In plants, several defense systems are found which help them to mitigate the problems of their survival against pest/ insect. A unique defense system found in members of Brassicaceae family which are known as “Glucosinolate – Myrosinase system”. This system is involved in a range of biological activities affecting herbivorous insects, plants and fungi. The compartmentalization of the components of the myrosinase‐glucosinolate system and the cell‐specific expression of the myrosinase represents a unique plant defense system.  The loss of cellular integrity as a result of wounding, insect or pathogen attack activates the binary glucosinolate-myrosinase system and leads to the generation of thioglucose, sulfate and an unstable intermediate which rearranges spontaneously into several degradation products.

Brassicaceae or Cruciferae L.(or cabbage) family have glucosinolates a secondary metabolite. For the generations of numerous compounds from glucosinolates, plants also possess the enzyme which catalyzes the hydrolysis of glucosinolates. This enzyme is myrosinase also known as thioglucosidase(EC 3.2.3.1). Cruciferous plants use a sophisticated defense system, known as “Mustard oil bomb”.Glucosinolates are found in plants at the strategic location mean the enzyme only comes into contact with its glucosinolate substrates if the plant tissues are disrupted as a result of wounding, insect or pathogen attack.Glucosinolates and their hydrolysis productsare very well studied as plant defense system against insects, herbivores and certain microbial pathogens. Besides, some of the hydrolysis products are serve as attractants to specialist insects feeding on crucifers while some of the volatile products are responsible for characteristic taste and smell of cruciferousvegetables. In some Brassica vegetables such ascauliflower, Brussels sprouts, cabbage and broccoli, glucosinolatedegradation products, especially isothiocyanateshave been shown to have anticarcinogenic properties (Redovnikovic et al., 2008).Glucosinolates contain sulfur and nitrogen in its structure, so glucosinolates may act as a storage pool for these elements which can be mobilized through hydrolysis by myrosinases.To overcome the toxicity of glucosinolates and their hydrolysis products some herbivores (Plutellaxylostella) possess sulfatase enzymes that removes the sulfate moiety from the glucosinolatestructure. The resulting desulfoglucosinolate does notserve as a substrate for myrosinase anymore and passesthrough the insect’s digestive tract.

Glucosinolates

Glucosinolates are sulhpur containing secondary metabolites synthesized by numerous species ofBrassicaceae family.Chemically, glucosinolates are composed of thiohydroximate-O-sulfonate group linked to glucose, and an alkyl, aralkyl, or indolyl side chain (R). Based on the side chain structure, glucosinolate are grouped into aliphatic, aromatic and indolylglucosinolateswhether they are derived from aliphatic amino acids (oftenmethionine), phenylalanine or tyrosine, or tryptophan, respectively[or chain-elongated homologues thereof (e.g. homophenylalanineand dihomomethionine)]. Further modifications in secondary side chains such as hydroxylations, methylations, oxidations and desaturations are extend the diversity of glucosinolates.More than 200 side-groups (-R) have been identified in different plants species.

All the genes related to glucosinolate biosynthesis, transcriptional regulation and breakdown are distributed across all nine chromosome of B. oleracea(Eun Yi et al., 2015).Glucosinolates have been detected in all organs of the plant, and are located within the vacuole of the cell.In mustard different types of glucosinolates are present (Table 1) like Sinigrin (in Black mustard) and Sinalbin (in white / yellow mustard).All these glucosinolates are play an important role in defense system against pest/ herbivores.Glucosinolates have been identified in different organs throughout the plant (from roots to flowers). Glucosinolates nature and quantity are depended on the organ, the developmental stage and several external factors.

Table 1. Glucosinolate in mustard.

Glucosinolate biosynthesis

The biosynthesis of glucosinolates (Figure 1) comprises threephases: (i) amino acid chain elongation, through methylene group addition into the side chain,(ii) conversion of the amino acid moiety to the glucosinolatecore structure, and (iii) subsequent side chainmodifications.

Myrosinase enzyme

Myrosinase enzyme also known as beta thioglucosidase which catalyze the thiol-glucose linkage in glucosinolate. It is the member of glycoside hydrolase family. Myrosinase is adimeric protein with a molecular weight in the range of62–75 kDaper subunit.In the presence of water, myrosinase cleaves off the glucose group from a glucosinolate. The remaining molecule then quickly converts to athiocyanate, an isothiocyanate, or a nitrile; these are the active substances that serve as defense for the plant. 

Glucosinolate-Myrosinase system

In plants of crucifereae family have a defence system against herbivores pest. Glucosinolate and myrosinase enzyme found in a cell but the compartmentalization of these molecules make them inactive. Glucosinolates in most of the plants are found in vacuole of plant cell where as myrosinase enzyme are found in myrosin cell.When plant cell attack by pest myrosinase enzyme work on glucosinolate.Myrosinase enzyme show its thioglucosidase activity by cleave the beta thioglucoside bond in glucosinolates. In mustard, myrosinase enzyme hydrolyses the sinigrin and remove glucose moiety from its structure and further produce aglycone as intermediate and allylisothiocyante as a final product (Figure 2). The role of glucosinolates in defense against pathogens is less clear than that for herbivores. The toxicity of glucosinolate is generally attributed to the isothiocyanates, thiocyanates,oxazolidinethiones and nitriles, originating from enzymatic cleavage of glucosinolates bymyrosinase(Burelet al., 2001).

Effect of glucosinolate on animals

Adverse effects in animals have been generally correlated to the amount of totalglucosinolatesin the diet, despite the fact that individual glucosinolates vary in their toxicity.

Conclusion

In nature plants face more diverse stresses compare to animals because plants can not able to move during stress condition. So plants have diverse and unique defense systems for combat against any stress conditions. In case of glucosinolates- myrosinase system, plants protect itself against herbivores by releasing several toxic intermediates which are helpful to repel the herbivores.

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