**4.3. Alkaloids**

Alkaloids are a group of chemical compounds that mostly contain basic nitrogen atoms. Saponins are a class of glycosylated triterpenes; steroids and steroidal alkaloids synthesized from the mevalonate or non-mevalonate pathway in plants. These compounds are absent in most monocotyledon plants and all cereals except in oat. The glycosylated form confers activity to avenacins, contrary to other compounds such as avenacosides, benzoxazanoids and other compounds with antifungal activity, then only with active in its form of aglycone [52]. Vacuoles are the reservoir of inactive avenosides, which allow them to be available when there is tissue damage caused by pathogenic fungi causing their activation; this results in alteration of the membranes and consequently the formation the biologically active aglycone. In a research performed by the homozygous mutant, *A. strigose* lines and the wild-type line were inoculated with fungal pathogens to assess the effects of the saponin-deficient mutations on plant disease resistance. The results exhibited that mutant plants showed increased susceptibility to *Fusarium culmorum* and *Fusarium avenaceum* revealing an implication of saponins in the plant resistance [53].

The best-known alkaloids of grasses are hordenine and gramine. Hordenine is found in many plant species and in cereals; it has been reported in barley, millet and sorghum. The reports of their allelopathic effects may imply a resistance to *Fusarium* and their mycotoxins; however, no specific reports have been found.

Several compounds within the monoterpene indole alkaloid class are known to exhibit antifungal properties. Secologanin production is induced by the application of methyl jasmonate in *C. roseus*, perhaps suggesting a link between defense-related signaling pathways and monoterpene indole alkaloid production. A study using double haploid barley lines differing in *Fusarium* head blight sensitivity observed metabolite accumulation and found secologanin was constitutively produced in resistant lines [54]. Few alkaloid compounds have been identified within wheat. A more detailed understanding of how cereal crops and related grass species respond to *Fusarium* pathogens will reveal novel mechanisms of resistance.

#### **4.4. Benzoxazinoid**

Benzoxazinoids (Bxs) are widely distributed in cereals discovered in the 1950s. A range of biological roles such as allelopathy, resistance to insects and defense against pathogens has been attached to them [55]. Benzoxazinoids are synthesized in the shikimate pathway from the amino acid tryptophan. They are present in maize; wheat, rye and certain wild barley species, however, have not been found in cultivated barley varieties, oat or rice. Bxs are stored in an inactive glucoside form in plant vacuoles or plastids to avoid toxicity to the plant itself; through the enzymatic activation and chemical degradation, the tissue disrupted form the active benzoxazinoid [56]. In a research using wheat, principal component analyses demonstrated a correlation between the susceptibility to FHB and the concentrations of range of Bxs [57]. The benzoxazinoid 2-β-glucopyranoside-2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3 one (DIMBOA-glc), α-tocopherol and the flavonoids homo-orientin and orientin were identified as potential inhibitors of (deoxynivalenol) DON accumulation in a study with wheat that correlates accumulation in *Fusarium*-infected winter and spring wheat cultivars [58].

On the other hand, in a cromatografy study, volatile organic compounds (VOCs) were identified using GC-MS in oats, barley and wheat infected by three species of *Fusarium*, including species that caused cortical rot disease in wheat, and two terpenes were identified (linalool and

The metabolomics as a tool helped in to identify the metabolites in barley that are related to resistance against *Fusarium* head blight FHB exposed that metabolites conferring resistance mainly belonged to phenylpropanoid, flavonoid, fatty acid and terpenoid metabolic pathways [50]. A research by Wang et al. [51] exposed a number of genes involved in secondary metabolites biosynthesis are specifically responsive to *F. verticillioides* inoculation in BT-1 kernels. Terpenoid biosynthesis and diterpenoid biosynthesis were particularly increased by *F. verticillioides* inoculation. See Ref. [29] to review a list of terpenoids conferring resistance to *Fusarium*.

Alkaloids are a group of chemical compounds that mostly contain basic nitrogen atoms. Saponins are a class of glycosylated triterpenes; steroids and steroidal alkaloids synthesized from the mevalonate or non-mevalonate pathway in plants. These compounds are absent in most monocotyledon plants and all cereals except in oat. The glycosylated form confers activity to avenacins, contrary to other compounds such as avenacosides, benzoxazanoids and other compounds with antifungal activity, then only with active in its form of aglycone [52]. Vacuoles are the reservoir of inactive avenosides, which allow them to be available when there is tissue damage caused by pathogenic fungi causing their activation; this results in alteration of the membranes and consequently the formation the biologically active aglycone. In a research performed by the homozygous mutant, *A. strigose* lines and the wild-type line were inoculated with fungal pathogens to assess the effects of the saponin-deficient mutations on plant disease resistance. The results exhibited that mutant plants showed increased susceptibility to *Fusarium culmorum* and *Fusarium avenaceum* revealing an implication of saponins in the plant resistance [53].

The best-known alkaloids of grasses are hordenine and gramine. Hordenine is found in many plant species and in cereals; it has been reported in barley, millet and sorghum. The reports of their allelopathic effects may imply a resistance to *Fusarium* and their mycotoxins; however,

Several compounds within the monoterpene indole alkaloid class are known to exhibit antifungal properties. Secologanin production is induced by the application of methyl jasmonate in *C. roseus*, perhaps suggesting a link between defense-related signaling pathways and monoterpene indole alkaloid production. A study using double haploid barley lines differing in *Fusarium* head blight sensitivity observed metabolite accumulation and found secologanin was constitutively produced in resistant lines [54]. Few alkaloid compounds have been identified within wheat. A more detailed understanding of how cereal crops and related grass spe-

Benzoxazinoids (Bxs) are widely distributed in cereals discovered in the 1950s. A range of biological roles such as allelopathy, resistance to insects and defense against pathogens has

cies respond to *Fusarium* pathogens will reveal novel mechanisms of resistance.

β-caryophyllene), which found higher concentrations with respect to the controls [49].

30 Fusarium - Plant Diseases, Pathogen Diversity, Genetic Diversity, Resistance and Molecular Markers

**4.3. Alkaloids**

no specific reports have been found.

**4.4. Benzoxazinoid**

A plethora of secondary metabolites have been reported to inhibit *Fusarium* and their mycotoxins; however, the molecular mechanisms of plant resistance to both are needed to provide a deeper understanding of the mode of actions of the metabolites as well as the mechanisms of detoxification.
