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How insect herbivores overcome 1,4-benzoxazin-3-ones in maize, D2, Tran Thi Thu Phuong

2014/06/24 22:10 に ユーザー不明 が投稿   [ 2014/06/24 22:12 に更新しました ]

Insect herbivores are often restricted by plant chemical defence compounds, such as, alkaloid, benzoxazinoid, cyanogenic and iridoid glucosides (1). DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one) is the main benzoxazinoids in maize and play an important role in the defence of maize against insects, fungi, and bacteria.  DIMBOA in intact tissues are stored in an inactive form as DIMBOA-glucoside. Upon tissue disruption, DIMBOA-Glc comes into contact with β-glucosidases form plastids, which hydrolyses the glucosidic bond and releases DIMBOA and their breakdown products called MBOA (2, 3, 4). In response to herbivorous insect, DIMBOA-Glc is methylated to form HDMBOA-Glc by an O-methyltransferase. HDMBOA is also released by plant derived β-glucosidase, however, HDMBOA is highly unstable and the conversion to MBOA is more rapid than for DIMBOA (5, 6). These 1,4-benzoxazin-3-ones of maize are toxic to a wide range of insect herbivores. However, many insect have evolved a diversity of counter-adaptations to overcome these chemical defence compounds in maize.

Larvae of insect herbivores have evolved a diversity of counter-adaptations to overcome maize chemical defence compounds. Larvae of some insect species have highly alkaline pH conditions in the midgut lumen, which may inhibit plant β-glucosidases and prevent activation of ingested defence compounds. A direct link between an alkaline midgut and reduced plant β-glucosidases activity towards benzoxazinoid glucosides was showed in the generalist fall armyworm S. frugiperda. When S. frugiperda larvae fed on maize plant, DIMBOA-Glc was present at high concentrations throughout digestion and HDMBOA-Glc was found in trace amounts in the fore- and midgut. The larval midgut lumen with a pH of 10 was shown to reduce plant β-glucosidases activity by more than 80%, which strongly reduced the release of toxic aglucones. Thus, insect herbivores with an alkaline midgut simply may have been pre-adapted to feed on plants protected by chemical defences (1). Moreover, even when plant β-glucosidases activate the corresponding glucosylated defence compounds into toxic aglucones, some insects have evolved enzymes for counteraction. Conjugating enzymes add a glucose to the aglucone to increase its water solubility and excretion efficiency. Activated glucose is then used to form O-glucosides, an enzymatic reaction catalysed by family1 UDP-glucosyltransferases (UGTs). Re-glucosylation of toxic aglucones in to non-toxic glucosides directly in the gut is an efficient adaptation that allows rapid and non-toxic excretion of the glucoside. The re-glucosylation of DIMBOA into non-toxic DIMBOA-Glc via UGTs in the gut and excrete was reported in S. frugiperda and Mythimna separata (6, 7). In addition, when larvae of S. frugiperda fed with purified DIMBOA and MBOA, DIMBOA-Glc and MBOA-Glc were also observed in the later stages of digestion and in its frass (2, 6).

In conclusion, insect herbivores have evolved a diversity of counter-adaptations to overcome plant chemical defence compounds. These adaptations include various physiological adaptations as well as metabolic enzymatic strategies of the insect’s digestive system. The glucosylation of DIMBOA and MBOA is a major detoxification mechanism that helps insects tolerate maize 1,4-benzoxazin-3-ones.



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