DIFFERENTIAL EXUDATION OF POLYPEPTIDES BY ROOTS OF ALUMINUM-RESISTANTAND ALUMINUM-SENSITIVE CULTIVARS OF TRITICUM-AESTIVUM L IN RESPONSE TO ALUMINUM STRESS

Cultivars of Triticum aestivum differing in resistance to Al were grow n under aseptic conditions in the presence and absence of Al and polyp eptides present in root exudates were collected, concentrated, and ana lyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Up on exposure to 100 and 200 mu M Al, root elongation in Al-sensitive cu ltivars was reduced by 30 and 65%, respectively, whereas root elongati on in resistant cultivars was reduced by only 15 and 30%. Accumulation of polypeptides in the growth medium increased with time for 96 to 12 0 h, with little additional accumulation thereafter. This pattern of e xudation 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 consiste nt with alterations in root elongation. Al-induced or Al-enhanced poly peptide bands were detected in Atlas 66 and Maringa after 72 h of expo sure to Al. Increased accumulation of 12-, 22-, and 33-kD bands was ob served at 75 mu M Al in Atlas 66 and 12-, 23-, and 43.5-kD bands start ed to appear at 50 mu M Al in Maringa. In the Al-sensitive cultivars R oblin and Katepwa, no significant effect on polypeptide profiles was o bserved at values up to 100 mu M Al. When root exudates were separated by ultrafiltration and the Al content was measured in both high molec ular mass (HMM; >10 kD) and ultrafiltrate (<10 kD) fractions, approxim ately 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 r eleased between 62 and 73% of total Al, with twice as much released fr om exudates of Al-resistant than of Al-sensitive cultivars. When plant s 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-sensiti ve cultivars. Saturation was not achieved in resistant cultivars. Diff erences in exudation of total polypeptides in response to Al stress, e nhanced accumulation of specific polypeptides, and the greater associa tion of Al with HMM fractions from Al-resistant cultivars suggest that root exudate polypeptides may play a role in plant response to Al.