The influence of oxygen deficiency on ethylene synthesis, 1‐aminocyclopropane‐1‐carboxylic acid levels and aerenchyma formation in roots of Zea mays

Abstract

The relationship between ethylene production, 1‐aminocyclopropane‐l‐carboxylic acid (ACC) concentration and aerenchyma formation (ethylene‐promoted cavitation of the cortex) was studied using nodal roots of maize (Zea mays L. cv. LG11) subjected to various O2 treatments. Ethylene evolution was 7–8 fold faster in roots grown at 3 kPa O2 than in those from aerated solution (21 kPa O2), and transferring roots from aerated solution to 3 kPa O2 enhanced ethylene synthesis within less than 2 h. Ethylene production and ACC accumulation were closely correlated in different zones of hypoxic roots, regardless of whether O2 was furnished to the roots through aerenchyma or external solution. Both ethylene production and ACC concentrations (fresh weight basis) were more than 10‐fold greater in the distal 0–10 mm than in the fully expanded zone of roots at 3 kPa O2. Aerenchyma formation occurred in the apical 20 mm of these roots.Roots transferred from air to anoxia accumulated less than 0. 1 nmol ACC (mg protein)‐1 for the first 1.75 h; no ethylene was produced in this time. The subsequent rise in ACC levels shows that ACC can reach high concentrations even in the absence of O2, presumably due to a de‐repression of ACC synthase. The hypothesis was therefore tested that anoxia in the apical region of the root caused enhanced synthesis of ACC, which was transported to more mature regions (10–20 mm behind the apex), where ethylene could be produced and aerenchyma formation stimulated. Surprisingly, exposure of intact root tips to anoxia inhibited aerenchyma formation in the mature root axis. High osmotic pressures around the growing region or excision of apices had the same effect, demonstrating that a growing apex is required for high rates of aerenchyma formation in the adjacent tissue.