Metabolic controls of Actinidia species fruit development: where are we at?

Actinidia fruit are botanically berries that can have very diverse phenotypes as well as excellent nutritional properties. Kiwifruit berry development has received much attention in recent years. Fruit development can be divided in three stages: stage I: time to seed maturity, including cell division and expansion, up to when seeds become competent to germinate; stage II: time to fruit maturity, when fruit growth ceases and they become competent to ripen; stage III: time to ripen, when fruit start to soften rapidly and become edible. While knowledge of key factors driving stages I and II is limited, we know that external factors such as ethylene and temperature have a well-defined role in modulating the progression of stage III. Kiwifruit provides a good model for starch metabolism during fruit growth and ripening. Despite being described as a climacteric fruit, the ethylene does not increase until fruit are eating-ripe, i.e., the flesh is soft and starch has been cleaved into soluble sugars. After a brief overview of the current knowledge of kiwifruit berry metabolism, this review will focus on the physiology of metabolic, genetic and hormonal control of fruit growth and development. Primary and secondary metabolism will be considered and their contribution to fruit quality discussed.