The High Level of Aluminum Resistance in Signalgrass Is Not Associated with Known Mechanisms of External Aluminum Detoxification in Root Apices

Abstract

Al resistance of signalgrass (Brachiaria decumbens Stapf cv Basilisk), a widely sown tropical forage grass, is outstanding compared with the closely related ruzigrass (Brachiaria ruziziensis Germain and Evrard cv Common) and Al-resistant genotypes of graminaceous crops such as wheat, triticale, and maize. Secretion of organic acids and phosphate by root apices and alkalinization of the apical rhizosphere are commonly believed to be important mechanisms of Al resistance. However, root apices of signalgrass secreted only moderately larger quantities of organic acids than did those of ruzigrass, and efflux from signalgrass apices was three to 30 times smaller than from apices of Al-resistant genotypes of buckwheat, maize, and wheat (all much more sensitive to Al than signalgrass). In the presence, but not absence, of Al, root apices of signalgrass alkalinized the rhizosphere more than did those of ruzigrass. The latter was associated with a shortening of the alkalinizing zone in Al-intoxicated apices of ruzigrass, indicating that differences in alkalinizing power were a consequence, not a cause of, differential Al resistance. These data indicate that the main mechanism of Al resistance in signalgrass does not involve external detoxification of Al. Therefore, highly effective resistance mechanisms based on different physiological strategies appear to operate in this species.Al toxicity is one of the most important constraints to crop production on acid soils (Rao et al., 1993; De la Fuente-Martínez and Herrera-Estrella, 1999). Al 3ϩ ions, the most toxic mononuclear Al species, inhibit root elongation by injuring the root apex, particularly the distal part of the transition zone (Ryan et al., 1993;Kinraide, 1997;Sivaguru and Horst, 1998). Despite considerable research, the mechanistic basis of Al toxicity is not well understood (Delhaize and

The Physiology and Biophysics of an Aluminum Tolerance Mechanism Based on Root Citrate Exudation in Maize

“…Exposure of the seedlings to solutions containing Al 3ϩ activities ranging from 0 to 80 m in full nutrient solution only caused a modest inhibition of root growth ( Fig. 2A), generally causing no more than 20% inhibition (relative to the Al control) at an Al 3ϩ activity of 39 m, and only 30% root growth inhibition in solutions with an Al 3ϩ activity of 80 m. These results established that the maize single cross (SLP 181/71 ϫ Cateto Colombia 96/71) was extremely Al tolerant compared with other maize and other crop species reported in the literature (for comparisons, see Wenzl et al, 2001). The same Al exposures triggered a significant activation of citrate exudation by the primary intact root (Fig.…”

“…Although the degree of Al tolerance displayed by cv Cateto-Colombia (as indicated by the root growth measurements) was higher than that reported for most other Al-tolerant plant species (Wenzl et al, 2001), comparisons should be made with caution, as there could be some degree of amelioration of Al toxicity by the high-ionic-strength full nutrient solutions used in this study (in contrast to some other studies where tolerance has been assessed in less physiological simple-salt solutions).…”

Aluminum tolerance measured by root growth and mucilage protection inUrochloa brizanthaandUrochloa decumbens

“…Mechanism of exclusion will include exudation of organic acids. Some species, such as U. decumbens, may have tolerance mechanisms based on exclusion of aluminum at the root apices, where exudation of organic acids does not seem to be related (Wenzl et al 2001). Despite some uncertainties, the root apex is the region most sensitive to aluminuminduced stress in plants (Sivaguru and Horst 1998;Huang et al 2009).…”

Evaluation of aluminum resistance in hybrids of Brachiaria decumbens Stapf.

“…At CIAT (International Center for Tropical Agriculture), a previous study has confirmed that Brachiaria showed genetic diversity for this trait, and B. decumbens cv. Basilisk proved to be the most Al-resistant (Wenzl et al 2001). Wenzl et al (2006) developed a methodology to evaluate the edaphic adaptation of brachiaria grass genotypes using vegetative propagules (stem cuttings) grown in solution culture.…”