Biochemical and molecular aspects of fruitlet abscission

Fruitlet abscission during fruit development is due to the activation of pr e-differentiated abscission zones (AZs) located between twig and pedicel, a nd/or pedicel and pericarp. Major advances on biochemical and molecular asp ects 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 isolat ed. AZ activation is preceded by an induction of ethylene biosynthesis, par alleled by a stimulation of ACO activity and transcript accumulation. Ethyl ene, besides a dramatic stimulation of PG and EG, up or down regulates seve ral other abscission related genes. The specific expression of genes encodi ng for ethylene receptors in the AZ would support the hypothesis that fruit let AZ specificity may depend on the ability of this region to sense ethyle ne.