Fruit quality traits

In recent years, methods based on the genetic manipulation of fruit ripening, have been developed as an alternative strategy to increase the storage life and improve the quality of fruits. Although most studies have been carried out with the tomato as model fruit, Cantaloupe melons represent good targets due to their fast ripening rate and short postharvest life. Postharvest losses largely due to over ripening have been estimated to be near 30%in the USA.³⁹

In climacteric fruits like melon, the expression of many genes involved in the ripening process is stimulated by ethylene. This plant hormone represents an obvious target for controlling fruit ripening by genetic manipulation. An antisense construct of acDNA encoding melon ACCoxidase(ACO)⁴⁰ driven by the 35S promoter has been used to generate transgenic melons of the Cantaloupe Charentais type (cv Vedrantais). Among several transformants, one line was selected that exhibited strong inhibition (over 99.5%) of ethyleneproduction.⁴¹ Ethylene suppression resulted in the inhibition of rind yellowing, flesh softening, climacteric respiration and peduncle detachment.⁴² However, coloration of the flesh, accumulation of sugars and organic acids, and the synthesis of the ethylene precursor ACC were not affected by ethylene suppression. Ethylene-inhibited fruit also evolve far fewer aroma volatiles than wild-type (WT) fruit.⁴³ However, ethylene treatment of antisense ACO fruit is capable of restoring the WT phenotype.

Antisense ACO Charentais cantaloupe melons enabled an assessment of the role of ethylene in some physiological disorders. These melons, in contrast to WT fruit, do not develop the characteristic pitting and browning of the rind associated with chilling injury either when stored at low temperature (2°C for 3 weeks) or upon rewarming to room temperature. Tolerance to chilling was clearly correlated with a lower accumulation of ethanol and acetaldehyde and a higher activity of activated oxygen scavenging enzymes in antisense ACO fruit. It appears that ethylene acts in conjunction with low temperature to induce metabolic shifts that participate in the development of chilling injury.⁴⁴

Another strategy for reducing ethylene synthesis has been used for cantaloupes of the American type commonly referred to as muskmelon. The T3 bacteriophage gene product S-adenosylmethionine hydrolase (SAMase) catalyses the degradation of SAM, a precursor to ethylene biosynthesis. The gene was expressed under the control of achimerical fruit-specific promoter.³⁹ It was confirmed that the chimerical promoter was capable of driving SAMase expression in a fruit-specific and ethylene-responsive manner. SAMase melons showed significant reduction in ethylene biosynthesis (up to 75%) both as inbred homozygous plants and as hybrids. The inhibition of ethylene production of SAMase fruit was not strong enough to dramatically alter the ripening and postharvest phenotype. However, although the onset of maturity was not significantly delayed, full maturity of transgenic fruit occurred over a shorter period. Also, the concentration of soluble sugars was frequently higher in transgenic fruit probably because fruit slip was delayed by one to three days, allowing more sugars to accumulate in the fruit before its harvest.

These examples indicate that two alternatives exist for the postharvest handling of ethylene-inhibited Cantaloupe melons: (i) use of lines and corresponding transgenic hybrids with very strong inhibition of ethylene production (in this case fruit can be stored in the cold with no risk of chilling injury and then ripened to the desired stage of maturity before distribution), or (ii) use of lines and corresponding hybrids with less reduction of ethylene production and showing delayed ripening and extended shelf-life so as to reach the consumer at the right maturity stage without ethylene treatment. A comparative evaluation of the ethylene-inhibited transgenic genotypes and already existing long-keeping varieties of melon deserves to be carried out from the points of view of quality of the fruit, storability and postharvest shelf-life.