Protecting the Vidalia Onion

The United States is one of the leading producers of Onion in the world.

Within the U.S., Georgia is famously known for its Vidalia onion industry. These onions were first grown in Toombs County, South Georgia. Their unique flat shape with a sweet taste led to immediate popularity among growers and consumers. The distinct quality of this onion is due to the low amount of sulfur in the soil which cuts down the acidity and pungency making them sweeter than most other varieties. The continued growth of the industry and Vidalia’s increasing popularity did create branding problems where onions brought from outside were bagged and sold as Vidalias. The Federal Market Order 955 in 1989 defined the growing regions and mandated the growers to register and use specific varieties that provided national protection to the industry. This branding support and advancements in storage conditions oversaw the further growth of the industry which accounts for a staggering $160 million farm gate value and is now spread across 12,000 acres in 20 counties of South Georgia. My research, as a plant pathology Ph.D. candidate at the University of Georgia, aims to help protect this industry from deadly bacterial diseases.

The sweet Vidalia is under threat.

Diseases caused by pathogens are causing severe losses in seed bed, production and onion storage conditions. Of the many pathogen groups, a bacterial genus called Burkholderia is a huge concern. Multiple species within Burkholderia can cause disease to onions. Among them is a species group Burkholderia gladioli pv. alliicola (Bga) which causes a disease called onion rot/slippery skin. This is an important field and storage disease of Vidalia and other onions grown in the United States. Although the other two closely related species Burkholderia cepacia and Burkholderia orbicola are more often associated with onion production, the economic impact of Bga is highly significant provided its ability to cause both foliar (leaf) and bulb symptoms. Despite being discovered in the 1940s as an onion bulb pathogen, the disease-causing mechanisms of this soil-borne bacterium are still not well understood.

Managing Bga is difficult because it can survive in the soil for a long period without its host onion. Limited available management options and the relatively low effectiveness of chemical control means that developing disease-resistant onion varieties could be the only viable long-term strategy against these pathogens. Understanding the disease-causing strategies of Bga is a fundamental step in developing these disease-resistant varieties.

This is where my research comes into play. We go deep to unravel the genetic basis of why this bacterium is a major cause of concern for Vidalia and onion farmers in the U.S. and dissect what different strategies the bacteria utilize to kill onion cells.

Our strategy is to look for predicted gene regions in the bacterium with the potential to make onions sick and remove or mutate their components. The modified bacterium is then checked to see if it still causes disease in onions. There are at least 7000 genes in many Burkholderia species. The usual suspects were selected first, meaning the genes in another closely related bacteria that were previously known to help kill their host. For example, one of the gene targets selected was a bacterial type III secretion system. The structure of type III secretion system looks like a syringe with a needle. Using this syringe and needle, bacteria inject proteins that eventually kill the plant cells. The gene that makes this syringe in Bga was then deleted or modified. To our great surprise, the bacteria could still kill onion cells despite not having a syringe. Similarly, many other suspects/targets were removed or modified but the bacterium was still able to kill onion cells.

So, what is that X-factor in bacteria without which the onions can’t be killed?

What looked like a straightforward question has now turned out to be a fascinating mystery. The mechanism bacteria utilize to kill onions could be more complicated than we’d first thought.

Our current work explores bacterial pathogens’ strategies against onion’s natural defense compounds. Bacteria must cross multiple levels of physical and plant defense barriers to successfully gain access to plant nutrients and reproduce. During this process, bacteria are exposed to toxic sulfur-defensive compounds produced by dead onion cells. We have likely all experienced this onion defense strategy:  the compounds are produced from the same precursors as another sulfur volatile compound that makes us cry when cutting an onion. To gain further access to nutrients and reproduce, bacteria must first withstand the effect of these toxic defensive onion compounds. Our lab found a gene cluster that helps a distantly related bacteria to tolerate the toxic effect of these defensive onion compounds.

Our next project looks at whether multiple members of the Burkholderia group utilize this similar strategy to tolerate the effect of onion defensive compounds.  One question is if the presence of this gene cluster enhances the ability of different Burkholderia members to cause disease in onions. Since our findings suggest that different distantly related bacteria utilize similar strategies to tolerate onion defensive compounds, we could potentially modify some other non-edible yet economically important crops like cotton to produce similar defensive sulfur compounds, minimizing crop loss from a wide range of bacterial pathogens.

Acknowledgment

The FFAR Fellows Program has been a wonderful experience helping me develop my skillsets in multiple facets of personal and professional development. Many thanks to my advisor Dr. Brian Kvitko for his support, The University of Georgia College of Agriculture and Environmental Sciences Office of Research for financial support, and Dr. Rebecca Dunning, Director of the FFAR Fellows Program, for her tireless work in preparing future leaders for food and agriculture.