Interrelationship of Polyamine and Ethylene Biosynthesis during Avocado Fruit Development and Ripening

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Concentrations of polyamines (PA) and the activities of the PA-synthesizing enzymes ornithine decarboxylase (ODC) and arginine decarboxylase (ADC) extracted from the mesocarp tissue of avocado (Persea americana Mill, cv `Simmonds') fruits at different stages of development were compared with DNA content and the activities of 5′-methylthioadenosine (MTA) nucleosidase and 5-methylthioribose (MTR) kinase. Putrescine, spermidine, and spermine were at their peak concentrations during the early stages of fruit development (362, 201, and 165 nanomoles per gram fresh weight, respectively, at 15 days from full bloom), then declined to 30% or less at full maturity. Agmatine showed only a slight change in concentration throughout the fruit development. The activity of ODC, which was low during flowering (8 nmoles per milligram protein per hour), increased more than threefold during the first 2 months then declined at the later stages of fruit development, while ADC activity showed only a slight increase. DNA content followed a similar pattern of change as that of PA and ODC. The decline in DNA and ODC activity suggest a lack of correlation between cell proliferation and PA at the later stages of the avocado fruit development. It is also possible that any cell division which may take place during the latter stages of the fruit development is not sufficient to alter the pattern of PA biosynthesis. MTA nucleosidase and MTR kinase activities increased during the first 15 days of fruit development followed by a slight decline at 60 and 90 days from full bloom. At 120 days (1 month before full maturity) both MTA nucleosidase and MTR kinase activities increased significantly. During maximum ethylene synthesis, MTA nucleosidase and MTR kinase activities were approximately fivefold and eightfold, respectively, higher than during maximum PA synthesis. The data indicate that the MTA molecules produced during PA and ethylene synthesis are actively metabolized to MTR and MTR-1-P, the two intermediates involved in the regeneration of S-adenosylmethionine from MTA. The data also suggest that the PA and ethylene biosynthetic pathways are not actively competing for the same substrates at any given stage of the avocado fruit development and ripening.

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