Occurrence and characterization of 2-hydroxy-1,4-benzoxazin-3-one and indole hydroxylases in juvenile wheat

Cyclic hydroxamic acids, 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA) and its 7-methoxy analogue (DIMBOA), occur transiently in high amounts in wheat an d maize during the juvenile, non-autotrophic stage of growth. To elucidate the biosynthetic enzymes operating for the transient production of these co mpounds, we examined the hydroxylating activities for 2-hydroxy-1,4-benzoxa zin-3-one (HBOA), the immediate precursor of DIBOA, and indole, the first i ntermediate of the biosynthetic pathway that branches off from the tryptoph an pathway, by using microsomes prepared from wheat seedlings. Both hydroxy lases occurred soon after germination, reached a maximum 48 h after germina tion, and decreased to finally disappear as the plants grew into the autotr ophic growth stage. The mode of appearance and disappearance similar to tha t of hydroxamic acids, suggesting that elevated expression of the whole set of enzymes involved in the biosynthesis after indole was responsible for t he transient occurrence of hydroxamic acids. The hydroxylating activity was also observed for 1,4-benzoxazin-3-one, a putative precursor of HBOA, but to significantly less extent than that for HBOA and indole.