Possible role of root border cells in detection and avoidance of aluminum toxicity.

Root border cells are living cells that surround root apices of most plant species and are involved in production of root exudates. We tested predictions of the hypothesis that they participate in detection and avoidance of aluminum (Al) toxicity by comparing responses of two snapbean (Phaseolus vulgaris) cultivars (cv Dade and cv Romano) known to differ in Al resistance at the whole-root level. Root border cells of these cultivars were killed by excess Al in agarose gels or in simple salt solutions. Percent viability of Al-sensitive cv Romano border cells exposed in situ for 96 h to 200 microM total Al in an agarose gel was significantly less than that of cv Dade border cells; similarly, relative viability of harvested cv Romano border cells was significantly less than that of cv Dade cells after 24 h in 25 microM total Al in a simple salt solution. These results indicate that Al-resistance mechanisms that operate at the level of whole roots also operate at the cellular level in border cells. Al induced a thicker mucilage layer around detached border cells of both cultivars. Cultivar Dade border cells produced a thicker mucilage layer in response to 25 microM Al compared with that of cv Romano cells after 8 h of treatment and this phenomenon preceded that of observed cultivar differences in relative cell viability. Release of an Al-binding mucilage by border cells could play a role in protecting root tips from Al-induced cellular damage.

Mechanism of aluminum tolerance in snapbeans : root exudation of citric Acid.

One proposed mechanism of aluminum (Al) tolerance in plants is the release of an Al-chelating compound into the rhizosphere. In this experiment, two cultivars of snapbeans (Phaseolus vulgaris L. "Romano" and "Dade") that differ in Al tolerance were grown hydroponically with and without Al under aseptic conditions. After growth in nutrient solutions for 8 days, aliphatic and phenolic organic acids were analyzed in the culture solutions with an ion chromatograph and a high pressure liquid chromatograph. The tolerant snapbean, "Dade", when exposed to Al, exuded citric acid into the rhizosphere in a concentration that was 70 times as great as that of "Dade" grown without Al, and 10 times as great as that of "Romano" grown with or without Al. The sensitive cultivar, "Romano", exuded only slightly more citric acid into the growing medium under Al-stress, compared to nonstressed conditions. Citric acid is known to chelate Al strongly and to reverse its phytotoxic effects. Also, citric acid has been shown previously to enhance the availability of phosphorus (P) from insoluble Al phosphates. Thus, one mechanism of Al-tolerance in snapbeans appears to be the exudation of citric acid into the rhizosphere, induced either by toxic levels of Al or by low P due to the precipitation of insoluble Al phosphates. Our experiment was not able to distinguish between these two factors; however, tolerance to both primary and secondary Al-stress injuries are important for plants growing in Al-toxic soils.

Mechanisms of Aluminum Tolerance in Wheat : An Investigation of Genotypic Differences in Rhizosphere pH, K, and H Transport, and Root-Cell Membrane Potentials.

Control of rhizosphere pH and exclusion of Al by the plasma membrane have been hypothesized as possible mechanisms for Al tolerance. To test primarily the rhizosphere pH hypothesis, wheat cultivars (Triticum aestivum L. ;Atlas 66' and ;Scout'), which differ in Al tolerance, were grown in either complete nutrient solution, or 0.6 millimolar CaSO(4), with and without Al at pH 4.50. A microelectrode system was used to simultaneously measure rhizosphere pH, K(+), and H(+) fluxes, and membrane potentials (E(m)) along the root at various distances from the root apex. In complete nutrient solution, the rhizosphere pH associated with mature root cells (measured 10-40 millimeters from the root apex) of Al-tolerant ;Atlas 66' was slightly higher than that of the bulk solution, whereas roots of Al-sensitive ;Scout' caused a very small decrease in the rhizosphere pH. In CaSO(4) solution, no significant differences in rhizosphere pH were found between wheat cultivars, while differential Al tolerance was still observed, indicating that the rhizosphere pH associated with mature root tissue is not directly involved in the mechanism(s) of differential Al tolerance. In Al-tolerant ;Atlas 66', growth in a CaSO(4) solution with 5 micromolar Al (pH 4.50) had little effect on net K(+) influx, H(+) efflux, and root-cell membrane potential measured in cells of mature root tissue (from 10-40 mm back from apex). However, in Al-sensitive ;Scout', Al treatment caused a dramatic inhibition of K(+) influx and both a moderate reduction of H(+) efflux and depolarization of the membrane potential. These results demonstrate that increased Al tolerance in wheat is associated with the increased ability of the tolerant plant to maintain normal ion fluxes and membrane potentials across the plasmalemma of root cells in the presence of Al.