Background: Agrobacterium tumefaciens has been successfully harnessed as the only natural vector for the incorporation of foreign genes into higher plants, but its use in the grain crops is often limited. Low transformation efficiency has been partly attributed to a failure in the initial events in the transformation process, specifically in the capacity of the VirA/VirG two-component system to induce expression of the virulence genes.
Results: Here we show that the root exudate of Zea mays seedlings specifically inhibits virulence gene expression, determine that 2-hydroxy-4,7-dimethoxybenzoxazin-3-one (MDIBOA), which constitutes > 98% of the organic exudate of the roots of these seedlings, is the most potent and specific inhibitor of signal perception in A. tumefaciens-mediated gene transfer yet discovered, and develop a model that is able to predict the MDIBOA concentration at any distance from the root surface. Finally, variants of A. tumefaciens resistant to MDIBOA-mediated inhibition of vir gene expression have been selected and partially characterized.
Conclusions: These results suggest a strategy in which a plant may resist pathogen invasion by specifically blocking virulence gene activation and yet ensure that the 'resistance factor' does not accumulate to levels sufficient to impose toxicity and selection pressure on the pathogen. The data further establish that naturally occurring inhibitors directed against signal perception by the VirA/VirG two-component regulatory system can play an important role in host defense. Finally, selected variants resistant to specific MDIBOA inhibition may now be used to extend the transformation efficiency of maize and possibly other cereals.