| Plants of Zea mays L. cv TX 5855 were grown in a complete, well oxygenated nutrient solution and then subjected to nutrient starvation by omitting either N or P from the solution. These treatments induced the formation of aerenchyma close to the apex of the adventitious roots that subsequently emerged from the base of the shoot. N- or P-starvation consistently depressed the rates of ethylene synthesis and many of the components of the ethylene biosynthetic pathway. Aerenchyma formation induced by nutrient starvation was blocked under noninjurious conditions, by addition of low concentrations of Ag{dollar}sp+{dollar} or AVG, well known inhibitors of ethylene action and ethylene synthesis. When extending roots were exposed to low concentrations of ethylene in air sparged through the nutrient solution, N- or P-starvation enhanced the sensitivity to exogenous ethylene at concentrations as low as 0.05 microliters ethylene per liter air, promoting a more rapid and extensive formation of aerenchyma than in unstarved roots. Hypoxia, N-deficiency or exogenous ethylene all induced a 10-fold increase in cellulase activity in the apical 10 mm segments, but at greater distance from the tip due to different treatments were small. Addition of AVG blocked induction of cellulase activity by hypoxia or N-deficiency; however, the effect could be overcome by simultaneous exposure to ethylene. The rise in ACC synthase activity was observed to precede that of cellulase activity in root tips. These results indicated that ethylene is implicated in the induction of cellulase activity which, in turn, breaks down cell walls leading to aerenchyma formation. Using labeled {dollar}sp{lcub}14{rcub}{dollar}C-ethylene, a linear increase of ethylene metabolism was found in roots of maize with increasing concentrations of exogenous ethylene for both control and N-deficient plants. In contrast, low temperature (4{dollar}spcirc{dollar}C) or N{dollar}sb2{dollar} atmosphere greatly inhibited ethylene metabolism. CS{dollar}sb2{dollar} showed very little effect on ethylene metabolism in roots of maize. At least two kinds of ethylene binding sites were demonstrated in roots of maize one with fast association and dissociation the other with slow association and dissociation. Treatment with 100 {dollar}mu{dollar}M AVG increased by 20% the bound ethylene for both control and N-deficient plant roots. There were no significant differences between controls and N-starved roots in the amount of ethylene metabolism, the abundance of ethylene binding sites or the association and dissociation characteristics of these sites. The increase in the sensitivity to ethylene of N-starved roots of maize appears to be due to an increase in the capacity of the signal transduction system from the ethylene-receptor complex to aerenchyma development. |
