Articles
ROLE OF CALCIUM IN FRUIT INVOLVED IN ETHYLENE PRODUCTION
Calmodulin, a calcium binding protein of ubiquitous occurrence, has been shown to mediate the biochemical action of calcium in both animals and plants.
The calcium-calmodulin complex appears to activate several plant enzymes, and thereby mediates the calcium signal and regulates cellular metabolism.
The final step in the biosynthesis of ethylene involves the catalytic conversion of 1-amino-cyclopropane-1-carboxylic acid (ACC) to ethylene by the ACC oxidase.
Excised tissue discs of Japanese pear produced ethylene at rates less than 0.2 nl·gFW-1·hr-1. However, the addition of ACC resulted in a rapid increase in the rate of ethylene production from the discs in a concentration- and time-dependent manner.
Over the pH range 5.2 to 8.3, ethylene production was highest at pH 5.5 and 5.8. The addition of Ca2+ to the reaction solution increased ethylene production from Japanese pear and apple discs by approximately 1.5 times with and without the addition of ACC.
The addition of the calmodulin antagonists, W-5 or W-7, to the reaction solutions inhibited the conversion of added ACC to ethylene by Japanese pear and apple discs.
The degree of inhibition in the presence of W-5 or W-7 increased with incubation time such that 200 μg·ml1 W-7 inhibited ethylene production by 91% at pH 5.5 and 89% at pH 7.0 after incubation for 4 – 6 h.
Inhibition of ethylene production was less in the presence of W-5 than in the presence of W-7. In apple tissue discs the addition of 1 mM W-7 with 0.04 mM ACC inhibited ethylene production 46% within 3 – 4.5 h.
The effect of inhibition of ACC oxidase activity by W-5 was less than that of W-7.
The naphthalenesulfonamide derivatives, W-5 and W-7, are known to inhibit binding between calmodulin and calmodulin-modulated proteins.
In the present study, these calmodulin antagonists inhibited the conversion of ACC to ethylene by Japanese pear and apple discs.
Although both antagonists inhibited ACC oxidase activity, W-5 was less effective than W-7. The passage of W-5, which has a lower hydrophobicity than W-7, through the plasma membrane will be more difficult than for W-7; and since calmodulin has been shown to be located in the cytosol of plant cells (Muto, 1982), this could explain the lower effectiveness of W-5. Therefore it was suggested that the enzyme required calcium and calmodulin, and differed from the ACC oxidase which required ascorbate and Fe2+. It is concluded that calcium and calmodulin may have a important roles in the conversion of ACC to ethylene during fruit senescence.