Microtubules in legume root hairs: cell polarity and response to rhizobial signal molecules

Microtubules, which occur as hollow protein tubes with a diameter of 25 nanometers, are an important compound of the cytoskeleton and occur in plant cells as a highly organized and dynamic array, which actual arrangement will depend on its tasks during the cell cycle. Microtubules play a key-role in the spatial distribution of the cytoplasm, cell growth, and cell division, and they respond to external or internal stimuli. In this thesis the microtubule cytoskeleton in developing Medicago truncatula root hairs is described. Cortical microtubules are present in all developmental stages, but the sub-apical cytoplasmic dense region of tip-growing root hairs contains a specific microtubule configuration that was not described for interphase cells in detail before. When tip growth begins, the hairs acquire an extensive endoplasmic microtubule array in their sub-apex, which they retain until growth stops. These endoplasmic microtubules may be specific for legume root hairs. They are essential in maintaining the highly polarized distribution of cytoplasm in growing root hairs, and in keeping the growth of these hairs at a high rate. Further, the endoplasmic microtubule cytoskeleton appears to be a target of early Nod factor signaling. Nod factors are rhizobacterial signal molecules, which cause several cellular responses in legume root hairs. The endoplasmic microtubule array in elongating hairs responds to Nod factors with a subtle and short termed shortening, whereas cortical microtubules are not obviously affected. The presented results demonstrate that the microtubule cytoskeleton contributes to sustaining tip-growth and to determining the growth direction of hairs after Nod factor application. The latter two features are crucial for root hair curling around rhizobia, which is one of the first steps in establishing symbiosis between legumes and rhizobia. A hypothesis how microtubules may determine the site of tip-growth and thereby the directionality of root hair growth is discussed.