Two transgenic lines of ‘Cantaloupe’ melon derived from the same wild type genotype were previously
generated using ACC oxidase antisense constructs from melon (pMEL1AS) and apple (pAP4AS). Both lines
yielded fruit with reduced ethylene production and low ACC oxidase (ACCO) expression. ACCO antisense
fruit also exhibited lower expression ofACC synthase genes,ACCS1 andACCS3, indicating thatthese genes
are positively regulated by ethylene and participate in the autocatalytic ethylene production process. In
contrast, a higher expression of ACCS5 was observed in antisense lines when compared to the wild type
indicating a negative feedback regulation of ACCS5 by ethylene. Fruit of both transformed lines exhibited
delayed ripening and reduction in ester volatile production but differed in their response to exogenous
ethylene supply. While postharvest ethylene application fully restored the ripening process in pMEL1AS
melon, it only restored flesh softening of pAP4AS melon but not rind color change or aroma volatile production.
Up-regulation of lipoxygenase pathway associated genes (hydroxyperoxide lyase, lipoxygenase,
and alcohol acyl transferases 1, 3 and 4) occurred in ethylene-treated pMEL1AS fruit but not in pAP4AS
melons. Polygalacturonase1 gene transcript accumulation increased in pMEL1AS and pAP4AS fruit upon
ethylene supply. Zeatin and zeatin riboside content of roots and fruit (rind and flesh) of pAP4AS plants
were 5-fold higher than the wild type and pMEL1AS counterparts. Higher relative transcript accumulation
of a gene involved in the cytokinin synthesis and a gene involved in cytokinin response were also
found in the roots and fruit of pAP4AS. In addition, polyamines, which are known to reduce sensitivity
to ethylene, remained unchanged in all fruit. Collectively the results suggest a putative role for the
increased endogenous cytokinin content in counteracting ethylene action in some aspects of the fruit
ripening process.