The role of cytokinin, auxin and abscisic acid metabolism in the control of Hass avocado fruit size

Avocado fruit growth is the result of sustained cell division and cell expansion. Studies have revealed that cell division is the limiting factor in the control of fruit growth and that the endogenous cytokinin to auxin to abscisic acid ratio is critical in supporting continued fruit growth. Detailed biochemical studies have led to the proposal that cytokinin regulates auxin and abscisic acid metabolism by impacting on the molybdenum cofactor-requiring aldehyde oxidase, responsible for catalysing the conversion of indole-3-acetaldehyde to indole-3-acetic acid and abscisic aldehyde to abscisic acid. In order to evaluate in more detail the biochemical interrelationship between cytokinin, auxin and abscisic acid metabolism a number of enzymes involved in all three processes were evaluated in both normal Hass fruit and its small-fruit phenotype, together with a number of compounds that form the vital link between these three metabolic pathways. Results indicate that the cytokinin and auxin level relative to the abscisic acid level is lowered in small-fruit compared to normal fruit.