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Abstract
Benzoxazinoids are secondary metabolites that are effective in defence and allelopathy.
They are synthesised in two subfamilies of the Poaceae and sporadically found in single
species of the dicots. The biosynthesis is fully elucidated in maize; here the genes
encoding the enzymes of the pathway are in physical proximity. This "biosynthetic
cluster" might facilitate coordinated gene regulation. Data from Zea mays, Triticum
aestivum and Hordeum lechleri suggest that the pathway is of monophyletic origin in
the Poaceae. The branchpoint from the primary metabolism (Bx1 gene) can be traced
back to duplication and functionalisation of the alpha-subunit of tryptophan synthase
(TSA). Modification of the intermediates by consecutive hydroxylation is catalysed
by members of a cytochrome P450 enzyme subfamily (Bx2-Bx5). Glucosylation by an UDP-glucosyltransferase
(UGT, Bx8, Bx9) is essential for the reduction of autotoxicity of the benzoxazinoids.
In some species 2,4-dihydroxy-1,4-benzoxazin-3-one-glucoside (DIBOA-glc) is further
modified by the 2-oxoglutarate-dependent dioxygenase BX6 and the O-methyltransferase
BX7. In the dicots Aphelandra squarrosa, Consolida orientalis, and Lamium galeobdolon,
benzoxazinoid biosynthesis is analogously organised: The branchpoint is established
by a homolog of TSA, P450 enzymes catalyse hydroxylations and at least the first hydroxylation
reaction is identical in dicots and Poaceae, the toxic aglucon is glucosylated by
an UGT. Functionally, TSA and BX1 are indole-glycerolphosphate lyases (IGLs). Igl
genes seem to be generally duplicated in angiosperms. Modelling and biochemical characterisation
of IGLs reveal that the catalytic properties of the enzyme can easily be modified
by mutation. Independent evolution can be assumed for the BX1 function in dicots and
Poaceae.