Antimicrobial coumarins and proteins to functionalizing the leaf surface for plant protection

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Soybean is one of the most important crops worldwide. Its major disease is Asian soybean rust (SBR) that is caused by the fungus Phakopsora pachyrhizi. In the major soybean-growing areas (Brazil, Argentina, and the USA), SBR causes annual yield losses of several billion Euros a year. Resistant elite varieties of the soybean are not available and cultivation practices as well as chemical or biological control measures ineffective, expensive, or potentially harmful. Therefore, alternative measures for SBR control are needed. We previously showed that the effective nonhost resistance of sunflower and Arabidopsis to SBR could be due to the accumulation of scopoletin. The coumarin effectively inhibits the germination of P. pachyrhizi spores. Scopoletin, however, seems to be absent from soybean. Reconstituting scopoletin biosynthesis in transgenic soybean plants slightly reduced the susceptibility to SBR. But, high scopoletin concentrations damaged the plants. We, therefore, would like to transfer the nonhost resistance of sunflower to soybean by secreting scopoletin to the leaf surface to effectively fight P. pachyrhizi spores without harming the plant. We intend to do the same for isoscopoletin, that in contrast to other coumarins is very stable in the light and effectively fights P. pachyrhizi spores. In addition, we aim to reconstitute the biosynthesis of sideretin in transgenic plants and investigate its mode of action. Finally, in a complementary approach we would like to synthesize and secrete to the leaf surface antimicrobial proteins that contribute to sunflower nonhost resistance to SBR. Soybean plants that surface-accumulate antimicrobial coumarins and/or proteins are promising for sustainable worldwide soybean production.