Resistance to Meloidogyne incognita Induced by Bacillus spp.

Abstract

Meloidogyne incognita race 3 is a major pest of hundreds of susceptible plant hosts around the world. Biological control agents are one management strategy that can be employed against this nematode. The goal of this research was to determine the mechanism of action by which five Bacillus spp. can manage M. incognita population density in cotton. The overall objectives were to 1) determine the efficacy and direct antagonistic capabilities of the Bacillus spp. and 2) determine the systemic capabilities of the Bacillus spp. The greenhouse in planta assay indicated B. amyloliquefaciens QST713 and B. firmus I-1582 could manage M. incognita similarly to the chemical standard fluopyram. An in vitro assay determined that B. firmus I-1582 and its extracted metabolites were able to directly manage M. incognita second stage juveniles by increasing mortality rate above 75%. A split root assay, used to determine systemic capabilities of the bacteria, indicated B. amyloliquefaciens QST713 and B. firmus I-1582 could indirectly decrease the nematode population density. Another species, B. mojavensis strain 2, also demonstrated systemic capabilities but was ruled out as a successful biological control agent because it had the second highest population density behind the control when in contact with the nematode in greenhouse in planta assay and the split root assay compared to any other treatment. A RT-qPCR assay was used to evaluate systemic activity observed in the split root assay. At 24 hours, both B. amyloliquefaciens QST713 and B. firmus I-1582 upregulated one gene involved in the initial stages of jasmonic acid (JA) synthesis pathway but not another gene involved in the later stages of JA synthesis. These results point to a JA intermediate molecule, most likely 12-oxo-phytodienoci acid (OPDA), stimulated by the bacteria rather than JA in a short-term systemic response. After 1 week, the Bacillus spp. stimulated a salicylic acid (SA) responsive defense related gene. The long-term systemic response to the Bacillus spp. indicates SA also plays a role in defense conferred by these bacteria. The final assay used qPCR to determine the concentration of the bacteria on the cotton roots after 24 days. Bacillus amyloliquefaciens QST713 and B. firmus I-1582 were able to colonize the root successfully, with the concentration after 24 days not significantly differing from the concentration at inoculation. This study identifies two bacteria that induce systemic resistance and will help aid in implementing these species in an integrated management system.