Survivor Trees and the Citrus Crisis: How Scientists Are Using Modeling to Save an Industry

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Picture this: sunshine, sandy beaches, and lush citrus groves. You’re in Florida. Naturally, you see the juicy oranges inextricably linked to the state. 

But underneath this idyllic image, a real crisis is brewing.

Citrus Huanglongbing (HLB), descriptively referred to as citrus greening, has caused irreparable damage in the Sunshine State. 

• Citrus production volumes have decreased by approximately 74%

• Annual Losses of over one billion dollars have occurred

• 7,900 jobs have been lost–hurting real people and families

Alarmingly, the disease is creeping west, putting California at risk. 

What Makes HLB So Devastating?

HLB is caused by an insect-vectored bacterium ‘Candidatus Liberibacter asiaticus’ (CLas). Once a tree is infected there is no effective cure. Death is inevitable, as the tree trudges down this trajectory: the leaves turn yellow, and the fruit becomes misshapen and bitter. 

The ineffectual management strategy farmers are currently trying is the regular application of insecticides, which are expensive and not environmentally sustainable. 

Additionally, traditional breeding methods for resistant trees can take years. Time we cannot afford to lose.

So now what? Enter a metabolic modeling-driven approach and a bit of botanical mystery.

A Perplexing Phenomenon: Survivor Trees

Picture this: a ravaged Florida orchard, where the majority of trees exhibit rapid HLB decline. But there are a couple of impervious trees–Survivor Trees(ST), which after four years, of monitoring have only declined by 0.5 in a 0 – 5 disease rating system. They’re an extraordinary exception that researchers from UC Riverside and UC San Diego are working to decode.

Cracking the Code:

Scientists are identifying the molecular pathways, molecules, and targets in citrus roots and citrus root microbes that are responsible for the ST phenotype in Florida.

Two techniques are being used:

• RNA sequencing (RNA-seq): Allows researchers to see which genes are active in the tree cells and which proteins the tree is making. This helps scientists deduce how the tree functions. The high-tech tool Illumina NexSeq Sequencing is being used to obtain RNA sequences for transcriptomics analyses.

• Metabolomics: This reveals what’s actually happening in real time by measuring the small molecules (metabolites) inside the tree.

Researchers are employing these methods to construct computational models for both non-survivors and ST to pinpoint what unique natural characteristic of the ST dictates its resistance. 

The Microbial Twist: Could a Fungus Be Helping?

The metabolic models are not limited to the molecular interactions between citrus rootstock and the HLB-associated pathogen (CLas). Models with and without the metabolic model for a fungus are being constructed. 

The fungus Exophiala pisciphila has been found near living citrus and the project team wants to know if it is playing a role–maybe helping the tree fight off infection or interfering with the bacterium’s survival.

The models simulate the whole ecosystem–the trees, the pathogen, and the microbes living in and around the citrus roots. By taking a holistic approach, the project team aspires to recognize the factors contributing to the Survivor Tree’s advantage. 

Turning Data Into Solutions:

The ultimate goal is to turn the findings from this project into actionable solutions for citrus nurseries including:

• Nutritional mixes: Delivering fertilizer mixtures or interjecting beneficial minerals into the growth media of young trees to optimize desirable traits like resilience and growth.

• Biostimulants: Developing biostimulants that boost a tree’s natural immunity

• Genetic Engineering: Using tools like CRISPR or agrobacterium-mediated transformation to modify genes in plants.

All in all, metabolic models are helping simulate the complex biological processes in citrus trees and their ecosystems to develop practical, future-focused interventions.

Final Thoughts:

Citrus greening has been devastating. It is one of the biggest threats US agriculture has ever faced. But by combining deep biological insight with high-powered modeling and collaborative outreach through organizations like LabtoFarm, there is hope. 

If we can crack the code behind those Survivor Trees, we might not just save citrus—we might change how we fight plant diseases across the board.