Putative role of cytokinin in differential ethylene response of two lines of antisense ACC oxidase cantaloupe melons

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 of ACC synthase genes, ACCS1 and ACCS3, indicating that these 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.