## 3 Discussion
Terpenoids play important roles in protecting plants against pests, microbes and pathogens. Monoterpenes and sesquiterpenes are the major volatile compounds released from plants after herbivore damage, attracting arthropods that prey on or parasitize herbivores [8]. Eucalyptus was also found to be a common constituent released from the 12 Brassiaceae vegetables studied. Structurally, eucalypt is not considered as a terpenoid; however, it has been found to be present at substantial levels during insect-repelling or pesticidal activities [37; 38]. Therefore, the results obtained could serve as a basis for the utilization of the volatile compounds from these Brassiaceae vegetables to exploit the specific activities of terpenoid constituents. In addition to the wide spectrum of volatile compounds mentioned, Brassiaceae vegetables are characterized by GLSs, a unique group of sulphur-containing secondary plant metabolites. When cells are disrupted, myrosinase is released and converts GLSs to various volatile breakdown products including isothiances (ITCs) and nitriles. The breakdown of GLSs and the release of ITCs are of potential significance in several aspects. In Brassiaceae vegetables, ITCs play important roles in the plant's defence against biotic stressors and act as effective deterrents against a multitude of pathogens due to their antifungal, antimicrobial and insecticidal properties [39]. Moreover, they are important for food sensory properties and have health benefits in the prevention of several human diseases. For example, ITCs are responsible for the sharp taste of mustard and radish [40], and several ITCs have cancer-preventing effects [20]. Among the ITCs, PEITC from watercress has been identified to be a pleiotropic protective agent that interacts with multiple carcinogenic pathways [41; 42; 43]. Many studies have shown the species-specific VOC emissions that reflects different genetic regulations among species [32]. Here we show that the activities of different pathways biosynthesizing volatile metabolites varied among these vegetable species. Evolution of Brassica crops may have been triggered by genome duplication events [44] and syntenic analysis further demonstrated that GLS genes may have increased their copy numbers through whole genome triplication events. In terms of genomic potential for glucosinolate biosynthesis, a pathway that shows major differences in VOCs between these species, there are key differences in the genes detected in these four species. Most of the GLS genes were present in multiple copies and _R. sativus_ having 144 GLS genes, which is slightly fewer than the 161 GLS genes present in _Brassica rapa_ and, and greater than the 117 GLS genes of _Brassica oleracea_[45]. Although GLS genes are mostly conserved among the Brassicas, some GLS genes differ in the GLS biosynthesis pathways, especially for the short-side-chain aliphatic GLSs. The biosynthesis pathways, of which involve MAM1, MAM3 and CYP79F2 genes that participate in the methionine side-chain elongation step and the AOP2 gene that catalyses the oxidation of aliphatic glucosinolate, present in the radish genome [44]. Finally, the expression levels of MYB28 and MYB29-like genes that are key regulators of glucosinolate biosynthesis in radish could contribute to the different species-specific glucosinolate profiles. Differences in VOCs associated with glucoerucin, glucoraphanin, glucoraphasatin and glucoarphenin could be due to the presence of GRS1 that is specific to radish. Interestingly, neither AOP2 nor AOP3 was detected in radish, this may lead to the accumulation of glucoarphasatin and glucoarphenin. While epithiospecific protein (ESP) and nitrile-specifier protein (NSP) can catalyse GSL to produce nitriles and epithionitriles, ESP is inactivated in radish and may lead to, the accumulation of isothiances, such as sulforaphene, instead of epithionitriles or nitriles ([PERSON] et al., 2017), while in _B. rapa_ and _B. oleracea_, the major GLS hydrolysis products are nitriles and epithionitriles.