D3443: Optimizing a bio-based seed treatment for immune activation in soybean

Seed treatments with rhizobacteria such as Bacillus subtilis are an increasingly common approach to promote plant growth or stimulate plant defenses against pests. Here, we used B. subtilis to deliver a Plant Elicitor Peptide (Pep) that combats soybean cyst nematode (SCN) infection, and we explored methods to improve the shelf life of B. subtilis seed treatments.  Peps are signals found throughout the flowering plants that activate plant immunity, and that can protect plants against pests and diseases when applied to seeds, roots, or foliage. Here, we 1) engineered B. subtilis to excrete a Pep from soybean (GmPep3); 2) established that treating seeds with B. subtilis vegetative cells enabled persistent colonization of soybean roots, and 3) demonstrated that SCN infection was significantly lower on plants treated with B. subtilis expressing GmPep3 than on plants that received the control solution or B. subtilis alone (the empty vector).  These findings are important because SCN (Heterodera glycines) causes yield losses of more than $1 billion annually on soybean. However, shelf life is a practical challenge for B. subtilis and other bio-based pest management products, and so we also investigated two approaches to increase the retention of viable bacteria on seeds: the binding agent carboxymethylcellulose (CMC), and the use of dormant B. subtilis spores rather than vegetative cells. While CMC did not have a strong or consistent benefit, the use of spores dramatically increased the long-termed viability of B. subtilis seed treatments. This information should assist in the development of many other rhizobacterial seed treatments.