Effect of auxin on fruit morphogenesis: physiological and molecular aspects in kiwifruit ripening

The synthetic auxin 3,5,6-trichloro-2-pyridyloxyacetic acid (3,5,6-TPA) was applied to ‘Hayward’ kiwifruit (Actinidia deliciosa) to verify the enhancement of fruit growth and changes in fruit quality attributes. The chemical, commercially available as Maxim, was tested at different concentration and application times during the fruit growth period. Application of 3,5,6-TPA caused a significant increase in kiwifruit weight and dimensions, particularly in length. At harvest and 1 month later, soluble solids content was lower and flesh firmness was higher in treated fruits compared to control fruit, while 3 months later flesh firmness of treated fruit was similar to that of controls, and soluble solid content remained lower. This change in ripening behaviour was confirmed by biochemical and molecular analyses. At harvest and 1 month later, treated fruit contained lower ACC-oxidase (ACO) activity and lower expression of ACO, polygalacturonase C (PGC) and expansin than control fruit, whereas 3 months later treated fruit reached higher expression levels of all 3 genes studied. This suggests that it would be useful to study further the role of 3,5,6-TPA in modifying ripening and affecting storage life of kiwifruit.