Cytoarchitecture and pattern of cytoplasmic streaming in root hairs ofMedicago truncatula during development and deformation by nodulation factors

Summary The cytoarchitecture and the pattern of cytoplasmic streaming change during the development of root hairs ofMedicago truncatula and after a challenge with nodulation (Nod) factors. We measured the speed and orientation of movement of 1–2 μm long organelles. The speed of organelle movement in cytoplasmic strands in the basal part of growing root hairs is 8–14 μm/s and is of the circulation type like in trichoblasts, bulges before tip-growth initiation, and full-grown hairs. In the subapical area of growing hairs, reverse-fountain streaming occurs discontinuously at a slower net speed. The reason for the slower speed is the fact that organelles often stop and jump. Reverse-fountain streaming is a pattern in which the main direction of organelle transport reverses 180 degrees before the cell tip is reached. Within minutes after their application to roots,Rhizobium leguminosarum-derived Nod factors, cause an increase and divergence in the subapical cytoplasmic strands. This phenomenon can best be observed in the growth-terminating hairs, since in hairs of this developmental stage, subapical cytoplasmic strands are transvacuolar. First, the tips of these hairs swell. The organelle movement in the swelling tip increases up to the level normal for circulation streaming, and the number of strands with moving organelles increases. When a new polar outgrowth emerges, reverse-fountain streaming is set up again, with all its characteristics like those seen in growing hairs. This outgrowth may obtain a new full root hair length, by which these hairs may become twice as long as nonchallenged hairs. Dedicated to Professor Walter Gustav Url on the occasion of his 70th birthday