Auxin, actin and growth of the Arabidopsis thaliana primary root

Abstract

SummaryTo understand how auxin regulates root growth, we quantified cell division and elemental elongation, and examined actin organization in the primary root of Arabidopsis thaliana. In treatments for 48 h that inhibited root elongation rate by 50%, we find that auxins and auxin-transport inhibitors can be divided into two classes based on their effects on cell division, elongation and actin organization. Indole acetic acid (IAA), 1-naphthalene acetic acid (NAA) and tri-iodobenzoic acid (TIBA) inhibit root growth primarily through reducing the length of the growth zone rather than the maximal rate of elemental elongation and they do not reduce cell production rate. These three compounds have little effect on the extent of filamentous actin, as imaged in living cells or by chemical fixation and immuno-cytochemistry, but tend to increase actin bundling. In contrast, 2,4-dichlorophenoxy-acetic acid (2,4-D) and naphthylphthalamic acid (NPA) inhibit root growth primarily by reducing cell production rate. These compounds remove actin and slow down cytoplasmic streaming, but do not lead to mislocalization of the auxin-efflux proteins, PIN1 or PIN2. The effects of 2,4-D and NPA were mimicked by the actin inhibitor, latrunculin B. The effects of these compounds on actin were also elicited by a 2 h treatment at higher concentration but were not seen in two mutants, eir1-1 and aux1-7, with deficient auxin transport. Our results show that IAA regulates the size of the root elongation zone whereas 2,4-D affects cell production and actin-dependent processes; and, further, that elemental elongation and localization of PINs are appreciably independent of actin.

Increase in Invaginated Vacuolar Membrane Structure Caused by Plant Cell Expansion by Genotoxic Stress Induced by DNA Double-Strand Breaks

“…The network of cytoplasmic strands would alter the nuclear position to the appropriate location to efficiently receive the materials for DNA repair in meristematic cells that are abundant in auxin. Previous studies indicated that auxin affects plant morphogenesis through the regulation of actin filaments (Maisch and Nick 2007, Nick et al 2009, Rahman et al 2007). This result would indicate that actin dynamics in the cytoplasmic strands are controlled by auxin.…”

Master and servant: Regulation of auxin transporters by FKBPs and cyclophilins

“…At low micromolar concentrations, auxin transport inhibitors are thought to impair directly the activity of auxin transporters [37,41,101], whereas at higher concentrations (>50 M) auxin transport inhibitors seem to interfere with the bundling status of the actin cytoskeleton, thus affecting the presence of auxin exporters on the plasma membrane in this manner [7,[102][103][104][105]. These two modes of actions for NPA on auxin transport, which could be even interconnected [7], might be represented by two distinct NPA binding affinities, as reported for zucchini plasma membrane and speculated to correspond to the auxin transporter itself and an additional NPA-binding regulatory subunit [106].…”

Gravitropism ofArabidopsis thalianaRoots Requires the Polarization of PIN2 toward the Root Tip in Meristematic Cortical Cells

“…Nevertheless, PIN2 is also expressed in the root cortex, where its localization pattern is puzzling. Although in the elongation zone, cortical PIN2 has a shootward localization, in the meristem, the protein has a rootward localization (Rahman et al, 2007). The functional significance of rootward localization of PIN2 in meristematic cortex cells remains enigmatic.…”