The role of ethylene metabolism in the short-term responses to aluminium by roots of two maize cultivars different in Al-resistance

The possible role of ethylene in the initial signal transduction of Al-indu ced root growth responses was investigated in two tropical maize (Zea mays) varieties that differ in Al resistance: ATP SR Yellow and HS 701 B. The in tensity of Al toxicity effects were evaluated after short (4 and 24 h) expo sure to 50 mu M Al in complete low ionic strength nutrient solution. Relati ve root elongation rates (RER) and callose formation in root tips were used as stress indicators. Ethylene production by the root tips and 1-aminocycl opropane-1-carboxylic acid (ACC) synthase and ACC oxydase activities were a nalysed. After 24 h exposure to Al, both less callose production and higher RER indicated that ATP SR Yellow was more Al resistant than HS 701 B. The Al resistance of ATP SR Yellow. however, was not expressed after 4 h exposu re to Al, when increased callose and decreased RER were observed. In any of the varieties and after any of the time-treatments an A-induced increase o f ethylene production was found. Our results indicate that the Al-resistanc e genes were not constitutively expressed in the absence of Al in the mediu m, but activated upon exposure to Al. An efficient protection against Al wa s achieved after a lag time of more than 4 h. Enhanced ethylene formation d oes not seem to play a role either in the Al-induced inhibition of root elo ngation or in the induction of the resistance mechanism. (C) 2000 Elsevier Science B.V. All rights reserved.