Physiological roles, Biosynthesis of Jasmonic acid, Salicylic acid & Brassinosteroids

Brassinosteroids:

Brassinosteroids are a recently discovered group of steroids that have distinct growth-promoting activity in some plants especially in stems. These compounds were first isolated in 1979 from bee collected pollen of rapeseed (Brassica napus). Brassinolide was the first isolated brassinosteroid. More than 70 brassinosteroid compounds have been isolated from plants.

Site of synthesis:

1. BS compounds are synthesized from different parts of plants such as pollens, seeds, leaves, stems, roots, and flowers.

Biosynthesis of Brassinosteroids:

Biosynthesis of brassinosteroids was studied using the cell culture system of Catharanthus roseus. It is synthesized from a precursor molecule, Campesterol. There are two pathways by which brassinosteroids are synthesized. The early C6- oxidation and late C6-oxidation pathway.

Physiological effects:

1. Promotes cell expansion and cell regeneration.
2. Protect plants during chilling and drought stress.
3. Inhibits root growth and development.
4. Accelerates senescence in dying tissue cultured cells.
5. Plays an important role in pollen tube formation.
6. Helps in the promotion of vascular differentiation.
7. It acts against abscisic acid and promotes seed germination.
8. Foliar application of brassinosteroids induces flowering in some plants like tomato, grapes, strawberry, etc.

Jasmonic acid:

Jasmonic acid is an organic compound, a member of the jasmonate class of plant hormones. These are signaling compounds involved in the regulation of cellular defense and development in plants. It was first isolated in 1957 as the methyl ester of jasmonic acid by Edouard Demole.

Site of synthesis:

Jasmonates are synthesized in chloroplast and peroxisomes.

Biosynthesis of Jasmonic acid:

Jasmonic acid is synthesized from linolenic acid by the octadecanoid pathway. It is synthesized in the chloroplast and peroxisomes in response to multiple stresses.

Physiological roles of Jasmonic acid:

1. Plays an important role in the regulation of biotic and abiotic stress.
2. Helps in plant growth and development.
3. Leads to growth inhibition and leaf senescence.
4. Inhibit root growth, pollen germination, and tendril coiling.
5. Stimulates ethylene biosynthesis and fruit ripening.
6. Acts against pests and herbivores, helpful in anti-pest defense.
7. Stimulates conversion of lycopene to carotene and is responsible for tuber formation in potatoes.

Salicylic acid:

Salicylic acid is a lipophilic monohydroxybenzoic acid, a type of phenolic phytohormone found in plants concerning plant growth and development, photosynthesis, transpiration. Salicylic acid is involved in endogenous signaling, mediating plant defense against pathogens. It is an important hormone that regulates many aspects of plant growth and development, as well as resistance to biotic stress.

Site of synthesis:

Salicylic acid is synthesized in chloroplast and cytosol.

Biosynthesis of Salicylic acid:

Salicylic acid is synthesized by two pathways.

1. Phenylalanine Ammonia Lyase pathway (PAL Pathway)
2. Isochorismate pathway (IC Pathway)

Chorismate acts as a precursor molecule for both pathways which is obtained from the Shikimate pathway.

Physiological roles:

1. The increasing level of salicylic acid induces thermogenesis which helps in pollination.
2. Provides resistance to subsequent pathogenic attacks.
3. Inhibit ethylene biosynthesis and seed germination.
4. Reverses the effect of ABA.
5. The application of salicylic acid enhances the longevity of flowers.