Tackling the olive-tree killer
LMU cell biologist and geneticist Silke Robatzek studies how plants become immune to bacterial infections – and how pathogens subvert these defenses. She recently received a prestigious grant from the European Research Council.
Among her earliest childhood memories, Silke Robatzek remembers laying leaves side by side and wondering whether their insides were as varied as their shapes and colors, or asking herself why – in contrast to gooseberry shrubs – the redcurrant shrub in the garden at home had so many feeding aphids on it. “Even then, I was curious about how plants defend themselves from disease, since they have no means of escape and can’t go to the doctor,” she says. This curiosity is something she has never lost, and it helps to explain why she is now one of the leading specialists in the field of plant-cell immunity, and leader of her own research group at LMU’s Biozentrum since 2018. Her primary interest focuses on the mechanisms that plants use to recognize pathogens, and the tricks used by the latter to disarm these defenses – and this interest extends to very diverse species of plants. For her innovative studies, Robatzek has now received one of the coveted Advanced Grants awarded by the European Research Council (ERC), after having obtained an ERC Starting Grant in 2012.
Unlike humans and many other animal species, plants do not possess specialized cell types that circulate in the vasculature and are dedicated to the repulsion of pathogenic invaders. In plants, virtually every cell type, whether in leaves, roots, and the vasculature, is in principle capable of defending itself against attack by pathogens. However, in common with us humans and most higher organisms, plants have an innate immune system that is based on receptor proteins which recognize different harmful pathogens and induce an immune response. This system works on several levels. The first line of defense is made up of immune receptors located on the surfaces of exposed cells, which recognize generalized structures that signal the presence of pathogens. If the attacker manages to get past this hurdle and continues to infect, a second battery of immune receptors inside plant cells awaits it. Indeed, the innate immune systems of plants are extremely versatile. Depending on the species concerned, plants can express between hundreds and several thousand immune receptors. Moreover, there are marked differences in the repertoires used by different species. Indeed, this can be a reason why, for example, redcurrant shrubs are more susceptible to aphid infestation than gooseberry shrubs.
A highly adaptable parasite
Her new ERC project addresses the question of how pathogens succeed in evading the clutches of immune receptor proteins in very different plant species. – And in the bacterium Xylella fastidiosa, now perhaps better known as the olive-tree killer, she has chosen an iconic example of such a parasite. Following transmission by sap-sucking insects, the bacterium replicates in the vessels that are responsible for water transport in plants. The accumulation of bacterial cells in the xylem vessels eventually kills the host by desiccation. X. fastidiosa infects more than 300 plant species – including economically significant plants such olive trees, grapevines, citrus, coffee, cherry and almond trees, as well as herbs like rosemary and thyme, and ornamentals like Oleander and lavender. Alarmingly, it is extending its geographic range in Europe, where it presents a potentially grave threat to olive production. “The bacterium may make use of a rather generalized virulence strategy that enables it to neutralize many different immune receptors,” says Robatzek. She therefore plans to systematically analyze the bacterial and plant factors involved in the development of the condition caused by X. fastidiosa, in order to understand what makes so many host plants susceptible to the infection and to identify immune receptors that can control it.
The study will use two plant model species, Arabidopsis thaliana (thale cress) and tobacco (Nicotiana). Both are susceptible to X. fastidiosa – and progression of the disease is much faster and easier to monitor in them than in the olive. In addition, both are established experimental models in plant biology. They can be cultivated under controlled conditions in the laboratory, and their genomes have been extensively characterized. “Among other approaches, my group and I will determine what sets of genes are activated in the plant and the parasite during the course of an infection, which will allow us to uncover the basic mechanisms that contribute to pathology,” Robatzek explains. “We will also carry out comparative studies of gene transcription in other plant species including olive trees and grapevines, because we would like to know whether the pathogen always activates expression of the same virulence factors, or varies its strategy depending on the particular host. Conversely, we will examine whether different hosts possess a common gene or set of genes that make(s) them susceptible to the bacterium and whether such genes might be specifically targeted by X. fastidiosa.”
The idea for the project was sparked off by an Italian postdoc in her group, whose family owns an olive orchard in Apulia. “It was he who alerted me to the plight of olive growers in Italy, and the fact that nobody seemed to have any idea what can be done to stop the disease from spreading.” That X. fastidiosa is the cause of the disease only became clear in 2013, and Robatzek hopes that her work will lead to the development of effective countermeasures that will help stem the loss of olive trees in Italy.
The spirit of discovery
Robatzek’s research is always focused on the biological process that she wishes to understand. “I am fascinated by the idea of discovering things that were hitherto unknown, and in this way giving something back to society,” she says. She is a great admirer of Alexander von Humboldt, the renowned naturalist “who discovered so much on his many expeditions in various parts of the world, from which society at large reaped many benefits”. This spirit, she says, has been her guide throughout her scientific career – though the path that led her to one of her own most important discoveries began with a decidedly frustrating experience. “As a postdoc at the Friedrich Miescher Institute for Biomedical Research in Basel I worked on a topic that proved to be too methodologically complex for the techniques that were available at the time,” she explains. “So I then concentrated on a project designed to observe the immune receptor for the bacterial protein flagellin, which I had begun in parallel with the other one, as a kind of insurance policy. I never expected that my ‘back-up’ project would become the basis for my later career.”
Robatzek’s work on the immune receptor for flagellin yielded important insights into its function – first in Basel, then at the Max Planck Institute Plant Breeding Research (MPIZ) in Cologne and later at The Sainsbury Laboratory in Norwich (UK), where she led an independent research group for 9 years before moving to LMU. As the name indicates, flagellins are the major components of flagella, which act like propellers that allow bacteria to swim over moist surfaces. Among other questions, she asked how the recognition of flagellin by its cognate receptor in plants triggers the closure of the stomata which are responsible for gas exchange on the leaf surface. This response makes sense, because stomata also serve as entry points for bacteria. Thus, closing them upon detection of intruders helps to fend off infection. “Actually, I was particularly interested in what happens to the receptor subsequently to signal activation,” she says. It took nearly 10 years of hard work with my group for us to establish that that, once the receptor has detected flagellin, it is taken up by the cell and degraded.” Once activated, it is not recycled, and rapid removal of the complex ensures that the plant does not remain blind to flagellin.
The long-term goal of her research is to develop disease-resistant strains of crop plants and so reduce the need for chemicals in agriculture. Robatzek is a dedicated practitioner of basic science, and her new ERC project is firmly anchored in this context. “But of course, we all hope that our findings will find practical applications,” she says. In the case of X. fastidiosa, it might be possible to develop a method that would help the plant to more effectively fight the disease by targeted activation of specific immune receptors. “Our results could also serve as the basis for similar strategies directed against other pathogens that attack plant vasculature systems.” Conversely, if she and her colleagues succeed in identifying a suitable susceptibility gene, it could be specifically knocked out. This has for example been used to develop resistant barley and rice cultivars. – Nevertheless, although Robatzek now has a basic understanding of why redcurrants are more susceptible to aphid infestations than gooseberries, the many ways plants interact with pathogens spark her interest to tackle yet unanswered questions in the field of plant immunity.