Aluminium rhizotoxicity in maize grown in solutions with Al3+ or Al(OH)4- as predominant solution Al species

Abstract

The rhizotoxicity of aluminium at low-pH with Al(3+) and at high pH with Al(OH)-(4) as the main Al species was studied. Aluminium reduced root growth to similar levels at pH 8.0 and pH 4.3, although the mononuclear Al concentration at pH 8.0 was three times lower than at pH 4.3. Al contents of root apices were much higher at pH 8 than at pH 4.3. Callose was induced only marginally at pH 8 and the formation was confined to the epidermis, whereas it proceeded through the cortex with time at pH 4.3. Well-documented genotypical differences in callose formation and Al accumulation could not be found at pH 8. The largest fraction of the root-tip Al was recovered in the cell-wall fraction independent of the solution pH. A sequential extraction of isolated cell walls suggests that most of the cell-wall Al was precipitated Al(OH)(3) at pH 8.0. This can be explained by a drastic pH reduction in the root apoplastic sap to 6.2, whereas at bulk solution pH 4.3 it rose to 5.6. Al precipitation was also confirmed by the microscopic localization of Al. At pH 8, Al could mostly be found in the epidermis, but in the apoplast of the outer cortex at pH 4.3. It is proposed here that at pH 4.3, Al(3+) inhibits root growth through binding to sensitive binding sites in the apoplast of the epidermis and the outer cortex. At pH 8, Al(OH)(3) precipitation in the epidermis causes a mechanical barrier thus impairing the root-growth control of the epidermis.

Rhizosphere pH difference regulated by plasma membrane H⁺-ATPase is related to differential Al tolerance of two wheat cultivars

“…Stass et al (2006) found that Al mainly existed as biologically toxic Al 3+ at rhizosphere pH < 4.3. Contents of active Al 3+ were decreased when the rhizosphere pH was increased from 4.0 to 5.0 (Zhu et al 2005).…”

The cell wall in plant cell response to trace metals: polysaccharide remodeling and its role in defense strategy

“…It functions as a physical barrier against many stress factors, which may threaten to penetrate the protoplast, e.g., biotic threats such as fungal pathogens (Bolwell et al 2002), or abiotic threats including wounding (Jacobs et al 2003) and TM stress (e.g., Hématy et al 2009). Callose appearance under TM stress has been observed in the response of a range of plant species, e.g., in response to Pb in Lemna minor (Samardakiewicz et al 1996) and F. hygrometrica protonemata (Krzesłowska et al 2009a), and to Al in Triticum aestivum (Sivaguru et al 2000), A. thaliana (de Cnodder et al 2005) and Zea mays (Stass et al 2006). In plants exposed to Al, it has been shown that callose deposition was inducted by the appearance of ROS in the CW and alteration of calcium homeostasis, mainly its increase within protoplast caused by the metal (Sivaguru et al 2000;Jones et al 2006).…”

Changes induced by aluminum stress in the organic acid content of maize seedlings: The crucial role of exogenous citrate in enhancing seedling growth

“…It was recognized that significant accumulation of Al in the cell walls may alter their structural and mechanical properties, and thus affect the root growth (Wang et al 2004). The Al binding to sensitive binding sites in the apoplast of the epidermis and the outer cortex of roots has been shown to inhibit root growth in maize (Stass et al 2006). The Al binding to these negatively charged sites can lead to displacement of Ca 2+ , fundamental for cell-wall stability (Rincon & Gonzales 1992;Tabuchi & Matsumoto 2001).…”