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Insect P450s and allelochemical tolerance D1 TRAN Thi Thu Phuong

2013/06/24 2:52 に Takashi Matsuo が投稿
2013/7/03

Plants produce a wide variety of secondary metabolites for protection against herbivores [1]. However, many herbivorous insects have developed counter-adaptations that enable them to feed on chemically defended plants without apparent negative effects [2]. How generalists cope with the diversity and unpredictability of plant defences remains largely unknown at both the genetic and molecular levels [3]. Insect cytochrome P450 monooxygenases (P450s or CYPs for encoding genes) play a paramount role in allelochemical metabolism and tolerance. Cytochrome P450 monooxygenases comprise a superfamily of heme-thiolate enzymes that catalyze the NADPH-associated reductive cleavage of oxygen to produce a functionalized product and water [4]. P450s are heme-dependent mixed function oxidases that constitute one of the major systems used by insects to detoxify plant allelochemichals [5]. Insect P450s metabolize all classes of plant allelochemicals because of their catalytic versatility and broad substrate specificity [6].

CYP genes are classified into different families (e.g. CYP6 family) and subfamilies (e.g. CYP6B subfamily) according to their amino-acid sequence homology, and have been isolated from various insect species. The CYP6B gene subfamily is involved in the metabolism of host plant toxins (i.e. furanocuoumarins). CYP6B transcripts are inducible to varying degrees by arrange of allelochemicals naturally encountered in host plant (xanthotoxin, indole 3-carbonol, chlorigenic acid, and flavone).  Similarities in structural and promoter regions of the CYP6B genes suggest that they are derived from a common ancestral gene [7, 8, 9]. CYP6B play a critical role in allowing Papilio to adapt to furanocoumarin containing host plants. Papilio polyxene, a specialist that feeds almost exclusively on furanocoumarins-containing species in Apiaceae and Rutaceae, contains two genes CYP6B1 and CYP6B3 for protection against plant allelochemicals [3] and two relate genes CYP6B1v3 and CYP6B3v2 [5]. Two closely polyphagous Papilio species, Papilio glaucus, which occasionally encounters plants containing furanocoumarins, and Papilio canadensis, which rarely if ever encounters furanocoumarins, have 16 CYP6B genes belonging to two groups CYP6B4 and CYP6B17 [10]. P. glaucus contains two nearly identical genes, CYP6B4v2 and CYP6B5v1 [5].

CYP6B genes are also found in the polyphagous noctuids Helicoverpa zea. In Helicoverpa zea, the CYP6B transcripts are also inducible to some extent by the plant defense signaling molecules, such as jasmonate and salicylate, allowing this species to eavesdrop on plant defense signals and activate their detoxification systems in advance of induced biosynthesis of host plant toxins [6, 9]. Four cytochrome CYP6B genes in the H. zea, CYP6B8, CYP6B9, CYP6B27, CYP6B28 are induced in response to a number of plant allelochemicals, including indole-3-carbinol and chlorogenic acid [1]. Molecular modeling indicates that CYP6B8 in H. zea has more flexible overall folding, a more elastic catalytic pocket, and one more substrate access channel than CYP6B1 in P. polysenes [3].

In conclusion, cytochrome P450 monooxygenases responsible for the metabolism of plant allelochemicals have been studied in only a few insect species and the molecular genetics of these metabolic reactions and induction responses, however, have been defined in a few systems. Therefore, further studies of P450s will be needed to provide critical information of genetic manipulation of insect species for inactivation of the monooxygenases activities mediating allelochemical detoxification.

 

REFERENCES

[1]. Li, X., Berenbaum, M. R., Schuler, M. A., (2002). Plant allelochemicals differentially regulate Helicoverpa zea clytochrome P450 genes. Insect Molecular Biology. 11: 343-351.

[2]. Wittstock, U., Agerbirk, N., Stauber, E. J., Olsen, C. E., Hippler, M., Mitchell-Olds, T., Gershenzon, J., Vogel, H., (2004). Successful herbivore attack due to metabolic diversion of a plant chemical defense, PNAS.101(14): 4859-4864.

[3]. Li, X., Baudry, J., Berenbaum, M. R., Schuler, M. A., (2004). Structural and functional divergence of insect CYP6B proteins: From specialist to generalist cytochrome P450. PNAS.101(9): 2939-2944.

[4]. Li, W., Schuler, M. A., Berenbaum, M. R., (2003). Diversification of furanocoumarin-metaboliizing cytochrome P450 monooxygenases in two papilionids: Specificity and substrate encounter rate. PNAS.100: 14593-14598.

[5]. Hung, C. F., Holzmacher, R., Connolly, E., Berenbaum, M. R., Schuler, M. A., (1996). Conserved promoter elements in the CYP6B gene family suggest common ancestry for cytochrome P450 monooxygenases mediating furanocoumarin detoxification. Proc. Natl. Acad. Sci. USA. 93: 12200-12205.

[6]. Li, X., Schuler, M. A., Berenbaum, M. R., (2007). Molecular mechanisms of metabolic resistance to synthetic and natural xenobiotic. Annu. Rev. Entomol. 52: 231-253.

[7]. Scott, J. G., Liu, N., Wen, Z., (1998). Insect cytochromes P450: diversity, insecticide resistance and tolerance to plant toxins. Comparative Biochemistry and Physiology, Part C. 121: 147-155

[8]. Schuler, M. A., (2011). P450s in plant-insect interactions. Biochimica et Biophysica Acta. 1814: 36-45.

[9]. Després, L., David J. P., Gallet, C., (2007). The evolutionary ecology of insect resistance to plant chemicals. TRENDS in Ecology and Ecolution. 22(6): 298-307.

[10]. Li, W., Berenbaum, M. R., Schuler, M. A., (2001). Molecular analysis of multiple CYP6B genes from polyphagous Papilio species. Insect Biochemistry and Molecular Biology. 31: 999-1011.
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