Date of Graduation

8-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Cell & Molecular Biology (PhD)

Degree Level

Graduate

Department

Cell & Molecular Biology

Advisor/Mentor

Goggin, Fiona

Committee Member

Pereira, Andy

Second Committee Member

Shi, Ainong

Third Committee Member

Rojas, Alejandro

Keywords

Ectopic expression; Soybean; Plant elicitor peptides

Abstract

Soybean (Glycine max) is the most beneficial legume crop because it is a source of protein as well as oil for humans and livestock. Soybean is the second most planted crop in the United States following corn and it is also the fourth planted crop worldwide. However, the soybean cyst nematode (Heterodera glycines, SCN) is the most damaging pest of soybean in the United States, accounting for more than twice as much yield loss as the following cause of yield loss in soybean. Therefore, there is a pressing need to develop alternative management strategies to control SCN infection in soybeans. Plant elicitor peptides (PEPs) are widely conserved signals that are produced by plants in response to stress. PEPs play an important role in controlling plants’ resistance to pathogens. PEPs were previously shown to reduce SCN infection on soybeans when applied as a seed treatment. At first, the expression of various genes in response to soybean seed treatment with a synthetic soybean PEP (GmPEP3) for overnight and 24 hrs were tested for both root and leaf tissues. The results indicate that the overnight or 24 hrs seed treatment did not have a constant effect on the expression of the tested genes at the early vegetative stages (VC). Therefore, utilizing a common plant growth-promoting rhizobacterium, Bacillus subtilis, as a bacterial expression system to deliver PEPs to soybean assure a continuous expression of PEPs throughout plants growth and may have a better effect on plants resistance to nematode than synthetic PEPs. The objectives of this study were to determine whether PEPs could be utilized to prime immune responses against nematodes, and whether a soil rhizobacterium, Bacillus subtilis, could be used to deliver exogenous PEPs to soybean roots for SCN suppression. Our results indicated plants that received a seed treatment with B. subtilis expressing a soybean PEP (GmPEP3) have 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. Because shelf life is a practical challenge for B. subtilis and other bio-based pest management products, we 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-term viability of B. subtilis seed treatments. These results indicate the ability of B. subtilis to deliver defense activators for nematode management on soybean and should assist in the development of many other rhizobacterial seed treatments.

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