A $1.5 million emergency grant is enabling UC Riverside scientists to find plants impervious to a disease threatening America's citrus fruit supply.
Citrus Greening Disease — also known as Huanglongbing, or HLB — results in fruit that is bitter and worthless. It has crippled Florida's citrus industry and has already been detected in California, which grows 80% of America's fresh citrus. An estimated 267,000 acres of Golden State oranges, lemons, grapefruits, and mandarins are at stake.
For these reasons, the National Institute of Food and Agriculture is supporting scientists at UCR, the University of Florida, and the U.S. Department of Agriculture's Agricultural Research Service in their search for plants with natural tolerance to HLB.
“If you find a disease affecting your crops, a good first step is to look for plants that are able to grow and produce despite infection,” said UCR geneticist Danelle Seymour. “Then you can start to identify the genetic basis of the disease tolerance and make sure the next generation of plants includes these genes.”
Following this recipe, Seymour and UCR plant pathologist Philippe Rolshausen will examine a set of 350 citrus hybrids developed and grown by project collaborators in Florida. All trees in the set are already infected with HLB, yet they live longer, are healthier, and yield more fruit than their infected relatives.
While there are a number of projects searching for different solutions to the threat of HLB, this project is different because the plants being tested were all grown in an environment endemic to the disease. Additionally, the number of plants they're able to test is unusually large.
“The environment in which these plants were grown means we can be confident that these rootstocks will enhance tree health and yield in HLB-affected areas,” Seymour said. “Also, because our data set is so large, we've got the opportunity to identify plants with levels of tolerance that exceed current commercial varieties.”
In addition to searching for parts of the hearty hybrids' genomes responsible for their tolerance to HLB, scientists will also be checking for plants that have resistance to other pathogens that are already in California.
Citrus in the state is also threatened by nematodes that chew up roots, preventing plants from taking up nutrients, and by phytophthora, a type of water mold that causes rotting roots.
By searching not only for a solution to the looming threat of HLB but also to problems that have already taken root in California, scientists are hoping to ensure that citrus won't need to be imported from HLB-free countries and costs stay low for both local growers and consumers.
“This way, we're making sure the next generation of rootstocks will include the right genes and that we're being as efficient as possible in our breeding practices,” Seymour said.
Citrus greening, also called Huanglongbing (HLB), is devastating the citrus industry. Florida alone has experienced a 50 to 75 percent reduction in citrus production. There are no resistant varieties of citrus available and limited disease control measures.
Some scientists think it is possible that orange juice could one day become as expensive and rare as caviar. In an effort to prevent this, three plant pathologists at the University of California-Berkeley and United States Department of Agriculture conducted research into ways to boost citrus immunity and protect the valuable fruit against citrus greening.
Because the bacteria that causes citrus greening cannot be grown in a lab, scientists have to find novel ways to conduct experiments. The University of California-Berkeley/USDA team looked at many different strains of the bacteria that cause citrus greening to see if they could identify peptides (a compound of two or more amino acids) that would trigger immune responses.
"This was a long list, so we narrowed it down by selecting small peptides that were a bit different in their peptide sequence, which might imply that the bacterium had made those sequence changes so that they wouldn't be recognized by the plant immune system," explained Jennifer D. Lewis, group leader of the research team. "Then we further narrowed that list to peptides from strains that caused disease in citrus."
Through this research, they showed that two peptides could trigger immune responses in multiple plant species, including citrus. These peptides may play a role in preventing or reducing yield loss from citrus greening.
According to Lewis, "We thought it was particularly interesting that some of the peptides predicted to elicit a response, could actually trigger immune responses in multiple plant species. This suggests that the immune response to these peptides is conserved across species."
ITHACA, NY, July 22, 2019 - Protecting crops from pests and pathogens without using toxic pesticides has been a longtime goal of farmers. Researchers at Boyce Thompson Institute have found that compounds from an unlikely source - microscopic soil roundworms - could achieve this aim.
As described in research published in the May 2019 issue of Journal of Phytopathology, these compounds helped protect major crops from various pathogens, and thus have potential to save billions of dollars and increase agricultural sustainability around the world.
Led by BTI Senior Research Associate Murli Manohar, a team around Professors Daniel Klessig and Frank Schroeder investigated the effects of a roundworm metabolite called ascr#18 on plant health.
Ascr#18 is a member of the ascaroside family of pheromones, which are produced by many soil-dwelling species of roundworms for chemical communication.
The researchers treated soybean (Glycine max), rice (Oryza sativa), wheat (Triticum aestivum) and maize (Zea mays) plants with small amounts of ascr#18, and then infected the plants with a virus, bacteria, fungus or oocmycete.
When examined several days later, the ascr#18-treated plants were significantly more resistant to the pathogens compared with untreated plants.
"Plant roots are constantly exposed to roundworms in the soil, so it makes sense that plants have evolved to sense the pest and prime their immune systems in anticipation of being attacked," says Schroeder.
Because they boost plants' immune systems instead of killing pests and pathogens, ascarosides are not pesticides. As a result, they are likely to be much safer than many current means of pest and pathogen control.
"Ascarosides are natural compounds that appear to be safe to plants, animals, humans and the environment," says Klessig. "I believe they could thus provide plants more environmentally friendly protection against pests and pathogens."
In previous work, Klessig and Schroeder demonstrated that ascr#18 and other ascarosides increased resistance against pest and pathogens in tomato, potato, barley and Arabidopsis.
"By expanding the work to major crops, and concentrating on their most significant pathogens, this study establishes the potential for ascarosides to enhance agriculture production worldwide," says Klessig.
Indeed, rice is the world's most important staple food for nearly half of the global population. Ascr#18 provided protection against Xanthomonas oryzae pv. oryzae, a bacterium that causes yield losses of 10-50% in Asian countries.
Wheat is close behind rice in importance as a food staple, and ascr#18 protected it against Zymoseptoria tritici, a fungus that is one of the most severe foliar diseases of the crop.
Maize is the most widely grown grain crop throughout the Americas with great importance for food, biofuel and animal feed. Ascr#18 provided protection against Cochliobolus heterostrophus, a fungal pathogen that causes southern corn leaf blight.
Soybean is a major high-protein, oil-rich seed crop used as a food source for humans and animals. Ascr#18 protected soybeans against Phytophthora sojae, an oomycete that can kill infected plants in days, as well as the bacterial pathogen Pseudomonas syringae pv glycinea and Soybean Mosaic Virus.
Extremely small concentrations of ascarosides are sufficient to provide plants with resistance against pathogens. Interestingly, the optimal concentration appears to be dependent on the plant species and not the pathogen.
The researchers believe the reason that different plant species have different optimal dosages is likely related to the plant cell's receptors for ascr#18. Different plant species may express different amounts of ascr#18 receptors, and receptors may have varying affinities for ascarosides. Such differences would affect the amount of ascr#18 needed to trigger the plant's immune systems.
The group is now working to determine the molecular mechanisms of how ascarosides prime the plant's immune systems.
These discoveries are being commercialized by a BTI and Cornell-based startup company, Ascribe Bioscience, as a family of crop protection products named PhytalixTM.
"This work is a great example of how the Institute is leveraging our technology through new start-up ventures, an important strategic initiative at BTI," says Paul Debbie, BTI's Director of New Business Development. "The Institute is proud of the opportunity to develop innovative technology in partnership with a new company that is having a positive economic impact here in our local community and for New York State."
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In addition to their BTI positions, Klessig is an adjunct professor in Cornell University's Department of Plant Pathology and Plant-Microbe Biology and Schroeder is a professor in Cornell's Department of Chemistry and Chemical Biology.
Collaborators included researchers at Cornell, University of Kentucky, Justus Liebig University in Germany, University of California, Davis, and Colorado State University.
The research was partially funded by the US Department of Agriculture (USDA) National Institute for Food and Agriculture, the USDA Agricultural Food and Research Initiative, the Colorado Agricultural Experiment Station, the Kentucky Soybean Promotion Board, and the German Minister of Education and Research.
Reference: Klessig, D.F., Manohar, M., Baby, S., Koch, A., Danquah, W.B., Luna, E., Park, H.J., Kolkman, J.M., Turgeon, B.G., Nelson, R. and Leach, J.E., 2019. Nematode ascaroside enhances resistance in a broad spectrum of plant-pathogen systems. Journal of Phytopathology, 167(5), pp.265-272.
There has been a good overall discussion of herbicide resistance found in plants and how they can affect orchard management. Check out this presentation by UC Cooperative Extension Weedologist, Brad Hanson, in the "past Webinars" section:
Rely too much on any one herbicide and you end up with weeds that will resist its effects—and that's just what is happening now with glyphosate (Roundup®). See how you can increase effectiveness by diversifying your weed-management strategies.
Glyphosate-resistant crops made farming a lot easier when they first came out, but many weeds have developed resistance. Herbicide-tolerant crops will only work as part of a more comprehensive, Integrated Pest Management plan.
Managing Glyphosate-Resistant Weeds in Orchard Crops
Description: One hour webinar about glyphosate-resistant weed management in orchards, delivered by Dr. Brad Hanson. One CEU (other) from the DPR is approved.
Time: Apr 24, 2019 3:00 PM in Pacific Time (US and Canada)
Recorded version will be published on UC IPM YouTube channel about a week after the webinar.
Cooperative Extension Weed Specialist @UC ANR / UC Davis
Dr. Hanson, an associate Cooperative Extension specialist in the Department of Plant Sciences, specializes in weed management in tree and vine cropping systems. Hanson completed his Ph.D. in plant sciences (with an emphasis in weed science) at the University of Idaho and worked as a research agronomist with the USDA-ARS before joining the UC Davis faculty in 2009. His research interests include weeds, weed control, herbicide resistance, weed biology, invasive plants, pest control in fruit and nut crops and other agricultural production systems.
University of California Cooperative Extension Ventura County 669 County Square Drive, Suite 100 Ventura, CA 93003 Phone: 805.645.1451 Fax: 805.645.1474
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