Alterations in the Cytoskeleton Accompany Aluminum-Induced Growth Inhibition and Morphological Changes in Primary Roots of Maize1

Abstract

Although Al is one of the major factors limiting crop production, the mechanisms of toxicity remain unknown. The growth inhibition and swelling of roots associated with Al exposure suggest that the cytoskeleton may be a target of Al toxicity. Using indirect immunofluorescence microscopy, microtubules and microfilaments in maize (Zea mays L.) roots were visualized and changes in their organization and stability correlated with the symptoms of Al toxicity. Growth studies showed that the site of Al toxicity was associated with the elongation zone. Within this region, Al resulted in a reorganization of microtubules in the inner cortex. However, the orientation of microtubules in the outer cortex and epidermis remained unchanged even after chronic symptoms of toxicity were manifest. Auxin-induced reorientation and cold-induced depolymerization of microtubules in the outer cortex were blocked by Al pretreatment. These results suggest that Al increased the stability of microtubules in these cells. The stabilizing effect of Al in the outer cortex coincided with growth inhibition. Reoriented microfilaments were also observed in Al-treated roots, and Al pretreatment minimized cytochalasin B-induced microfilament fragmentation. These data show that reorganization and stabilization of the cytoskeleton are closely associated with Al toxicity in maize roots.

Impacts of Aluminum on the Cytoskeleton of the Maize Root Apex. Short-Term Effects on the Distal Part of the Transition Zone1

“…Our study confirms the findings of Blancaflor et al (1998) that MTs of elongating cells are relatively insensitive to Al (at least as can be inferred from static immunofluorescence images) during the first 3 h of the Al exposure. However, focusing on the elongation zone, Blancaflor et al (1998) missed those critical cells of the epidermis and outer cortex in the DTZ, which initiated degeneration of peripheral tissue domains in less than 3 h. Al-mediated alterations to the MT cytoskeleton and to the organization of the maize root apex can be mimicked by auxin over-supplies (Blancaflor and Hasenstein, 1995a;Baluška et al, 1996b) and by auxin-transport inhibitors (Kerk and Feldman, 1994;Ruegger et al, 1997; this study).…”

“…However, high auxin treatment temporarily depolymerized the MTs in all of the root cells (Baluška et al, 1996a). Although most of the elongating cortex cells preserved their transverse MT arrays, some inner cortex cells acquired randomly arranged CMTs after 1 h of Al treatment, and this feature became even more prominent after 6 and 12 h of Al treatment, when longitudinally arranged CMT arrays appeared in some inner cortex cells (see also Blancaflor et al, 1998). This confirms that the two to three outermost cortical layers, representing the outer cortex, are unique tissues, differing in many aspects from cells of the middle and inner cortex (Baluška et al, 1993a).…”

“…However, focusing on the elongation zone, Blancaflor et al (1998) missed those critical cells of the epidermis and outer cortex in the DTZ, which initiated degeneration of peripheral tissue domains in less than 3 h. Al-mediated alterations to the MT cytoskeleton and to the organization of the maize root apex can be mimicked by auxin over-supplies (Blancaflor and Hasenstein, 1995a;Baluška et al, 1996b) and by auxin-transport inhibitors (Kerk and Feldman, 1994;Ruegger et al, 1997; this study). For instance, as already mentioned, high auxin levels induced temporary depolymerization of MTs.…”

“…Another typical auxin-induced response of the MT cytoskeleton was a rearrangement of CMTs, which typically changed from transverse to random and longitudinal arrays (Blancaflor and Hasenstein, 1995a;Baluška et al, 1996b). Imbalances to endogenous auxin levels could be relevant for morphological disorders, such as the newly induced, periclinal divisions and subapical root swellings (Kerk and Feldman, 1994;Ruegger et al, 1997;Blancaflor et al, 1998). Al-induced inhibition of root growth, associated with progressively increased growth anisotropy, might be expected to involve disruption of putative auxincytoskeleton interactions (for review, see Shibaoka, 1994;Baluška et al, 1998).…”

“…Grabski and Schindler (1995), using a novel "cell optical displacement technique," reported that exposure of plant cells to Al increased the stability of actin MFs in suspension-cultured soybean cells; and Sasaki et al (1997) reported an Al-induced depolymerization of MTs in wheat cells. Recently, Blancaflor et al (1998) have studied Al-induced effects on MTs and actin MFs in elongating cells of maize root apices, and related the Al-induced growth inhibition to the stabilization of MTs in the central elongation zone.…”