Molecular biology of ethylene biosynthesis and its application in horticulture

Abstract

Ethylene is produced by all higher plants and in trace amounts regulates many aspects of growth and development, ranging from seed germination to flower fading, fruit ripening and leaf senescence; its effects are often spectacular and are commercially important in agriculture. Ethylene is biosynthesized via the following pathway: methionine → S-adenosylmethionine (AdoMet) → 1-aminocyclopropane-1-carboxylic acid (ACC) → C2 H4. The committed steps in ethylene biosynthesis are the conversion of AdoMet to ACC catalyzed by ACC synthase, and the conversion of ACC to ethylene by ACC oxidase. ACC synthase is a pyridoxal phosphate (PLP) enzyme, which mediates a unique α, γ-elimination of the substrate AdoMet to form ACC. Biochemical and site-directed mutagenesis studies confirm that Lys-272 in apple ACC synthase is the active centre that binds the PLP coenzyme and functions as the base in the enzymic reaction. ACC oxidase, which catalyzes the aerobic oxidation of ACC to ethylene, CO2 and HCN, requires Fe2+, and CO2 as essential cofactors, and ascorbate as its co-substrate. The cloning of the ACC synthase and ACC oxidase genes has permitted scientists use antisense RNA technology to genetically engineer perishable crops, such as tomato and melon fruits and carnation flowers, with suppressed ethylene production rate and thereby to extend the shelf-life and to reduce spoilage. Recent molecular genetic approach using Arabidopsis thaliana has resulted in the isolation of genes responsible for ethylene insensitivity. Transgenic biotechnology has demonstrated the feasibility to confer ethylene insensitivity in horticultural crops resulting in delay of the senescence process.