Kiwifruit: a unique carbohydrate metabolism

Kiwifruit vines have many unusual attributes including unique carbohydrate metabolism and transport systems. Two unusual carbohydrates, the polyol myo-inositol and the trisaccharide planteose, play important roles in kiwifruit vines. myo-Inositol is found in all tissues; it may be a precursor for synthesis of planteose in mature leaves, and concentrations are elevated in response to applied stresses. Trace amounts of myo-inositol transported in the phloem do not account for the elevated levels of myo-inositol found in developing fruit, indicating it is synthesized in fruit. We postulate the role of myo-inositol in the fruit is in development of seeds rather than synthesis of planteose. Kiwifruit is the only species reported where planteose is transported in phloem from source to sink tissues in similar or greater quantities than sucrose. This enables more efficient transport of soluble carbohydrate to the fruit compared with using sucrose alone. However, when planteose arrives at the fruit it is rapidly metabolized. Planteose and myo-inositol are major soluble carbohydrates ubiquitous in species of the genus the Actinidia. Identifying critical enzymes and genes for these unusual metabolic pathways is important for developing screening tools to select genotypes with higher dry matter and better resistance to abiotic and biotic stresses.