The below mentioned article provides a study note on jasmonate. After reading this article you will learn about 1. Biosynthesis of Jasmonate 2. Role of Jasmonate as Plant Growth Regulator and 3. Role of Jasmonate as Signalling Agent in Plant Defense Responses.
Jasmonate (Jasmonic acid or JA) is a 12-C unsaturated fatty acid with a cyclopentane ring structure and a keto group that occurs along with its methyl ester, methyl jasmonate (Fig. 23.7) in wide variety of organisms including fungi, mosses, ferns and higher plants and appears to be ubiquitous in plant kingdom.
Higher concentrations of jasmonate are found in young actively growing tissues. Methyl jasmonate is chief component of oil of jasmine (Jasminum, Family Oleaceae) Jasmonates are known to accumulate in wounded plants and plants treated with elicitors.
Jasmonate is biosynthesized from 18-C unsaturated fatty acid, the linolenic acid and along with its methyl ester is known to play important role as:
(i) Plant growth regulator, and
(ii) An important signalling agent in plant defense responses against fungal pathogens, insects and other herbivores.
Biosynthesis of Jasmonate:
As mentioned earlier, jasmonate is synthesized in plants from linolenic acid (18: 3). Membrane lipids are rich sources of linolenic acid in the form of phospholipids from where it is released by the action of enzymes phospholipases. Linolenic acid is then converted into jasmonate through a pathway called as octadecanoid signalling pathway (Fig. 23.8).
The conversion of linolenip acid into jasmonate is completed in chloroplast and peroxisome. The first three steps of this pathway occurs in chloroplast which result in the formation of cyclized intermediate called 12-oxo-phytodienoic acid. The latter now moves to peroxisome where it is first reduced and then converted into jasmonate by β-oxidation that is repeated thrice. (Striking similarities have been observed by scientists in structure and biosynthesis of jasmonate with some eicosanoids which are central to inflammatory responses and other physiological processes in mammals).
Role of Jasmonate as Plant Growth Regulator:
i. Jasmonates have been shown to inhibit growth of certain plant parts and to strongly promote senescence of leaves. In-fact, jasmonates were first recognized for their ability to promote senescence of detached barley leaf segments.
ii. Jasmonates are believed to modulate the action of a host of genes, influencing a number of other physiological processes in plants such as, (i) seed and pollen germination, (ii) vegetative protein storage and (iii) root development and tendril coiling. In most of these effects jasmonates appear to act in close harmony with ethylene.
iii. Wide ranging physiological effects of jasmonates on plants have prompted some scientists to suggest that they may represent another class of plant growth substances.
Role of Jasmonate as Signalling Agent in Plant Defense Responses:
Jasmonates are known to activate genes that encode defensive proteins (such as α-amylase inhibitors, lectins, and proteinase inhibitors) and promote the rapid buildup of phytoalexins that provide strong deterrents to feeding insects and other herbivores.
The precise mechanism of action of jasmonates in plant defense responses is not clearly understood. However, extensive studies done by scientists in recent years on legumes, tomatoes and other plants have thrown some light on it.
It is believed that on being wounded by insect or other herbivore, the host plant produces a short polypeptide (of 18 amino acids) called systemin. The latter is released from the wounded cells to the apoplast and transported out of wounded leaf via phloem. In adjacent intact tissues and elsewhere, systemin binds with its receptor present on plasma membrane. This receptor is a leucine rich repeat (LRR) protein with kinase activity.
The systemin-receptor complex now gets auto phosphorylated and in turn activates a phospholipase A2 (PLA2) also located on plasma membrane. Action of PLA2 results in release of linolenic acid from the plasma-membrane. Linolenic acid is now converted into jasmonic acid through octadecanoid pathway as mentioned earlier.
Jasmonic acid thus produced activates expression of genes that encode defensive proteins (such as α-amylase inhibitors, lectins, proteinase inhibitors). These proteins are anti-digestive which greatly hamper digestion of insects and other herbivores and thus provide strong deterrents to them.