International Sugar Journal

Targeting soft rot of tuber crops with soil-dwelling phagocytic predators of bacteria

The Problem: Potato soft rot

Each year, approximately 20% of the total potato crop is lost to potato soft rot (1-3). Currently, there is no effective treatment for the disease once soft rot bacteria have infected plant tissue. Potato bacterial soft rot occurs worldwide and causes a greater total loss of produce than any other bacterial disease. In fact, according to data from the United States Department of Agriculture (USDA), the cost of potato loss in the U.S. alone likely exceeded $225 million in 2017 (4). AmebaGone is developing proprietary technologies to prevent crop losses due to potato soft rot and other major bacterial diseases of crops.

Dictyostelids: An effective bacterial predator

Dictyostelids, or Dicty, are soil-dwelling amoeba that feed on bacteria (5, 6). Dicty hail from across the globe and thrive in many distinct ecological niches: aerial plant surfaces, cultivated fields, natural prairie grasses, deciduous forest soils, wet marsh soils, and mesa sands (5, 7, 8). Dicty even inhabit cold environments such as those found in Alaska, miles north of the arctic circle, where frost covers the ground more than 80% of the year (9). This is particularly relevant to potato soft rot since most potatoes are stored at temperatures around 10°C. The causative agents of potato soft rot, Dickeya and Pectobacterium species, can proliferate and cause disease even at these low temperatures.

In the presence of an abundant bacterial food source, Dicty exist as single-celled phagocytic organisms that chase and engulf their bacterial prey. When food becomes limited, these single-celled organisms merge together to ultimately form a multicellular structure called a sporangium (Figure 1) (5). The sporangium may contain thousands of spores that remain dormant until conditions are appropriate for the next generation of Dicty to emerge and begin feeding on surrounding bacteria. Dormant spores are particularly resilient to environmental challenges such as temperature extremes, desiccation, and decades of storage when freeze-dried (5).

The ability of Dicty to degrade bacterial biofilms, feed on a diverse array of bacterial pathogens, and produce large quantities of resilient spores allows for many fundamentally novel, highly innovative applications (10, 11). AmebaGone has licensing rights to a collection of a few thousands Dicty strains collected from a few dozen countries. We are leveraging the diversity of this collection to identify Dicty strains that can be used to treat major bacterial diseases in agriculture. We have identified a few Dicty strains that are able to feed on biofilms of both Dickeya dianthicola (Dd) and Pectobacterium carotovorum subsp. carotovorum (Pcc) on potato surfaces at temperatures relevant to storage conditions. Ongoing investigations are aimed at testing whether soft rot severity is significantly reduced when seed potatoes inoculated with Dd or Pcc are treated with these Dicty strains (Figure 2).

AmebaGone’s solution to soft rot of potatoes and sugar beets

Freeze-dried Dicty spores remain viable for up to 80 years when stored at a wide range of temperatures and are readily rehydrated with water (M. Filutowicz and K. Borys unpublished data). We envision the prophylactic treatment will consist of semi-dormant Dicty spores that are reconstituted in water before being applied to the surface of storage crops in the form of a fine mist. Early efforts at AmebaGone are focused on developing a treatment for potato soft rot on tubers during storage (Figure 3). Interestingly, Pectobacterium carotovorum subsp. betavasculorum (Pcb) is responsible for bacterial soft rot in sugar beets, which presents AmebaGone with a great opportunity to target this devastating disease as well (12). Pending the results from trials on potatoes in storage, AmebaGone will conduct storage trials on sugar beets exposed to Pcb.

The competition

The only products currently available to mitigate the effects of soft rot in storage use the broad-spectrum disinfectants chlorine dioxide, hydrogen peroxide, and ozone as their active ingredient (Figure 4). While they have shown some potential to prevent soft rot from developing and spreading in storage, they are ineffective once disease has developed. These products also require frequent or continuous applications that can be challenging to administer in storage facilities containing millions of pounds of potatoes layered upon each other. All of these active ingredients are inactivated by residual soil on the potato surface. This is particularly problematic as potatoes are not washed prior to storage and potatoes harvested from muddy portions of a field are particularly prone to soft rot. Finally, none of these chemicals are approved for use on seed potatoes.

Our vision

For all stages of the potato and sugar beet supply chain that require safe and effective protection from soft rot, AmebaGone’s Dicty-based treatment is an all-natural solution that is non-toxic and environmentally safe. Our product is an entirely new biological approach that uses the natural predatory power of Dicty instead of harsh chemical treatments. Dicty can penetrate and eliminate bacterial biofilms, which pose a challenge for even the strongest disinfectants, while leaving plant surfaces completely unharmed (13). Dicty are hardy, environmentally tolerant, and self-propagating, making them an excellent choice for sustainable agriculture. 

References

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10 Raper KB. Dictyostelium discoideum, a new species of slime mold from decaying forest leaves. J Agr Res. 1935;50:135-47.

11 Raper KB. Growth and development of Dictyostelium discoideum with bacterial associates. J Agr Res. 1937;55:289-316.

12 Ozturk M, Eroglu Z, Soylu S. First report of Pectobacterium betavasculorum associated with bacterial vascular necrosis and root rot disease of sugar beet in Turkey. New Disease Reports. 2019;39(20). doi: http://dx.doi.org/10.5197/j.2044-0588.2019.039.020.

13  Sanders D, Borys KD, Kisa F, Rakowski SA, Lozano M, Filutowicz M. Multiple Dictyostelid Species Destroy Biofilms of Klebsiella oxytoca and Other Gram Negative Species. Protist. 2017;168(3):311-25. Epub 2017/04/12. doi: 10.1016/j.protis.2017.04.001. PubMed PMID: 28499132.