Over a decade after US agriculture authorities provoked a legal storm among wealthy homeowners in Florida by ordering the destruction of tens of thousands of healthy trees to halt a deadly tree blight called citrus canker, scientists in Brazil have just discovered exactly how this disease spreads.
An article published in the journal Structure takes forward the science of this economically-significant plant disease by showing the structure and function of a key protein in the development of citrus canker. An invading bacterium is able to reprogramme cell growth in the plant, resulting in the characteristic lesions or canker.
Just as orange cultivation is one of the mainsprings of Florida’s economy, so oranges are also vital to Brazil’s agro-industry (in 2016, approximately 2.32 billion metric tons of orange juice were exported from Brazil, up from 2.01 billion metric tons a year earlier). And just like Florida, Brazil has been plagued by citrus canker.
In São Paulo state, the heartland of orange production, you can see from this graphic where the disease is most prevalent.
As of today, there are no totally effective ways to eliminate citrus canker. Citrus canker is not vectored by any organism but is spread by wind-driven rain. However, wounds caused by feeding of the Asian citrus leaf miner (Phyllocnistis citrella Stainton) may serve as an entry point for the bacterium if citrus canker is present.
Now, fresh scientific insights may encourage a review of sanitary rules governing outbreaks of this disease, whether in the US, Brazil or other locations where oranges are planted commercially. Right now, eradication is the first choice (as it was in Florida). There, American homeowners with healthy trees in their gardens were forced to destroy them part of a defensive strategy to protect billions of dollars’ worth of investment in Florida’s commercial groves. Their claim for reimbursement from the state dragged on for 11 years before it was finally defeated.
As well as copper spraying of citrus groves, replacement of infected plants with healthy saplings and more resistant varieties, authorities try to control the citrus leafminer (Phyllocnistis citrella), an insect that facilitates the spread of citrus canker. New legislation in Brazil also prohibits the sales of any fruit with lesions or other symptoms of the disease.
Thanks to a group of researchers at the National Bioscience Laboratory (LNBio) which is attached to the National Energy and Materials Research Centre (CNPEM) in Campinas, São Paulo State, for the first time the bacterium Xanthomonas citri which causes runaway plant cell growth that produces lesions on leaf, fruit and branch surfaces, has been effectively mapped. This will eventually allow orange grove owners to develop more effective prevention and control strategies.
Specifically, the disease is the result in warfare within the host cell between invading pathogens and a host cell protein called MAF1. The bacteria triggering citrus canker spread via lesions, and when the infection is severe enough, the disease can cause leaves and fruit to drop prematurely, decreasing crop yield.
Mapping MAF1 and the ways invading Xanthomonas citri bacteria successfully circumvent the host cells’ defence systems, is the first step in controlling it. Yet although the MAF1 protein family has been widely studied in yeast, fruit flies and mammals, especially humans and mice (because of its involvement in diseases such as cancer and obesity), citrus MAF1 had never been studied.
“For the lesions to develop, the pathogen must neutralize a host cell protein called MAF1, which is responsible for controlling cell growth. Our research explored the MAF1 regulation pathways in citrus as well as the regions of the plant’s molecular structure that are important for anti-tumor function,” said Celso Benedetti, principal investigator for the study, which was supported by FAPESP, the leading regional research council funding science in Brazil.
You can read in this separate SFB article how Brazilian scientists (also funded by FAPESP) are working in important fields to unravel and neutralise other disease threats to economically important crops such as coffee and olives.
“In a previous study, we showed that bacteria inject host cells with proteins known as TAL (transcription activator-like) effectors, which reprogramme cell growth in the plant. Our results for citrus suggest this reprogramming is driven largely by inactivation of MAF1,” Benedetti said.
Evidence in the scientific literature suggests MAF1 controls gene transcription and cell growth by binding to RNA polymerase 3 (also known as Pol III), a protein complex that produces the “ingredients” needed for protein synthesis.
Until publication of the article in Structure, the scientific literature offered only a description of the structure of human MAF1. Now, however, the crystal structure of citrus MAF1 obtained by the group at LNBio shows structural elements not found in human MAF1.
You can read a copy of the Structure article by clicking here. But you will have to pay publisher Elsevier to do so. You can read a more detailed article summarizing the work of Benedetti and his LNBio team authored by Brazilian journalist Karina Toledo by clicking here.