Reorientation of Microfibrils and Microtubules at the Outer Epidermal Wall of Maize Coleoptiles During Auxin‐Mediated Growth

Abstract

Auxin‐mediated elongation growth of isolated subapical coleoptile segments of maize (Zea mays L.) is controlled by the extensibility of the outer cell wall of the outer epidermis (Kutschera et al., 1987). Here we investigate the hypothesis that auxin controls the extensibility of this wall by changing the orientation of newly deposited microfibrils through a corresponding change in the orientation of cortical microtubules. On the basis of electron micrographs it is shown that cessation of growth after removal of the endogenous source of auxin is correlated with a relative increase of longitudinally orientated microfibrils and microtubules at the inner wall surface. Conversely, reinduction of growth by exogenous auxin is correlated with a relative increase of transversely orientated microfibrils and microtubules at the inner wall surface. These changes can be detected 30–60 min after the removal and addition of auxin, respectively. The functional significance of directional changes of newly desposited wall microfibrils for the control of elongation growth is discussed.

The Auxin-induced Maize Gene ZmSAUR2 Encodes a Short-lived Nuclear Protein Expressed in Elongating Tissues

“…3) one may infer that induction starts earlier than 5 min, similar to soybean SAUR mRNAs. Thus, ZmSAUR2 transcripts precede auxin-induced cell elongation, for which in corn coleoptiles a lag phase of growth of about 15 min was measured (34,48). Compared with the induction kinetics of the recently identified ZmSAUR1, the ZmSAUR2 gene is more rapidly induced; in addition, ZmSAUR2 is not expressed in roots (13).…”

“…Already after 10 min of induction (the earliest time point taken in this experiment) it is higher than the in situ level and increases further within the following 60 min. Bergfeld et al (34) show that exogenous application of IAA to auxin-depleted coleoptile segments reinduces elongation after a lag period of about 16 min. Thus, ZmSAUR2 transcripts appear before the segments start to elongate.…”

Hormones act downstream of TTG and GL2 to promote root hair outgrowth during epidermis development in the Arabidopsis root.

“…60th auxin and ethylene are known to influence cell expansion (Davies, 1987;Abeles et al, 1992). Auxin may act by inducing proton efflux leading to acidification of the cell wall and changes in wall plasticity (Evans, 1984;Cleland, 1988) and/or by altering the orientation of microtubules (Bergfeld et al, 1988;Nick -et al, 1994;Shibaoka, 1994). Ethylene has been shown to induce radial cell expansion and reorientation of microtubules (Lang et al, 1982;Roberts et al, 1985;Shibaoka, 1994).…”

An auxin-binding protein is localized to the plasma membrane of maize coleoptile cells: identification by photoaffinity labeling and purification of a 23-kda polypeptide.

“…Although the chemical structure of the major natural auxin, indole-3-acetic acid (IAA), has been known for many years (2), the primary biochemical and molecular events responsible for auxin action are not yet understood. Among the first physiological changes observed after application of this phytohormone to plant tissues are an alteration of electrical potential of plasma membranes, an activation of H+-ATPase leading to H+ secretion, and a reorientation of cortical microtubules (3,4).…”