Ethylene Synthesis and Sensitivity in the Formation of Aerenchyma in Response to Deficiencies of N and P in Roots of Zea Mays

Abstract

Like hypoxia, temporary shortages of nutrient ions induce formation of aerenchyma in nodal (adventitious) roots of Zea mays L. The effect of short-term nutrient deficiency (-N, -P) in solution culture was expressed quickly in collapse and lysis of cortical cells and formation of gas spaces in root tips. These changes in root anatomy were prevented by non-toxic levels (0.5 μM) of the ethylene action inhibitor, Ag+. However, unlike hypoxia, nutrient deficiency reduced rather than stimulated ethylene synthesis by excised root tips. The inhibitory effect on ethylene production was detectable after only 1 day of N-deprivation and persisted through a 20-day period. N-deprivation was most effective in promoting aerenchyma formation but was also most effective in inhibiting ethylene synthesis. The pattern for ethylene production was generally paralleled by ACC levels, ACC synthase activity and EFE activity, the only minor exception being that ACC synthase activity was similar in -N and -P treated roots. Since increased ethylene synthesis did not explain the induction of aerenchyma in nutrient-ion deprived maize roots, we tested their sensitivity to exogenous ethylene. At 0.05 μl ethylene liter−1 air, -N roots were strongly induced to form aerenchyma, to a degree indicating saturation of the response. The apparent sensitivity was up to a 100-fold greater than in control plants, which showed a modest response at 1 μl and saturated response at 5 μl ethylene liter−1. Roots from -P plants were one half saturated in aerenchyma formation at 1 μl and fully saturated at 5 μl ethylene liter−1 indicating a 2- or 3-fold increase in sensitivity. Based on data in the literature relating ethylene production rates to air concentrations of ethylene in tissues, and the sensitivity data presented here, it seems likely that nutrient deficiency induces aerenchyma formation by markedly increasing sensitivity to endogenous ethylene while its production rate is reduced.