Léo Ramos Transmitted by the leafhopper insect, Xylella bacteria prevent water and nutrients from nourishing the plant…Léo Ramos
During the mid-1990s, orange seedling production in São Paulo State was done out in the open, which exposed the plants to insect vectors of harmful microorganisms. Among them was Xylella fastidiosa, a type of bacteria that causes citrus variegated chlorosis (CVC), known as “yellowing,” for its characteristic spots left on leaves and fruit. At the time, the disease was found in 34% of the state’s orange orchards, causing damage estimated at $100 million (equivalent to R$327 million) per year to São Paulo’s citriculture. Thanks to research efforts undertaken in the next two decades, knowledge of the biology of the bacteria advanced and a disease management model was developed, which today is based on seedlings grown in protected nurseries, pruning or elimination of contaminated plants, and vector control. As a result, the percentage of orange trees threatened by the bacteria in São Paulo State and the Minas Triangle (western region of Minas Gerais State) fell from 42.58% in 2008 to 3.02% in 2016.
“Progress in controlling CVC would have been impossible without sequencing the Xylella genome and studying the mechanisms that allow the bacteria to trigger the disease,” said agronomist Antonio Juliano Ayres, general manager of the Citriculture Defense Fund (Fundecitrus), at a seminar on the impact of bacterial sequencing on controlling CVC, held on July 13, 2016, at FAPESP.
Data generated from sequencing the bacteria, according to Ayres, were important to advancing knowledge of the epidemiology of the disease. Since then, experiments conducted in the affected areas have shown that both Xylella and the leafhopper, the vector responsible for disseminating the bacteria, were propagated more intensely in warmer regions, where there were water shortages. This led to a change in the way orange trees are planted, with the use of seedlings grown in protected nurseries and increased irrigation in the orchards. The measure, together with the use of insecticides against the vectors and the elimination of more infected plants, or pruning of the less affected, resulted in suppression of contamination by the bacteria in the nurseries and reduced the incidence of the problem in the orange groves of São Paulo State.
The bacteria obstruct the vessels responsible for transporting water and nutrients from the root to the canopy, resulting in hard, small, burnt fruit that ripens faster and becomes unsuitable for sale. Sequencing of the bacteria paved the way for research on breeding aimed at obtaining varieties that are more resistant to phytopathogens and insecticides against the leafhopper, according to molecular biologist Jesus Aparecido Ferro of the School of Agriculture and Veterinary Sciences of São Paulo State University (FCAV-Unesp), in Jaboticabal. Ferro notes that there are now several studies along these lines, but none thus far have resulted in an effective strategy for controlling the bacteria through genetic engineering. “Management of the disease remains the principal strategy for controlling Xylella,” says Ferro, who was a participant in the FAPESP Genome Project.
Fundecitrus…resulting in fruit that is hard, small and burntFundecitrus
Ambitious project
FAPESP, supported by Fundecitrus, launched the Xylella genome project on October 14, 1997. It involved a network of 60 laboratories and more than 190 researchers from various São Paulo institutions and different disciplines, which were integrated virtually through the Organization for Nucleotide Sequencing and Analysis Network (Onsa Network). The work, completed in November 1999, consisted of sequencing 2.7 million base pairs of the Xylella chromosome and identifying the genes present in the genome of the bacterium that allowed it to trigger the disease. According to FAPESP’s Scientific Director, Carlos Henrique de Brito Cruz, who attended the event, demonstrating the capacity of Brazilian scientists was the main objective of sequencing the Xylella genome.
Mapping the genome of the bacterium expanded the horizons of genetic research in Brazil. Further sequencing was done, such as the Cana Genome project, which began in July 1999, to sequence selected parts of sugarcane DNA and identify genes with characteristics of economic interest. The Cancer Genome project came about at almost the same time, a partnership with the Ludwig Institute for Cancer Research, and the sequencing project of the genetic code of the bacterium Xanthomonas axonopodis pv. citri, which causes citrus canker.
The development of a CVC management model also helped in developing strategies to control Huanglongbing disease, known as citrus greening disease, which affects fruit ripening. When greening disease appeared in the orange groves of São Paulo, the growers realized that measures used to control it would have to differ from those employed to combat citrus yellowing disease. “Managing the greening disease would have to be done on a regional scale, because the number of disease vectors for the bacteria that cause the disease is much higher than for yellowing disease and bacteria colonization is much faster,” says engineer-agronomist Armando Bergamin Filho, of the Luiz de Queiroz College of Agriculture at the University of São Paulo (Esalq-USP). He points out that there is still much to do for greening disease, like yellow-linen disease, to become a thing of the past in São Paulo citriculture.
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