Citrus Greening Still Throwing New Curveballs at Scientists and Growers
November 25, 2022 Citrus greening (aka, HLB) has been a major pain for Florida growers and a puzzle for scientists from more than 15 years. Despite all the time and money put into fighting and studying the disease, it’s still revealing itself. And at the heart of the situation is a complex exchange between the tree and the CLas (Candidatus Liberibacter asiaticus) bacterium.
In a new paper authored by University of Florida researchers and published in the journal Plant Pathology, discoveries of new ways the bacteria interact with a citrus tree’s natural defenses are highlighted. The findings shed light on the complexity of the disease path within the tree and what it means for scientists looking to mitigate its deadly impact.
Amit Levy, UF/IFAS Assistant Professor of Plant Pathology, and post-doctoral researcher Chiara Bernardini discovered how the bacteria and citrus tree engage in a “back-and-forth” reactionary relationship. He and others showed that once infected with the CLas bacterium, the tree’s defense system starts to generate callose in the phloem, a material that essentially “plugs” the phloem and generates something called “reactive oxygen species” or ROS.
In plants, ROS is involved in a plant’s defense systems and impacts a plant’s tolerance to various types of stress. Presence of a pathogen like CLas can increase ROS production to a negative effect and eventually cause cell death.
Levy’s research concluded that the CLas bacteria responded to the generation of callose and reactive oxygen species by actually reducing them, allowing bacteria to once again replicate and transport throughout the tree.
This back-and-forth repetitive relationship of callose plugging and reactive oxygen species accumulation — and then their elimination by CLas — is a complicated one and replicates an immune response competition between hosts and pathogens found in many other diseases.
Citrus varieties that maintain a fine balance between callose and reactive oxygen species generation and then elimination without either side gaining “control” may be more inclined to continue to produce fruit over many years, according to the scientists.
“This research demonstrates the complicated, intertwined relationship between the HLB bacteria and the tree’s immune defense system.” Levy says. “The fact that CLas developed mechanisms to suppress the immunity tells us that the plant immunity is critical to stop the bacteria. The HLB disease is about both the pathogen and the immune response, and their interaction. It is a fine balance.”
To read the entire research paper in detail, visit academic.oup.com.
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