1. Phenylalanine as a Precursor:
- SA biosynthesis starts with the amino acid phenylalanine. Phenylalanine ammonia-lyase (PAL), a key enzyme in the phenylpropanoid pathway, converts phenylalanine into trans-cinnamic acid.
2. Formation of Benzoic Acid:
- Trans-cinnamic acid is then hydroxylated by cinnamate 4-hydroxylase (C4H) to produce 4-coumaric acid.
- 4-Coumaric acid is further hydroxylated by coumaric acid 3-hydroxylase (C3H) to form caffeic acid.
- Caffeic acid is then O-methylated by caffeic acid O-methyltransferase (COMT) to produce 5-O-methyl caffeic acid (5-OMCA).
- Finally, 5-OMCA is oxidized by 5-OMCA oxygenase (5-OMCA-O) to form benzoic acid.
3. Conversion to Salicylic Acid:
- Benzoic acid is the immediate precursor of SA in plants. It undergoes hydroxylation by benzoic acid 2-hydroxylase (BA2H) to produce 2,3-dihydroxybenzoic acid (2,3-DHBA).
- 2,3-DHBA is then spontaneously dehydrated to form salicylic acid.
The biosynthesis of SA is regulated by several factors, including environmental stimuli such as pathogen attacks, drought, UV light, and wounding. These factors can trigger the activation of specific enzymes involved in the SA biosynthetic pathway, leading to increased production of SA.
Additionally, SA can also be synthesized through an alternative pathway involving the isochorismate synthase enzyme. This pathway is considered a minor route for SA biosynthesis and is less well-studied compared to the main pathway described above.
The precise regulation and compartmentalization of SA biosynthesis allow plants to control the levels and localization of this important signalling molecule, enabling them to respond appropriately to various environmental cues and developmental processes.
