Citrus Industry

The Australian finger lime, a citrus relative, could be a new specialty crop for Florida citrus growers.

Traditionally, finger limes have remained rare in the United States, grown few and far between. However, the fruit's unique tolerance to HLB is becoming increasingly attractive to Florida growers. Manjul Dutt, research assistant scientist at the  University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) believes finger limes could secure Florida's position in the global citrus market.

In the field, finger limes have a low HLB infection rate. Early on, researchers noticed these trees were much more tolerant to HLB than any of the traditional citrus varieties being grown in the state.

“We have a number of theories as to why this finger lime could be tolerant to HLB,” Dutt says. “It could be due to the presence of physical barriers, or it could be due to the presence of certain toxins or certain chemicals in the phloem that the Candidatus Liberibacter asiaticus' (CLas) doesn't like.” CLas is the bacterium that causes HLB.

The young flesh of finger limes contains high levels of anthocyanins, producing a dark-red color on the leaves of the tree.  Studies have indicated that insects, including the Asian citrus psyllid, move according to visual cues. It's possible the high levels of anthocyanins can discourage psyllid feeding and thus prevent transmission of HLB.

Additionally, the phloem of the finger lime contains high levels of aldehyde compounds. According to Dutt, citronellol, a compound of growing interest and present in the phloem, has shown to have anti-bacterial activity, which could also be preventing the replication of CLas.

One of the pressing issues limiting commercial production of finger limes in Florida is the lack of knowledge about the crop. Dutt and his team of researchers are currently evaluating different rootstocks in hopes of finding varieties suitable for Florida's growing conditions.

Furthermore, they are developing new cultivars that are crosses between conventional citrus and finger limes to incorporate HLB tolerance into traditional citrus varieties. Dutt says thousands of trees are currently being evaluated and quite a few appear promising

Tapping into their genetics
While finger limes aren't exactly set out to be the new crop replacing Florida's longstanding orange and grapefruit industry, Dutt believes finger lime trees can provide a strong assist. “Hybrids between finger limes and sweet orange down the road may have sweet orange-like traits that can be acceptable to the grower and consumer. It would create a sweet orange-like fruit with finger lime genetics that allow it to be tolerant to HLB,” he says. “Many people in the industry realize it's a long-term process. Some are skeptical but overall, people are hopeful that the finger-lime genetics play an important role in providing HLB-tolerant trees in the future.”

To date, finger limes are more of a niche crop in North America with only a few growers in California, Hawaii and Florida.

In the meantime, Dutt has produced a finger lime hybrid that looks like a larger finger lime. “We'll be releasing it this summer—it's similar to the finger lime but it has more pulp and the same “pearls” that finger limes do,” he says. He adds that it's a commercial release as a niche crop and hopes the limes will be available in stores in the next three to four years

 more about finger limes

 and finger lime resistance

For more information:
Dr. Manjul Dutt
University of Florida
Tel: +1 (863) 956-8679
manjul@ufl.edu
https://crec.ifas.ufl.edu/

https://news.ucr.edu/author/jules-bernstein

New research reveals an essential step in scientists' quest to create targeted, more eco-friendly fungicides that protect food crops.

Scientists have known for decades that biological cells manufacture tiny, round structures called extracellular vesicles. However, their pivotal roles in communication between invading microorganisms and their hosts were recognized only recently.

UC Riverside geneticist Hailing Jin and her team found plants use these vesicles to launch RNA molecules at fungal invaders, suppressing the genes that make the fungi dangerous.  

“These vesicles shuttle small RNAs between cells, like tiny Trojan horses with weapons hidden inside,” said Jin, a professor of genetics and the Cy Mouradick Chair in the Department of Plant Pathology and Microbiology. “They can silence pathogenic fungal gene expression.”

Using extracellular vesicles and small RNAs has several advantages over conventional fungicides. They're more eco-friendly because they are similar to naturally occurring products. Eventually, they degrade and do not leave toxic residues in the soil. Also, Jin explained, this method of fighting fungi is less likely to breed drug-resistant pathogens.

A sticking point for scientists in creating these fungicides has been figuring out how to load their desired small RNAs into the vesicles.

“We've wondered how these weaponized small RNAs get into the bubbles,” Jin said. “Now, we think we have an answer.”

Her laboratory has identified several proteins that serve as binding agents, helping to select and load small RNAs into the vesicles. The lab's research is detailed in a new Nature Plants journal article.

The Jin laboratory has been working for several years on the development of gene-silencing RNA fungicides. Work toward this goal led to the team's landmark discovery in 2013 that gene-silencing RNA messages can be sent from the fungal pathogen to the plant host to suppress host immunity. Later, the team learned small RNAs can move both ways — from plants into pathogenic invader cells as well. In 2018, the team worked out that extracellular vesicles were the major delivery system for these small RNAs. They observed that Arabidopsis plants secrete extracellular vesicles into Botrytis cinerea, a fungus that causes grey mold disease and destroys millions of crops every year.

“This was the first example of a host using these vesicles to deliver small RNAs to another organism,” Jin said. “Previously we saw movement of RNA, but didn't know how the small RNA are selected and transported.”

Now, she and her colleagues have identified several RNA-binding proteins in Arabidopsis that bind to specific small RNA molecules and load them into extracellular vesicles. This suggests the proteins play an important role in loading and stabilizing small RNAs in the vesicles. The finding can help increase the payload of gene-silencing RNAs that make it into vesicles and enhance the efficiency of disease control.

Some scientists have taken inspiration from the RNA communication in plant vesicles to design human therapies. For example, some are attempting to load anti-cancer RNAs and drugs into extracellular vesicles in fruits or vegetables, so people can eat or drink them. Jin is hopeful that her lab's discovery can aid these efforts.

https://news.ucr.edu/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.