plasma membrane. It is well documented that these cortical microtubules determine the orientation of cell growth in unidirectional intercalary, growing cells, in which the microtubules are always found perpendicular to the axis of cell elongation. There are indications that bast fiber cells of flax have two types of cell growth within one single cell: unidirectional intercalary and tip growth. Since the ultimate length of fiber cells determines the quality of the flax fiber for industry, we study the growth of these cells. In order to determine whether tip growth occurs or not, we use the following tip growth indicators: the type of cell wall at the cell tip, vesicle accumulation at the cell tip, calcium gradient at the cell tip, configuration of the actin cytoskeleton, and configuration of the microtubule cytoskeleton. Here we report on the microtubule cytoskeleton. We started to study the microtubules in fixed flax fiber cells using immunocytochemistry. Both, sections, as well as enzymatically isolated fibers, were analyzed at increasing distances from the shoot apex. Young fibers from the subapical region exhibit microtubules that are positioned approximately perpendicular to the cell's long axis. After prolonged elongation they are found in a helical orientation. In the two tapering regions of the elongating fiber, the microtubules are more often in a perpendicular orientation than in the middle zone of the cell indicating that within a single fiber more than one growth rate might occur. After elongation ceased, all cortical MTs are positioned approximately parallel to the long axis of the fiber. Based on the changes of the configurations of MTs in growing fibers, it is concluded that flax fibers exhibit coordinated growth first, and then also begin to exhibit intrusive growth at both ends as well. Until now, no observations support tip growth.