1-aminocyclopropane-1-carboxylate oxidase activity limits ethylene biosynthesis in Rumex palustris during submergence

Submergence strongly stimulates petiole elongation in Rumex palustris, and ethylene accumulation initiates and maintains this response in submerged ti ssues. cDNAs from R. palustris corresponding to a 1-aminocyclopropane-1-car boxylate (ACC) oxidase gene (RP-ACO1) were isolated from elongating petiole s and used to study the expression of the corresponding gene. An increase i n RP-ACO1 messenger was observed in the petioles and lamina of elongating l eaves 2 h after the start of submergence. ACC oxidase enzyme activity was m easured in homogenates of R. palustris shoots, and a relevant increase was observed within 12 h under water with a maximum after 24 h. We have shown p reviously that the ethylene production rate of submerged shoots does not in crease significantly during the first 24 h of submergence (L.A.C.J. Voesene k, M. Banga, R.H. Thier, C.M. Mudde, F.M. Harren, G.W.M. Barendse, C.W.P.M. Blom [1993] Plant Physiol 103: 783-791), suggesting that under these condi tions ACC oxidase activity is inhibited in vivo. We found evidence that thi s inhibition is caused by a reduction of oxygen levels. We hypothesize that an increased ACC oxidase enzyme concentration counterbalances the reduced enzyme activity caused by low oxygen concentration during submergence, thus sustaining ethylene production under these conditions. Therefore, ethylene biosynthesis seems to be limited at the level of ACC oxidase activity rath er than by ACC synthase in R. palustris during submergence.