Differential Exudation of Polypeptides by Roots of Aluminum-Resistant and Aluminum-Sensitive Cultivars of Triticum aestivum L. in Response to Aluminum Stress.

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Cultivars of Triticum aestivum differing in resistance to Al were grown under aseptic conditions in the presence and absence of Al and polypeptides present in root exudates were collected, concentrated, and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Upon exposure to 100 and 200 [mu]M Al, root elongation in Al-sensitive cultivars was reduced by 30 and 65%, respectively, whereas root elongation in resistant cultivars was reduced by only 15 and 30%. Accumulation of polypeptides in the growth medium increased with time for 96 to 120 h, with little additional accumulation thereafter. This pattern of exudation was virtually unaffected by exposure to 100 [mu]M Al in the Al-resistant cultivars Atlas 66 and Maringa, whereas total accumulation was reduced in sensitive cultivars. Changes in exudation were consistent with alterations in root elongation. Al-induced or Al-enhanced polypeptide bands were detected in Atlas 66 and Maringa after 72 h of exposure to Al. Increased accumulation of 12-, 22-, and 33-kD bands was observed at 75 [mu]M Al in Atlas 66 and 12-, 23-, and 43.5-kD bands started to appear at 50 [mu]M Al in Maringa. In the Al-sensitive cultivars Roblin and Katepwa, no significant effect on polypeptide profiles was observed at values up to 100 [mu]M Al. When root exudates were separated by ultrafiltration and the Al content was measured in both high molecular mass (HMM; >10 kD) and ultrafiltrate (<10 kD) fractions, approximately 2 times more Al was detected in HMM fractions from Al-resistant cultivars than from Al-sensitive cultivars. Dialysis of HMM fractions against water did not release this bound Al;digestion with protease released between 62 and 73% of total Al, with twice as much released from exudates of Al-resistant than of Al-sensitive cultivars. When plants were grown in the presence of 0 to 200 [mu]M Al, saturation of the Al-binding capacity of HMM exudates occurred at 50 [mu]M Al in Al-sensitive cultivars. Saturation was not achieved in resistant cultivars. Differences in exudation of total polypeptides in response to Al stress, enhanced accumulation of specific polypeptides, and the greater association of Al with HMM fractions from Al-resistant cultivars suggest that root exudate polypeptides may play a role in plant response to Al.

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