Research

Interactions between plants and bacterial pathogens

 

Plant diseases caused by bacterial pathogens pose an important threat to food security worldwide, and efficient strategies to combat them are urgently required. To achieve this, it will be necessary to understand the mechanisms of infection by pathogens and the counter-measures employed by resistant plants. Bacterial pathogens, from both animals and plants, need to manipulate host functions in order to promote disease. In most gram-negative bacteria, an essential virulence factor is the Type III secretion system (T3SS), which injects Type III effector proteins (T3Es) into host cells. T3Es collectively perturb cellular processes in host cells and modulate host immunity to enable bacterial infection. However, resistant plants have developed ways to detect specific T3Es and subsequently trigger immune responses that prevent disease. In plant pathogens, we are beginning to determine the molecular mechanisms underlying the subversion of host processes by T3Es, but several important questions remain poorly understood. This constitutes an important handicap for our knowledge of fundamental aspects of bacterial diseases in plants and the engineering of efficient plant resistance against them.

 

Ralstonia solanacearum is considered the most destructive plant pathogenic bacterium worldwide due to its lethality, persistence, wide host range and broad geographic distribution. Bacterial wilt caused by R. solanacearum affects over 250 plant species in over 50 families, including important crops such as potato, tomato, tobacco, banana, pepper and eggplant, among others, being responsible for enormous economic losses worldwide. The incidence of the disease is particularly dramatic for agriculture in many countries in inter-tropical regions, where R. solanacearum is endemic, although it is currently a continuing menace also in temperate climates, due to the incessant emergence of cold-tolerant strains and to global warming. R. solanacearum is a soil-borne pathogen that secretes more than 70 T3Es into the plant cell, and colonizes different plant organs, such as roots, stems and leaves. Our work aims to characterize plant processes in a tissue-specific manner, and to understand specific bacterial virulence factors in different plant organs and steps of the infection process. We use a wide array of host plants, including the model plant Arabidopsis thaliana, Nicotiana benthamiana and tomato.

 

Our laboratory was created in 2015 with the overall purpose of deciphering the molecular mechanisms of infection by plant-pathogenic bacteria (with special emphasis on the role of T3SS), the plant processes targeted by them and the associated plant signaling pathways. In recent years, it became evident that pathogens manipulate plant stress signaling to achieve infection, and research in this field constitutes a valuable opportunity to understand both bacterial virulence and plant signaling. For our research, we use an integrated multidisciplinary approach combining biochemistry, molecular biology and genetics, on both pathogen and host plants.

 

1- Manipulation of plant immunity by bacterial type-III effectors

 

Successful pathogens must evade or inhibit host immunity to cause disease. In the past decade, a number of T3Es from plant pathogenic bacteria have been shown to suppress plant innate immunity. More recently, the detailed mechanisms of action have been defined for several of these effectors, mostly from the genera Pseudomonas and Xanthomonas. Interestingly, effectors display a wide array of virulence targets, being able to prevent activation of immune receptors and to hijack immune signaling pathways. However, the mechanisms of immune suppression of pathogens with a more complex lifestyle, such as R. solanacearum remain poorly understood. We are especially interested on the identification and characterization of R. solanacearum T3Es that interfere with the different layers of plant immunity in a direct or indirect manner.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                                                                                        Macho & Zipfel, Current Opinion in Microbiology, 2015

 

 

2- Understanding the bases of biotic stress: type-III effector manipulation of plant signaling pathways

 

Besides being a fascinating example of pathogen-host co-evolution, effectors have also emerged as valuable tools to dissect important biological processes in host cells. In recent years, research has uncovered T3E functions different from direct immune suppression, including the modulation of plant hormone signaling, metabolism, or organelle function. These activities may contribute to the modulation of plant cells in order to promote bacterial survival, nutrient release, and bacterial replication and dissemination, constituting an important component of plant biotic stress and causing important losses in relevant crops. We study T3Es that target plant hormone signaling, development, metabolism and organelle functions, with the goal of understanding both bacterial virulence activities and the associated plant signaling pathways.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                                                                                                                                         Macho, New Phytologist, 2015

 

 

3- Plant integration of environmental signals: common elements involved in biotic and abiotic stress responses

 

In nature, plants have to integrate multiple environmental signals from abiotic or biotic origin. Additionally, responses to stress need to be balanced with the plant developmental program. As a consequence, plant signaling is highly interconnected and common elements or ‘hubs’ play an important role in the plant adaptation to multiple environmental cues. Pathogens may take advantage of this signaling integration, by enhancing cellular processes in host plants that indirectly trigger a suppression of immune responses. We aim at identifying and characterizing these hubs at the molecular level, as well as deciphering the molecular cross-talks between plant signaling pathways involved in different stress responses. Understanding the integration of plant stress signaling will be essential to achieve sustainable solutions for modern agriculture.

4- Plant immunity against R. solanacearum

In spite of the extensive research on plant immunity over the last years, the perception of molecular patterns from R. solanacearum that activate immunity in plants is still poorly understood, which hinders the development of strategies to generate resistance against bacterial wilt disease. Most plants have evolved to perceive bacterial flagellin as an indicator of bacterial infection, which in turn activates immune responses. Work from our lab and others indicates that R. solanacearum has developed a polymorphic flagellin, which is able to avoid the recognition by plant receptors, while keeping the function of the flagellum in bacterial movement. We are currently working on the identification and improvement of potential plant receptors that could mediate recognition to flagellin from R. solanacearum, which would enable us to increase plant disease resistance against this and other pathogens with polymorphic flagellin sequences. Among other bacterial elicitors perceived by plants, we have found that specific plants within the Solanaceae family are able to perceive a peptide named csp22, which constitute an epitope in the bacterial cold shock protein. The perception of csp22 from R. solanacearum is mediated in tomato by the receptor LsCORE. Our work showed that perception of csp22 by LsCORE activates resistance against R. solanacearum and transgenic expression of LsCORE in non-responsive plants, such as Arabidopsis, leads to increased resistance against R. solanacearum colonization and slower development of disease symptoms. This work sheds light on the mechanisms for perception of R. solanacearum by plants, paving the way for improving current approaches to generate resistance against R. solanacearum.

 

5- Unbiased approaches to understand plant stress: making the most of the R. solanacearum model

R. solanacearum is an extremely versatile pathogen. It normally enters the plant through the roots, moving through different cell layers to reach the vascular system. Once inside xylem vessels, it colonizes the whole plant and replicates until it saturates plant tissues with bacteria, causing plant wilting and the spread of bacteria back to the soil, to water streams, or to other plants. This offers the possibility of studying plant-bacteria interactions in a cell type- and organ-specific manner. In this context, we have developed unbiased approaches to understand the interplay between the infection by R. solanacearum and:

            - Plant hormone signalling

            - Cell wall biogenesis and stability

            - Root development

            - Epigenetic basis of the regulation of gene expression

This work is done in collaboration with several other groups in and out of PSC, and will contribute to an integrated understanding of plant responses to environmental stresses and the molecular mechanisms underlying them.

 

@2019 Alberto Macho - Shanghai Center for Plant Stress Biology - Chinese Academy of Sciences