Biosynthesis and regulation of jasmonic acid & salicylic acid in plant stress tolerance
By
SADAF SALEEM
Department of Plant Breeding and Genetics, University of Agriculture Faisalabad.
Phytohormones and their derivatives are considered as major regulators of plant growth and development and systemic response mediators in different physiological actions under stressful environments. Jasmonic acid (JA) and salicylic acid (SA) are plant hormones characterized as common signaling molecules under biotic and abiotic stress resistance and function in systemic acquired resistance. Plant responses and adaptations to heat, cold, drought, salinity, heavy metals toxicity and pathogens attack are governed by complex interactions and coordinated regulation (synergistic /antagonistic relationships) of phytohormones. JA and SA have key roles in regulating plant signaling pathways and responses under stress conditions.
Jasmonic Acid
JA was first identified from jasmine essential oil as odorant compound in 1962. JA and its conjugate with isoleucine (JA-ileu) serve as major signal molecular under stressful conditions. JA is synthesized through octadecanoid pathway as following:
Biosynthesis
Triunsaturated fatty acids (18:3 and 16:3) after getting oxygenate by Lipoxygenases converted into 13-hydroperoxyfatty acids. They are catalyzed by allene oxide synthase and resulted in production of two compounds (12-oxo-phytodienoic acid (OPDA) and dinor-OPDA) which are transferred to peroxisomes.
Peroxisomal enzyme OPDA reductase 3 reduce OPDA and dinor-OPDA into 3-oxo-2-( 2- pentenyl)-cyclopentane-1-octanoic acid (OPC8) and 3-oxo-2-(2-pentenyl)- cyclopentane-1-hexanoic acid (OPC6) respectively. These compounds upon β- oxidation produce jasmonic acid which is transferred to cytosol.
Regulation
In case of plant growth and development, JA has roles in growth, secondary metabolism, chlorophyll production, tuber formation, sexual reproduction and pollen germination.
JA also involved in petiole abscission, senescence, root growth inhibition and stimulation of ethylene biosynthesis.
A key regulator of plant signaling pathways under salinity, drought, cold and osmotic stress, develop immunity against microbial pathogens and wounding.
Promote synthesis of plant defense proteins (proteinase inhibitor) against herbivore attack.
Salicylic Acid
SA (2-hydroxy benzoic acid) is a phenolic compound, firstly identified as a medicinal compound “salicin” from a willow (Salix alba) bark and used to relieve fever and pain. Later salicin is converted into an acid and sugar and thus named as salicylic acid. Acetylsalicylic acid (ASA) commonly known as aspirin (world famous drug) is a synthetic derivative of SA.
Biosynthesis
SA is synthesized through phenyl alanine ammonialyase (PAL) pathway in cytosol. In PAL pathway, phenylalanine is converted into trans-cinnamic acid by action of PAL and subsequently to benzoic acid. By the action of benzoic-acid-2-hydroxylase (BA2H), benzoic acid is converted into SA. After synthesis SA undergo a number of modifications; glucosylation, methylation, and amino acid (AA) conjugation for proper accumulation, mobility and functioning. SA is also converted into its volatile derivative “methyl salicylate (MeSA)” which activates systemic acquired resistance (SAR) in uninfected tissues of related plants or distant parts of the same plant.
Regulation
SA has roles in cell growth, respiration, stomatal aperture, ion transport, seed germination, seedling development, flowering induction, senescence and fruit ripening.
SA serve as key signaling molecule under extreme heat, ozone, UV-radiation, salinity, drought and heavy metals toxicity stresses in plants.
SA has major role in regulating plant immune responses at local and systemic levels against several pathogens along with mediating systemic acquired resistance (SAR).
References
Pedranzani, H. and A. Vigliocco (2017). “REGULATION OF JASMONIC ACID AND SALICYLIC ACID LEVELS IN ABIOTIC STRESS TOLERANCE: PAST AND PRESENT.” DOI 10.1007/978-981-10-4732-9_4
Khan, M. I. R., et al. (2015). “Salicylic acid-induced abiotic stress tolerance and underlying mechanisms in plants.” Frontiers in Plant Science 6(462).
