- Author: Ben Faber
Translating the science of managing HLB
Your resource for learning about existing and new tools being developed by researchers to protect and manage citrus from the causative agent of huanglongbing (HLB).
Research Snapshots
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See how scientists are working to control HLB by reading short snapshots about their research. Topics include:
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Click on the links below to explore the snapshots!
Questions?
We have the answers!
Growers and end-user audiences can use this PowerPoint presentation which has been tailored for their use. It provides general background on the genetics that are used to modify crops and information on state and national regulatory approaches and consumer attitudes.
Examples of the questions that are addressed:
- What are the general processes of genetically modifying plants and insects?
- When and where did huanglongbing (HLB) come from?
- What are some genetic approaches to controlling HLB?
- What is consumer response to engineered crops and foods?
- Author: Brad Buck, UF/IFAS
LAKE ALFRED, Fla. — Sometimes in science, a new perspective brings an “a ha!” moment. That's what one senior researcher at the University of Florida Institute of Food and Agricultural Sciences believes happened with his latest research on Huanglongbing (HLB), or citrus greening.
HLB is worldwide, devastating citrus disease caused by Candidatus Liberibacter asiaticus (CLas), a bacterium that settles into the tree's phloem — its interior vascular system — eventually killing the tree. Since first found in Florida in 2005, it has infected virtually every grove in Florida and cost the citrus industry billions of dollars.
UF/IFAS' Nian Wang's most recent research describes in detail how HLB causes damage to citrus trees and presents the case that HLB is a pathogen-triggered immune disease. A pathogen-triggered immune disease is a disease that results from the activation of an organism's immune cells fighting a pathogen (a virus, bacteria, or parasite) that invades an organism, in this case, the citrus plant.
This is the first time that this explanation of HLB symptoms as pathogen-triggered immune responses has been presented and defended. Seeing HLB in this new context may mean finding new solutions to the disease, faster. Pathogen-triggered immune diseases have not been reported in the world of plants that includes over 250,000 species but are common in humans.
Scientists don't fully understand how the pathogen that causes HLB damages infected citrus plants because it has not been cultured in artificial media. With this new evidence that supports a premise that HLB is an immune-mediated disease, researchers can see new light on how to manage HLB.
Through his research Wang, has shown HLB infection stimulates systemic and chronic immune responses in phloem tissue, especially overproduction of reactive oxygen species (ROS), which are part of the plant's immune response. Chronic and excessive ROS production is responsible for systemic cell death of phloem tissues, which in turn causes HLB symptoms. This supports the hypothesis that HLB is an immune-mediated disease.
Antioxidants and immunoregulators are commonly used to treat human immune-mediated diseases. They halt or reduce the process that results in cell death. In citrus, the researchers tested whether growth hormones like gibberellin acid (GA) and antioxidants (uric acid and rutin) could impact cell death triggered by the infection and, therefore, block or reduce HLB symptoms.
The researchers found the GA and uric acid had an encouraging positive impact on infected trees. This has also been supported by other research in process at the UF/IFAS Citrus Research and Education Center.
“Our findings allow us to control HLB by mitigating ROS with integrated horticultural measures, genetic improvements of citrus varieties with antioxidant enzymes, generating non-transgenic HLB resistant/tolerant citrus varieties by editing key genes required for CLas-triggered ROS production, and using CTV-mediated expression of antioxidant enzymes and silencing of key genes required for CLas-triggered ROS production,” said Wang, a professor of microbiology and cell science at UF/IFAS.
Wang's research also tested existing grove management practices that integrate antioxidants, micronutrients (activating antioxidant enzymes), gibberellin (mitigating ROS, regulating immune response, and promoting new growth) and optimized fertilization and irrigation (reducing ROS production) that are available in Florida. All the products tested, except the antioxidants, have already been registered for citrus production, thus can be easily adopted by citrus growers.
Wang's research has also identified the RBOHD gene to be the main producer of CLas-triggered ROS. With this, the researchers can now “edit (using CRISPR technology)” or “silence” RBOHD or specific receptor gene(s) responsible for RBOHD activation and the resulting cell death of phloem tissues and HLB symptom development. It is expected these approaches will allow economic and efficient citrus production in HLB endemic citrus production regions such as Florida and Texas and provide the long-term solution against HLB for all citrus production regions of U.S. (including California) in case that HLB becomes endemic in the future.
Read the whole story:
Key Points About Plant "Immunity"
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Unlike vertebrates, plants do not have an adaptive immune system. Nonetheless, plants can launch specific, self-tolerant immune responses and establish immune memory.
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To promote virulence, pathogens inject effector molecules that target conserved immune signalling hubs into the plant cell. In response, plants have evolved resistance (R) proteins that detect effector-induced perturbations in these hubs, providing the potential to specifically recognize a large number of pathogens with similar infection strategies through a smaller number of R proteins.
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Intraspecific and interspecific plant crosses suggest that autoimmunity can arise from self-reacting R proteins, illustrating the threat of uncontrolled R protein activity. Dynamic transcriptional and post-transcriptional regulation of R protein levels is thought to minimize the risk of autoimmunity in plants.
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Pathogen-infected tissues generate a mobile immune signal consisting of multiple proteins as well as lipid-derived and hormone-like molecules. These signal molecules are transported to systemic tissues, where they induce systemic acquired resistance (SAR). SAR is associated with the systemic reprogramming of thousands of genes to prioritize immune responses over routine cellular requirements.
- Author: Ben Faber
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New ‘Candidatus Liberibacter' Pathosystems Focus Issue from APS (American Phytopathological Society) |
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Read the great review of the bacteria that causes Citrus Huanglongbing and then the abstracts of the articles in this edition of Phytopathology. The review itself is pretty comprehensive, however you can't read the full articles contained in the editon without paying. But this gives you an idea of the extent of work being done, even though the language may be quite technical. The 18 articles in this Phytopathology Focus Issue showcase the enormous research efforts made by the scientific community, giving rise to major advances and achievements in a short time often through multidisciplinary approaches applied to the bacterium, psyllid vector, and plant host. Preview two editors' pick below or see the full Focus Issue. |
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Transcriptome Profiling of ‘Candidatus Liberibacter asiaticus' in Citrus and Psyllids |
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Alves et al. explored Huanglongbing (HLB) presence and absence over 13 years in citrus orchards in Brazil and compared two hierarchical Bayesian modeling approaches to link climatic factors to the spatial distribution of HLB prevalence. They found an inverse relationship between HLB prevalence and mean temperature during the dry season, but wind speed, rainfall, and proximity of other HLB contributed to HLB prevalence. The results further our understanding of environmental factors associated with disease distribution and spread and assists policymakers in defining regions at risk of HLB outbreaks to help guide monitoring strategies that mitigate further spread of HLB. |
HLB symptoms
- Author: Ben Faber
Citrus greening disease can infect an entire tree weeks before symptoms appear
Ashley Carlin
American Phytopathological Society
For the first time ever, scientists have been able to measure the speed of a bacterium that causes the incurable citrus greening disease. Citrus greening disease (also known as Huanglongbing) is the most devastating citrus disease in the world. Afflicted trees grow yellow leaves and low-quality fruit and eventually stop producing altogether, resulting in enormous economic losses to farmers.
Small insects carry the causal bacteria, Candidatus Liberibacter asiaticus (CLas), and inject it when they feed on a tree's sap. CLas also relies on this sap to grow and spread throughout the tree. Using a new statistical modeling analysis and measurement approach, plant pathologists were able to follow CLas on its journey through a tree.
“We found that CLas can move at average speed of 2.9 to 3.8 cm per day. At these speeds a tree that is 3 meters in height will be fully colonized by CLas in around 80 to 100 days, and this is faster than the symptoms appear, which generally takes at least 4 months,” explained Silvio A. Lopes, a plant pathologist based at Fundecitrus, research institution maintained by citrus growers of the State of Sao Paulo in Brazil.
In other words, by the time growers see the symptoms of citrus greening disease, the tree has already been infected from the roots on up, which explains why removing symptomatic branches isn't enough to cure a diseased tree.
Lopes and colleagues also studied the impact of temperature on the speed of colonization. They already knew that CLas does not multiply well in hot or cold environments—but now they have more specific numbers.
“We estimated that 25.7°C (78°F) was the best condition for CLas to move from one side to the other side of the tree,” said Lopes. This is the first time impact of temperature on plant colonization of CLas has been experimentally demonstrated. “The grower can use this information to select areas less risky for planting citrus trees.”
- Author: Ben Faber
Huanglongbing (HLB) and its causal agent Candidatus Liberibacter asiaticus (CLas) are a serious problem for the US citrus industry, with Florida and Texas already heavily affected, and California at an early stage. Rootstock cultivars with genetic tolerance to HLB improve tree health, fruit production, and fruit quality in HLB-affected orchards, but even the best rootstock available suffers large yield losses when infected. Further gains in tolerance or resistance to HLB are urgently needed to sustain the industry. Field trees in Florida are now all infected with CLas, providing an opportunity to screen thousands of trees for overall performance in environments with high disease pressure. Previous work to create and evaluate over 300 new rootstock hybrids across three HLB- endemic regions in Florida in replicated sweet orange field trials will be leveraged to deliver the best performing HLB-tolerant rootstocks for commercial release, including expanded collection of performance information over the next two years (Objective 1). Top performing rootstocks will be further screened for resistance to Phytophthora and other relevant soil-borne pathogens to ensure adoption in HLB-endemic and HLB-threatened regions. This extensive set of phenotypic data will be integrated with genetic information to identify the genetic control of HLB-tolerance and other important traits, enabling rapid selection of superior tolerant hybrid rootstocks in future breeding cycles (Objective 2). Commercial testing and release of rootstocks will occur in close consultation with industry members and will be disseminated to stakeholders at all levels through virtual and in- person seminars and large industry events.
PERFORMANCE OF 300 HYBRIDS IN ESTABLISHED TRIALS TO MAP HUANGLONGBING TOLERANCE/RESISTANCE GENES AND RELEASE SUPERIOR NEW ROOTSTOCKS
Seymour, D. K.; Deng, ZH, .; Rolshausen, PH, .; Bowman, KI, D.; Albrecht, UT, .
