The innate immunity of maize and the dynamic chemical strategies regulating two-component signal transduction in Agrobacterium tumefaciens

The naturally occurring 2-hydroxy-4,7-dimethoxybenzoxazin-3-one (HDMBOA), essentially the sole component of maize seed ling organ ic exudate, was shown to be a potent inhibitor of the VirA-VirG two-component system which mediates host recognition and activates virulence gene transcription in the soil pathogen Agrobacterium, turnefaciens. The hydrolytic lability of HDMBOA creates a steady-state zone of inhibition circumscribing the young maize seedling. We now show that rather than the HIDMBOA natural product, an o-imidoquinone decomposition intermediate, (3Z)-2,2-dihydroxy-N-(4-methoxy-6-oxocyctohexa2,4-dienylidene)acetamide, can function as an inhibitor of virulencegene expression in A. turnefaciens. Structural characterization of this o-imicloquinone intermediate clarifies several issues related to the decomposition pathways available to this class of antibiotics. Of direct ecological importance, this species is pr oduced rapidly and quantitatively within the more neutral pH ranges of the A. tumefaclens cytoplasm, while HDMBOA is more persistent at the slightly acidic pH common to many soils. These results suggest the rather intriguing possibility that the physical instability of the benzoxazinone antibiotics may not only create a steady-state local defense, but also enable a pro-drug strategy directed against bacterial environmental sensing.