In the cultivation of citrus fruits such as lemons, oranges, and mandarins, citrus canker is one of the most economically significant plant diseases. It is caused by infection with Xanthomonas citri, which leads to brown spots and fluid-filled pustules on affected leaves and fruits. These resemble small ulcers - hence the name citrus canker. Plants lose infected leaves and fruit prematurely, resulting in significant yield losses.
How does it work?
The pathogen activates selected parts of the fruit-ripening program inside infected leaves. Normally, this program causes citrus fruits to soften and sweeten as sugars are released. However, the bacterium hijacks this fruit-specific machinery in infected leaf tissue, causing the host to unlock sugars that would otherwise not be accessible to Xanthomonas as a nutrient source. As a result, the pathogen can grow up to a hundred times faster.
The sophisticated strategy used by Xanthomonas bacteria to infect citrus plants was discovered by an international research team at the University of Tübingen’s Center for Plant Molecular Biology.
It was known that many pathogens – including Xanthomonas bacteria – target the sugars bound up in the plant cell wall. We were interested in the tricks these tiny bacteria use to extract nutrients from the plant's cell walls, because these walls represent an almost insurmountable protective barrier for many pathogens”, explains Dr. Trang Phan from the Center for Plant Molecular Biology at the University of Tübingen, the first author of the study.
The study provides new insights into how microbial pathogens hijack the host plant's developmental programs to access otherwise inaccessible nutrients.
“The bacterium Xanthomonas citri uses a syringe-like protein complex to inject various effector proteins into plant cells. These bacterial proteins specifically manipulate cellular processes in host cells to promote infection", explains professor Thomas Lahaye, the team leader. “Because the bacterium forces the host to run this fruit-specific ripening program in leaves, cell wall-bound sugars are released directly into the area where the bacteria grow – a clear nutritional advantage that accelerates bacterial growth", completes Lahaye.
Similarly structured ripening programs in both tomato and citrus fruits
Trang Phan and co-author Dr. Jan Grau from the Institute of Computer Science at the University of Halle-Wittenberg jointly evaluated the extensive project data, especially the sequencing datasets, using bioinformatics.
A comparison of the genes activated in infected citrus leaves (native program) and in ripening fruit (hijacked program) revealed striking similarities between the two gene sets. These parallels showed that the program triggered by the bacterium in leaves essentially mimics the natural fruit-ripening program.
Building on this insight, the team showed that the citrus control protein – and its tomato counterpart – activate similarly structured ripening programs in both tomato and citrus fruits.
Practical tips for growing citrus
These detailed insights into both the process of citrus plant infection by Xanthomonas citri and the regulation of fruit ripening provide the research team with new leads for developing disease-resistant plants.
“While our research aimed to clarify how the pathogen manipulates the host, we also gained new insights into how plants orchestrate fruit ripening. This knowledge could now serve as a basis for using standard genetic modification methods to fine-tune the ripening process. In the future, our findings could help to adjust both the firmness and sweetness of tomatoes and citrus fruits,” concludes Lahaye.
Latest Highlights:
Antarctic expedition searches for clues to future sea-level rise
Scientists track adaptation of bird flu in dairy cattle
University of Salzburg wins Internationalisation Award for Staff Mobility and Development
Astronomers discover a superheated star factory in the early universe