The effects of Bacillus subtilis expressing a plant elicitor peptide on nematode infection on soybean

There is a pressing need to develop alternative management strategies for the soybean-cyst nematode (Heterodera glycines, SCN), the most costly pathogen to soybeans. Plant elicitor peptides (PEPs), which are produced by plants in response to stress and stimulate broad-spectrum disease resistance, were previously shown to reduce SCN infection on soybeans when applied as a seed treatment. Here, we introduce an alternative method to deliver PEPs to soybean using a common plant growth-promoting rhizobacterium, Bacillus subtilis, as a bacterial expression system. Similar to the empty vector control, B. subtilis engineered to express a PEP from soybean (GmPEP3) was able to colonize soybean roots and persisted on roots more than a month after treatment. Compared to water or the empty vector control, plants that received a seed treatment with B. subtilis expressing GmPEP3 (B.+GmPEP3) were significantly taller early in vegetative growth (V1 stage) and had lower chlorophyll content in the reproductive stage (R3/R4); these results suggested that GmPEP3 may hasten growth and subsequent senescence. When plants were inoculated with SCN at the V1 stage, those pre-treated with B.+GmPEP3 supported significantly fewer nematode eggs at the reproductive stage (R3/R4) than plants treated with water or the empty vector. The effects of B.+GmPEP3 on nematode infection and plant growth appeared to be due primarily to the peptide itself because no significant differences were observed between plants treated with water or with B. subtilis expressing the empty vector. These results indicate the ability of B. subtilis to deliver defense activators for nematode management on soybean.