- Author: Jules Bernstein
UC Riverside scientists are betting an ancient solution will solve citrus growers' biggest problem by breeding new fruits with natural resistance to a deadly tree disease.
New hybrid citrus fruit bred for disease resistance and flavor. (Chandrika Ramadugu/UCR)
The hybrid fruits will ideally share the best of their parents' attributes: the tastiness of the best citrus, and the resistance to Huanglongbing, or HLB, displayed by some Australian relatives of citrus.
There is no truly effective commercial treatment for HLB, also called citrus greening disease, which has destroyed orchards worldwide. The disease has already been detected in California, where 80 percent of the country's fresh citrus is grown. However, it has not yet been detected in a commercial grove.
To prevent that from happening, the National Institute of Food and Agriculture has awarded a UC Riverside-led research team $4.67 million. Chandrika Ramadugu, a UCR botanist leading the project, helped identify microcitrus varieties with natural resistance to HLB about eight years ago.
Cross section of a hybrid fruit bred for this project. (Chandrika Ramadugu/UCR)
“HLB is caused by bacteria, so many people are trying to control it with antimicrobial sprays,” Ramadugu said. “We want to incorporate resistance into the citrus trees themselves through breeding, to provide a more sustainable solution.”
Part of the challenge with this approach to solving the HLB problem is that it's possible to breed hybrids that are resistant to the disease but don't taste good, Ramadugu said. “Hence the need to generate a lot of hybrids and screen them for the ones that will be most ideal for the citrus industry.”
Microcitrus, such as the Australian finger lime, tends to have a sharper, more bitter taste than its relative citrus fruits, like oranges. The perfect cross will have just the right mix of genes to give it sweetness and HLB resistance.
Ramadugu's team includes collaborators from Texas A&M University, the University of Florida, Washington State University and the U.S. Department of Agriculture, as well as scientists from UC Riverside's Department of Botany and Plant Sciences.
Breeding project team members from UC Riverside's Department of Botany and Plant Sciences. (Chandrika Ramadugu/UCR)
Currently, the team is studying differences in the genetic makeup of the hybrids they've already bred. Analyzing the new plants' DNA will help the team see whether enough disease resistance has been bred into the fruit, but not so much that the flavor is compromised.
Another challenge with breeding is the time it takes for new citrus varieties to flower naturally, which can be several years. With the help of Sean Cutler, UCR professor of plant cell biology, the team is hoping to accelerate the time it takes for the hybrid plants to bear fruit in a greenhouse.
This way the hybrids can be analyzed for taste much sooner. Clones of the best hybrid plants will then be grown in Florida and Texas field trials.
UC Riverside scientists are using a variety of approaches to fight HLB. While some hope that altering soil and root bacteria will improve plants' immunity to the disease, others are trying to improve HLB resistance by tweaking citrus metabolism, or by using an antibacterial peptide to clear HLB from an infected plant.
The fruit produced through Ramadugu's method will appeal to many consumers because it will not have genes introduced into them by scientists. Breeding has been done for thousands of years to improve crops and is considered a more natural practice.
Additionally, Ramadugu says she's excited about her approach because it will ultimately produce a product useful for growers and consumers.
Researchers at the California Data Analysis and Tactical Operations Center (DATOC) have analyzed Asian citrus psyllid (ACP) trapping data along major transportation routes before and after tarping regulations for bulk citrus shipments were enacted. The purpose was to determine the effectiveness of the policy.
DATOC is an independent group of scientists sponsored by the Citrus Research Board and the California Citrus Pest and Disease Prevention Program. The group was formed in 2016 to create and amend tactical response plans for huanglongbing (HLB) suppression and management for California citrus.
DATOC found a significant reduction in the rate of ACP finds throughout the San Joaquin Valley (SJV) after tarping regulations went into effect. The SJV contains more than 70% of California's packinghouses. Coastal and Southern California counties ship more than 63 million pounds of bulk citrus into the SJV annually for processing.
In years past, ACP populations have soared as they presumably “hitchhiked” on trucks that weren't properly covered, coming from Southern California into the SJV and threatening the livelihood of commercial groves throughout California along the way. However, after the California Department of Food and Agriculture (CDFA) required tarping in 2017, DATOC data shows that tarping has effectively reduced ACP movement.
While these results are encouraging, scientists say that growers must continue to remain vigilant. In a recent letter, Citrus Pest & Disease Prevention Committee (CPDPC) chairman Jim Gorden stated that ACP populations are expected to “flare up” occasionally, such as the late 2020 ACP detections in Kern, Madera, San Luis Obispo, Santa Barbara, Santa Clara, Tulare, Contra Costa and other counties.
The CPDPC emphasizes that growers, packers, transporters and other stakeholders must continue to stay on top of this elusive ACP pest and the dangerous HLB disease it spreads. The upfront cost to manage ACP is much less than the potential hit to the citrus industry if HLB spreads throughout the state.
In order to move bulk citrus from an ACP regional quarantine zone or a HLB quarantine area under the terms of the permit(s), growers, grove managers, haulers and harvesters must comply with the CDFA's transporting requirement as detailed in their order. Get specific details here.
Source: Citrus Pest & Disease Prevention Program
Science for Citrus Health spring webinar series
Management of Asian Citrus Psyllid (ACP) and Huanglongbing (HLB) in the field – March 11, 10 a.m.–12 p.m.
- Relation of ACP density and tree stress: what is the threshold to take control measures? (Dr. Lukasz Stelinski, Professor, Entomology and Nematology, University of Florida)
- Biological control of ACP using predators and parasitoids (Dr. Jawwad Qureshi, Assistant Professor, Entomology and Nematology, University of Florida)
- Importance of citrus phenology-based sprays for ACP control and Implementation of ACP area-wide management in Texas (Dr. Mamoudou Sétamou, Professor, Citrus Entomology, Texas A&M University)
- Q&A and panel discussion.
1.5 DPR and CCA CEUs were requested. Register at https://ucanr.zoom.us/webinar/register/WN_I7KPgo3STaqwKwJXJQ9Fkw
Author: Jules Bernstein, UCR,Senior Public Information Officer
New research affirms a unique peptide found in an Australian plant can destroy the No. 1 killer of citrus trees worldwide and help prevent infection.
Huanglongbing, HLB, or citrus greening has multiple names, but one ultimate result: bitter and worthless citrus fruits. It has wiped out citrus orchards across the globe, causing billions in annual production losses.
All commercially important citrus varieties are susceptible to it, and there is no effective tool to treat HLB-positive trees, or to prevent new infections.
However, new UC Riverside research shows that a naturally occurring peptide found in HLB-tolerant citrus relatives, such as Australian finger lime, can not only kill the bacteria that causes the disease, it can also activate the plant's own immune system to inhibit new HLB infection. Few treatments can do both.
Research demonstrating the effectiveness of the peptide in greenhouse experiments has just been published in the Proceedings of the National Academy of Sciences.
The disease is caused by a bacterium called CLas that is transmitted to trees by a flying insect. One of the most effective ways to treat it may be through the use of this antimicrobial peptide found in Australian finger lime, a fruit that is a close relative of citrus plants.
"The peptide's corkscrew-like helix structure can quickly puncture the bacterium, causing it to leak fluid and die within half an hour, much faster than antibiotics," explained Hailing Jin, the UCR geneticist who led the research.
When the research team injected the peptide into plants already sick with HLB, the plants survived and grew healthy new shoots. Infected plants that went untreated became sicker and some eventually died.
Arrows point to areas of fluid leakage from the bacterial cell after treatment with the antimicrobial peptide. (Hailing Jin/UCR)
"The treated trees had very low bacteria counts, and one had no detectable bacteria anymore," Jin said. "This shows the peptide can rescue infected plants, which is important as so many trees are already positive."
The team also tested applying the peptide by spraying it. For this experiment, researchers took healthy sweet orange trees and infected them with HLB-positive citrus psyllids -- the insect that transmits CLas.
After spraying at regular intervals, only three of 10 treated trees tested positive for the disease, and none of them died. By comparison, nine of 10 untreated trees became positive, and four of them died.
In addition to its efficacy against the bacterium, the stable anti-microbial peptide, or SAMP, offers a number of benefits over current control methods. For one, as the name implies, it remains stable and active even when used in 130-degree heat, unlike most antibiotic sprays that are heat sensitive -- an important attribute for citrus orchards in hot climates like Florida and parts of California.
In addition, the peptide is much safer for the environment than other synthetic treatments. "Because it's in the finger lime fruit, people have eaten this peptide for hundreds of years," Jin said.
Researchers also identified that one half of the peptide's helix structure is responsible for most of its antimicrobial activity. Since it is only necessary to synthesize half the peptide, this is likely to reduce the cost of large-scale manufacturing.
The SAMP technology has already been licensed by Invaio Sciences, whose proprietary injection technology will further enhance the treatment.
Following the successful greenhouse experiments, the researchers have started field tests of the peptides in Florida. They are also studying whether the peptide can inhibit diseases caused by the same family of bacteria that affect other crops, such as potato and tomato.
"The potential for this discovery to solve such devastating problems with our food supply is extremely exciting," Jin said.
Untreated citrus plants on the left, as compared to treated ones on the right. (Hailing Jin/UCR)/span>
Provided by American Phytopathological Society
While humanity is facing the COVID-19 pandemic, the citrus industry is trying to manage its own devastating disease, Huanglongbing (HLB), also known as citrus greening disease. HLB is the most destructive citrus disease in the world. In the past decade, the disease has annihilated the Florida citrus industry, reducing orange production for juice and other products by 72%. Candidatus Liberibacter asiaticus (CLas) is the microbe associated with the disease. It resides in the phloem of the tree and, like many plant pathogens, is transmitted by insects during feeding events. Disease progression can be slow but catastrophic. Symptoms begin with blotchy leaves, yellow shoots, and stunting, and progress into yield decline, poor quality fruit, and eventually death.
Currently, the only thing citrus growers can do to protect their crops from HLB is control the insect vector. Dozens of researchers are trying to find ways to manage the disease, using strategies ranging from pesticides to antibiotics to CLas-sniffing dogs. Understanding the plant microbiome, an exciting new frontier in plant disease management, is another strategy.
Dr. Caroline Roper and first author Dr. Nichole Ginnan at the University of California, Riverside led a large research collaboration that sought to explore the microbiome's role in HLB disease progression. Their recent article in Phytobiomes Journal, "Disease-Induced Microbial Shifts in Citrus Indicate Microbiome-Derived Responses to Huanglongbing," moves beyond the single-snapshot view of the microbial landscape typical of microbiome research. Their holistic approach to studying plant-microbe interactions captured several snapshots across three years and three distinct tissue types (roots, stems, and leaves). What is so interesting about this research is the use of amplicon (16S and ITS) sequencing to capture the highly intricate and dynamic role of the microbiome (both bacterial and fungal) as it changes over the course of HLB disease progression.
Ginnan et al. surmised that HLB created a diseased-induced shift of the tree's microbiome. Specifically, the researchers showed that as the disease progresses, the microbial diversity increases. They further investigated this trend to find that the increase in diversity was associated with an increase in putative pathogenic (disease-causing) and saprophytic (dead tissue-feeding) microbes. They observed a significant drop in beneficial microbes in the early phases of the disease. Arbuscular mycorrhizal fungi (AMF) were one such beneficial group that the authors highlighted as showing a drastic decline in relative abundance.
The depletion of key microbial species during disease might be opening the door for other microbes to invade. Certain resources may become more or less available, allowing different microbes to prosper. Dr. Roper and Dr. Ginnan hypothesize that when HLB begins, this depletion event triggers a surge of beneficial microbes to come to the aid of the citrus tree. They suspect that the microbes are initiating an immune response to protect the host.
As the disease proliferates, the citrus tree and its microbiome continue to change. Dr. Ginnan, the lead author on this study, found that there was an enrichment of parasitic and saprophytic microorganisms in severely diseased roots. The enrichment of these microbes may contribute to disease progression and root decline, one side effect of HLB.
Survivor trees, or trees that did not progress into severe disease, had a unique microbial profile as well. These trees were enriched with putative symbiotic microbes like Lactobacillus sp. and Aureobasidium sp. This discovery led the researchers to identify certain microbes that were associated with slower disease progression.
Dr. Ginnan says their "aha" moment during the research was in the data analysis. "Originally we were looking for taxa that increased and decreased in relative abundance as disease rating increased," she said. "Our differential abundance analysis ended up revealing clear enrichment patterns replicated in multiple taxa." This is the moment they began to develop the individual patterns they were seeing into a broader disease model.
This research is the foundation for future projects and collaborations that the authors are excited to continue to develop. They are motivated by the potential function of the microbiome to manage crop diseases. In the near future, they hope that these discoveries and an understanding of beneficial microbes can help establish a microbiome-mediated treatment plan to protect crops from diseases like HLB. In addition, the model they've developed can be applied to understanding diseases of other tree crop systems.