Flavones, C-glycosyl

Maysin (2 -0-a-l-rhamnosyl-6-C-(6-deoxyxv/o-hexos-4-ulosyl)-lutcolin 6.17a), apimaysin (6.17b) and methoxymaysin (6.17c) are C-glycosyl flavones that confer resistance against the com earworm (Helicoverpa zea (Boddie)), a major silk- and kernel-feeding insect pest in the United States. 

Maysin is generally the most abundant of these three compounds, and is typically present at concentrations of 0.3% fresh silk weight, which is very high for a single compound. As a consequence, the C-glycosyl flavones can be considered preformed defense compounds. 

The genetic control of the substitution of the C-glycosyl flavones was investigated in further detail by Cortes-Cruz et al. (2003). Two F2 populations were generated from maize inbred lines that differed from each 

The biosynthetic pathway leading to maysin starts with flavanone (6.18), which is hydroxylated by flavone 3 hydroxylase to yield di-hydroxyl flavanone (6.19) and is the reduced by flavone synthase to the flavone luteolin (6.20). The next steps were recently investigated in more detail by McMullen et al. (2004) using two salmon silk mutants, sml (Anderson, 1921) and a newly discovered mutant sm2. These mutants have salmon colored silks instead of green silks as a result of pigment accumulation throughout the shaft of the silks, as opposed to only in the silk hairs, but do require a functional PI gene in order for the mutant phenotype to be apparent (see also Chapter 3, Section 9.2). 

Compounds formed in response to stress may occur in at least two ways. In one response, the plant may form compounds throughout the tissue at a considerable distance from the infection site (Hammerschmidt, 1999). In another response, the plant may form compounds specifically at the infection site. This may include only a few cells and in rare cases, as few as one or two cells. (Snyder and Nicholson, 1990 Nicholson and Wood, 2001). In general, such compounds are referred to as either stress metabolites or more often as phytoalexins. By definition phytoalexins are formed in response to infection (Aguero et al., 2002 Lo et al., 2002 Hammerschmidt and Nicholson, 2001 Lo and Nicholson, 1998). Phytoalexins often exhibit toxicity to specific pathogens. In this case there is a genetic relationship between the expression of phytoalexin synthesis and the organism that induces that synthesis (Essenberg et al., 1985). 

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Ferreres F, Gil-Izquierdo A, Andrade PB, Valentao P and Tomas Barberan FA. 2007. Characterization of C-glycosyl flavones O-glycosylated by liquid chromatography—tandem mass spectrometry. J Chromatogr A 1161 (1—2) 214—223. 

Diets rich in millet have been associated with endemic goiter in parts of West Africa where millet is a staple. The damage has been attributed to vitexin, a C-glycosyl flavone, that in rats has antithyroid activity and that in vitro inhibits thyroid peroxidase and the free radical iodination step in thyroid hormone biosynthesis. Isoflavones have produced similar antithyroid effects in rats, but clinical studies in adults have not. " However, this remains a possible concern in infants fed soya-based milk-replacers, especially if iodine supply is compromised. 

Zea mays Maysin (C-glycosyl flavone) Corn earworm, Helicoverpa zea 369... 

Stems and leaves C-glycosyl-flavones and Myzus persicae ... 

Cortes-Cruz, M., Snook, M., and Mac Mullen, M.D., The genetic basis of C-glycosyl flavone B-ring modification in maize (Zea mays L.) silks, Genome, 46, 182, 2003. 

Thus, in mono-C-glycosyl flavones the presence of ion [(M—H) —120], and the simultaneous absence of ion [(M—H)—60], indicate that a hexose is the sugar involved in the C-glycosylation. In these cases the ion [(M—H) —90] can also be present, which is more relevant in the 6-C-hexosyl derivatives (being, sometimes, the base peak) than in the 8-C-hexosyl derivatives (being, sometimes, very small). The ion [(M—H) — 18], which ismore frequent in 6-C-hexosyl derivatives than in 8-C-hexosyl derivatives, can also be observed (Ferreres et al., 2003) (Figure 7.10). 

Figure 7.10 MS2[M—H] of (a) mono-C-glycosyl flavonoid isomers 8-C-glucosyl luteolin and 6-C-glucosyl luteolin and (b) asymmetric di-C-glycosyl flavone isomers 6-C-arabinosyl-8-C-glucosyl apigenin and 6-C-glucosyl-8-C-arabinosyl apigenin (Ferreres et al., 2003).

Figure 7.11 General fragmentation of 0-glyeosyl-C-glycosyl flavones (a) 0-glyco-sylation on phenolie hydroxyls (b) 2"-0-glyeosyl-C-glycosyl derivatives (e) 6"-0-glyeosyl-C-glyeosyl derivatives.

Figure 7.12 MS2[M—H] and MS3[(M—H) Yq] of C-glycosyl flavones O-glycosylated on phenolic hydroxyl (a) 4 -0-glucosyl-6-C-glucosyl luteolin (b) 4 -O-glucosyl-8-C-glucosyl luteolin (Ferreres et ah, 2007b).

Figure 7.13 Fragmentation and MS2 [M—H] of X"-0-glycosyl-C-glycosyl flavones (a) 2"-0-hexosyl-6-C-hexosyl luteolin (b) 6"-0-hexosyl-6-C-hexosyl luteolin (Ferreres et al., 2007b).

Ferreres, F, Silva, B.M., Andrade, P.B., Seabra, R.M., Ferreira, M.A. 2003. Approach to the study of C-glycosyl flavones by Ion Trap FIPLC-PAD-ESI/MS/MS application to seeds of quince (Cydonia oblonga). Phytochem. Anal. 14 352-359. 

C-Glycosyl flavones have been reported from green algae, mosses, ferns, and both monocotyledonous and dicotyledonous angiosperms (Chopin and Dellamonica, 1988 Markham, 1988) records from algae are questionable... 

Ferreres, F. Andrade, P.B. Valentao, P. Gil-Izquierdo, A. Further knowledge on harley (Hordeum vulgare L.) leaves O-glycosyl-C-glycosyl flavones hy liquid chromatography-UV diode-array detection-electrospray ionisation mass spectrometry. J. Chromatogr. A 2008,1182 (1), 56-64. 

Based on these steps, the biosynthetic pathway for C-glycosyl-flavones, which commonly occur in pearl millet, is outlined in Fig. 8. 

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