Calcium regulation of exogenous and endogenous 1‐aminocylopropane‐1‐carboxylic acid bioconversion to ethylene

Ethylene production and overall levels of free and conjugated 1‐aminocyclopropane‐1‐carboxylic acid (ACC) were studied in parenchymatous tissues, excised from clmacteric apples (Malus domestica Borkh. cv. Granny Smith) and infiltrated with an incubation medium containing 0, 1, 10 or 100 mM Ca2+, with or without exogenous ACC (2 mM). Irrespective of whether exogenous ACC was applied or not, ethylene production was inhibited to the same extent (40%) by an apoplastic effect of 100 mM Ca2+. In the absence of external ACC, the inhibition was associated with an increase in total endogenous ACC and may be related to a reduction in the rate of the last step of ethylene pathway. This suggests that the ethylene‐forming enzyme (EFE) is localized in the plasma membrane. Low Ca2+ concentrations (1 mM) enhanced basal ethylene synthesis due to influx of Ca2+ into the cytosol, while overall concentrations of ACC in the tissue decreased. However, 1 mM Ca2+ did not stimulate ACC‐dependent ethylene formation. Thus, Ca2+ influx may stimulate the translocation of endogenous ACC from synthesis or storage compartment (s) to reactive site(s) of the plasma membrane. The concentration of 10 mM Ca2+ had no effect on basal ethylene production and appears to represent a balance point between the stimulating and inhibiting effects of 1 and 100 mM Ca2+, respectively, Furthermore, the charge‐times of exogenous ACC observed with 0, 1 and 10 mM Ca2+ suggest that EFE is located on the inner side of the plasma membrane.