Biochemical and molecular aspects of fruitlet abscission

Fruitlet abscission during fruit development is due to the activation of pre-differentiated abscission zones (AZs) located between twig and pedicel, and/or pedicel and pericarp. Major advances on biochemical and molecular aspects are related to beta-1,4-endoglucanase (EG) and polygalacturonase (PG), two cell hydrolases involved in the cell wall disassemblement responsible for fruit shedding. AZ activation is accompanied by an increase in activity and transcript accumulation of one or both enzymes. Expression of PG genes specifically related to abscission has been found in tomato flower AZ. In peach, an EG gene highly expressed in leaf and fruitlet AZs has been isolated. AZ activation is preceded by an induction of ethylene biosynthesis, paralleled by a stimulation of ACO activity and transcript accumulation. Ethylene, besides a dramatic stimulation of PG and EG, up or down regulates several other abscission related genes. The specific expression of genes encoding for ethylene receptors in the AZ would support the hypothesis that fruitlet AZ specificity may depend on the ability of this region to sense ethylene.