Auxin and ethylene interaction during fruit growth and ripening of Actinidia deliciosa

The complex series of mechanisms that characterized the ripening process depends on a interplay of several classes of plant hormones among them a central role is played by ethylene. Relative to the current knowledge about ethylene, much less is known on the interaction between this hormone and other growth substances, in particular auxins, during fruit growth and ripening in some species as for example Actinidia deliciosa. In the present work we investigated the effect of the synthetic auxin 3,5,6-trichloro-2-pyridyloxyacetic acid (3,5,6-TPA) on fruit growth and quality traits of cv Hayward (Actinidia deliciosa) and the relationship between the obtained effect and ethylene emission, biosynthetic enzyme activity and gene expression. Moreover, expression studies of genes encoding for protein involved in cell wall disassembly (polygalacturonases and expansin) were also performed. During a three-years trial, the auxin, commercially available as Maxim\u2122, was applied at different concentrations and application times and its effects were studied throughout the fruit growing season, at harvest and during a cold storage period. The application of chemical caused a significant increase in kiwifruit weight and affected also ripening dynamics. In fact, at harvest and 1 month later, soluble solids content was lower and flesh firmness was higher in treated fruits as compared to control ones, while 3 months later an opposite trend was observed. This ripening dynamic modification was also confirmed by biochemical and molecular analyses. In fact, at harvest and 1 month later, treated fruit reached ACC-oxidase (ACO) activity and ACO, polygalacturonase C and expansin expressions lower than control fruits, while 3 months later treated fruit reached higher transcript levels in all the 3 considered genes. This suggest to further study the relationship between auxin and ethylene in modifying kiwifruit ripening and its influence on the duration of the storage