Endoplasmic microtubules connect the advancing nucleus to the tip of legume root hairs, but F‐actin is involved in basipetal migration

Abstract

A prominent feature of tip growth in filamentous plant cells is that the nucleus often migrates in step with the tip as it extends. We have studied this long‐recognized but unexplained relationship in root hairs of the legume Vicia hirsuta by a variety of microscopic techniques. Using rhodaminyl lysine phallotoxin, and antitubulin antibodies, root hairs are shown to contain axial bundles of F‐actin and a complex microtubular system. To the basal side of the nucleus the microtubules are cortical and net axial but in the region between nucleus and tip the arrangement is more complicated. Electron microscopic thin sections demonstrate that internal bundles of microtubles exist in addition to the plasma membrane‐associated kind. Computerized deblurring of through‐focal series of antitubulin stained hairs clarifies the three‐dimensional organization: bundles of endoplasmic microtubules progress from the nuclear region toward the apical dome where they can be seen to fountain out upon the cortex. The relationship between nucleus and tip can be uncoupled with antimicrotubule herbicides. Time lapse video microscopy shows that these agents cause the nucleus to migrate toward the base. This contrary migration can be inhibited by adding cytochalasin D, which fragments the F‐actin bundles. It is concluded that microtubules connect the nucleus to the tip but that F‐actin is involved in basipetal migration as is known to occur when symbiotic bacteria uncouple the nucleus from the tip.

The maize actin‐depolymerizing factor, ZmADF3, redistributes to the growing tip of elongating root hairs and can be induced to translocate into the nucleus with actin

“…The elongation of root hairs is dependent on the cooperative function of the microtubule and actin arrays (Lloyd et al, 1987). The reorganization of the actin cytoskeleton, shown here in maize root hairs, is very striking.…”

“…These data demonstrate that ZmADF3 concentrates at the tip of the root hairs during elongation. Bar = 20 p.m. present at the site of tip growth (Lloyd and Wells, 1985;Lloyd et al, 1987) but there is uncertainty about the organization of the microtubule and actin networks at this point. In this context the localization of ZmADF3 at the tips of maize root hairs is interesting.…”

“…The cytoskeleton plays a central role in root hair formation. Nuclear migration and tip growth are processes common to both root hairs and pollen tubes in plants and are coordinated by microtubule and actin networks (Heslop-Harrison and Heslop-Harrison, 1989a;Lloyd and Wells, 1985;Lloyd et aL, 1987;Shimmen etal., 1995). The nucleus is moved into the root hair during hair elongation and wall forming vesicles travel acropetally through cytoplasmic streaming and fuse to the cell's growing tip.…”

Nuclear Dynamics inArabidopsis thaliana

“…In other eukaryotes, nuclear movement is associated primarily with cell division or with a phenomenon termed nuclear migration or nuclear positioning (Reinsch and Gö nczy, 1998;Morris, 2000), typified by mating and cell division in yeast (Shaw et al, 1997;Yamamoto et al, 1999), hyphal tip-associated growth in filamentous fungi (Morris, 2000), early embryogenesis and eye differentiation in Drosophila (Foe and Alberts, 1983;Baker et al, 1993), and development in Caenorhabditis elegans (Morris, 2000). Nuclear positioning has also been described in legume root hairs (Lloyd et al, 1987), Micrasterias (Meindl et al, 1994), and Spirogyra (Grolig, 1998) and during fertilization in Pelvetia (Swope and Kropf, 1993). Other cases of nuclear positioning have been reported within the syncytial embryo (von Dassow and Schubiger, 1994) and nurse cells (Guild et al, 1997) of Drosophila.…”

“…The elongated appearance of the nuclei and the observations of strand-like connections between subnuclear components favor the former possibility, as does the observation of no clear correlation between patterns of cytoplasmic streaming (compare Movies 1, 3, and 4) and overall nuclear movement. It was not possible to configure the confocal microscope for the simultaneous examination of cytoplasmic streaming with the use of either phase contrast or Nomarski illumination.Within root hairs, the lack of involvement of microtubules in nuclear movement generally distinguishes this phenomenon from that of most cases of nuclear migration, nuclear positioning, and nucleokinesis (Foe and Alberts, 1983;Lloyd et al, 1987;Baker et al, 1993;Swope and Kropf, 1993;Meindl et al, 1994;Shaw et al, 1997;Grolig, 1998;Reinsch and Gö nczy, 1998;Yamamoto et al, 1999;Morris, 2000). The situation in Drosophila may be different, with nuclear positioning in nurse cells and the syncytial embryo being reported to be exclusively actin associated (von Dassow and Schubiger, 1994;Guild et al, 1997).…”