Organic chemial approach to the elucidation of the mechanism of plant transformation induced by Agrobacterium tumefaciens

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Plant transformation by Agrobacterium tumefaciens has been dramatically developed to create plants acquiring new characters, e.g., herbicide- or virus-resistance. Many scientists have been exploring the mechanism for the plant transformation, although the entire mechanism is not yet fully understood. As the plant transformation consists of several complex steps, a specific chemical probe affecting each step is required to elucidate its mechanism. The discovery of transformation regulators and the classification of these probes based on their mode of action are both helpful. We devised a rapid and quantitative bioassay method for regulators of plant tranformation by A.tumefaciens with an intron-GUS binary vector as an alternative method to the time-consuming potato tuber disk assay. Among several host plants tested, tobacco BY-2 suspension cells were the most preferable for this GUS expression assay. Furthermore, as the phytotoxicity of the test material, if any, is apparent from the growth inhibition of BY-2 suspension cells, this method can exclude phytotoxic compounds.In the course of our screening by the potato tuber disk assay for inhibitors of crown gall formation, oxazolomycin was isolated. Oxazolomycin inhibited crown gall formation owing to strong anitbacterial activity to the pathogen, A.tumefaciens. Interestingly, oxazolomycin esters had no antibacterial activity, although they inhibited crown gall formation. Curromycins, oxazolomycin analogs, were reproted as antibiotics. We isolated curromycin B from the curromycin-producing strain and prepared curromycin B diacetate. Curromycin B showed both crown gall formation inhibitory and antibacterial activities against A.tumefaciens. Curromycin B diacetate did not show any antibacterial activity, although it inhibited crown gall fromation. These results indicated that curromycin B diacetate was a promising chemical probe for studying the mechanism of plant transformation by A.tumefaciens.