Hormone homeostasis and induction of the small-fruit phenotype in ‘Hass’ avocado

Plant hormones play an important regulatory role in growth and development of fleshy fruits and often these are synthesized in a sequence that is tissue- and/or fruit-specific. Previous studies using ‘Hass’ avocado and its small-fruit phenotype as a model demonstrated a relationship between the cytokinin/abscisic acid ration and final fruit size. More detailed studies revealed that differential activity of molybdenum-cofactor-containing enzymes was in part responsible for the observed increase in abscisic acid in small ‘Hass’ fruit and that the same suite of enzymes likely impacted levels of indole-3-acetic acid. The present study confirms that the inability of the seed to produce/accumulate indole-3-acetic acid is a major contributing factor and characteristic of fruit that display a small-fruit phenotype, that reduced indole-3-acetic acid levels occur concomitant with increased abscisic acid, and that cytokinin metabolism appears unchanged irrespective of fruit size. Results are discussed in relation to the role of molybdenum-cofactor-requiring enzymes in indole-3-acetic acid and abscisic acid metabolism.