Ethylene, unlike the rest of the plant hormones is a gaseous hormone. Like abscisic acid, it is the only member of its class. Of all the known plant growth substance, ethylene has the simplest structure. It is produced in all higher plants and is usually associated with fruit ripening and the triple response. The triple response on dark grown pea seedlings includes, reduced stem elongation, increased stem thickening, and horizontal growth habit.

Biosynthesis and transport

Ethylene is produced by almost all parts of higher plants. In general, meristematic regions are active sites for ethylene, biosynthesis. The rate of production also depends on the type of tissue and stage of development. Its synthesis increases in tissues undergoing senescence or ripening. Ethylene biosynthesis is increased by stress conditions such as drought, flooding, chilling, or mechanical wounding. Auxins also promote ethylene synthesis. Being a gas ethylene moves by diffusion from its site of synthesis.

The amino acid methionine is the precursor of ethylene. In the first step of ethylene biosynthetic pathway, methionine is converted into S-adenosylmethionine (AdoMet). The rate-limiting step In the pathway is the conversion of AdoMet to 1-Aminocyclopropane-1-carboxylic acid (ACC), which is catalyzed by the enzyme ACC synthase. The last step in the pathway is the conversion of ACC to ethylene. This step requires oxygen and is catalyzed by the enzyme ACC oxidase. Amino ethoxy-vinyl glycine and amino-oxyacetic acid block the biosynthetic pathway of ethylene. Both inhibit the conversion of AdoMet to ACC. The cobalt ion also acts as an inhibitor of ethylene biosynthesis, blocking the conversion of ACC to ethylene.

Physiological effects

Abscission: The shedding of leaves, fruits, flowers, and other plant organs is termed abscission. It occurs at a specialized layer of cells – the abscission layers. Auxin apparently prevents leaf abscission by maintaining cells in the abscission zone insensitive to ethylene. When auxin levels in the leaf decline, the tissues become sensitive to ethylene that promotes abscission by producing and secreting cellulases and other enzymes.

Flowering: Ethylene induces flowering in mango and pineapple family (Bromeliaceae). In monoecious plants that have separate male and female flowers, ethylene may change the sex of developing flowers. The promotion of female flower formation in cucumber is one example of this effect.

Epinasty: The downward curvature of leaves that occurs when the upper adaxial) side grows faster than the lower (abaxial} side is termed epinasty. Ethylene induces epinasty.

• Rooting: Ethylene induces adventitious root and root hair formation. Root hairs are tubular projections originating from a specialized subset of root epidermal cells that increase the surface area of the roots, thereby increasing their absorptive capacity for water and nutrients.

Fruit ripening: Ethylene accelerates the processes associated with ripening in many fruits. A dramatic increase in ethylene production also accompanies the initiation of ripening. All fruits that ripen in response to ethylene exhibit a characteristic respiratory rise before the ripening phase called a climacteric. Apples, bananas, ‘and tomatoes are examples of climacteric fruits. In contrast, fruits such as citrus fruits and grapes do not exhibit the respiration and ethylene production rise and are called noncalimacteric fruits.

Ethylene signaling pathway

Ethylene binds to membrane-bound receptors. Ethylene receptors are grouped in a family of five members (EFRI, ETR2, ERS1, ERS2, EIN4), which have similarity to two-component regulators from bacteria. These receptors are present on the plasma membrane (except receptor ETR1 which present on endoplasmic reticulum). These are negative regulators. Binding of ethylene inactivates the receptors. Ethylene receptors are dimeric transmembrane proteins which function as histidine kinases. Ethylene receptors have an extracellular domain, which contains a copper atom that binds ethylene, and an intracellular histidine-kinase domain. The binding of ethylene inactivates ethylene receptors, inhibiting the kinase domain and the downstream signaling pathway emanating from it. In its unbound, active state, the receptor activates CTR1 (Constitutive Triple Response), a serine-threonine kinase. CTR1 initiates a protein kinase cascade similar to MAP-kinase signaling pathway. CTR1 is equivalent to a MAP-kinase. The activation of MAP-kinase pathway leads to the inactivation of gene regulatory proteins in the nucleus that is responsible for stimulating the transcription on of ethylene-responsive genes. The binding of ethylene to the receptors inactivates this signaling pathway by preventing the interaction of CTR1 and ETR, thereby turning these genes on. Trans-cyclooctene and 1~methyl cyclopropene act as an inhibitor of ethylene action because they block ethylene action because they block ethylene binding to its receptor.