Theme 2: Interactions between Plants and Biotic Agents

Molecular, biochemical and physiological studies on plant-pathogen and plant-herbivore interactions are a major field of research within the graduate school. Interactions between plant pathogens and their host plant are highly specific. Often only one plant species or sometimes even only one cultivar of a plant species becomes infected/attacked. Specific molecules or signals are involved in communication between host plants and their pathogens/herbivores and eventually determine whether a plant becomes infected/attacked or remains healthy. Modern crop plants need to be protected against pathogens/herbivores. In practice various ways of crop protection are being used. They include chemical control, resistance breeding and biological control. For efficient crop protection pathogens should be detected by the plant before or during early stages of infection. In order to achieve this, sensitive diagnostic tools are being developed to detect particularly viruses and micro-organisms in plants.

To obtain durable crop protection alternatives for the presently used chemical control measures, which often have unwanted side effects, are needed. For the advancement of durable crop protection it is important to unravel the mechanisms and genes underlying the specific interactions between host plants and their pathogens. Detailed insight in how pathogens recognize and attack their hosts and how host plants defend themselves provide tools to develop new strategies to protect our crop plants.

In a few cases fundamental research has generated new strategies already. Expression of genes encoding various viral proteins in plants protect them against several viruses. This type of resistance is becoming intensively exploited in engineered resistance breeding. In a similar way studies on the molecular basis of gene-for-gene systems led to a better understanding of how pathogens overcome or avoid resistance in plants. The recent cloning of plant disease resistance genes has shown that plants have mechanisms to defend themselves against highly diverse pathogens which are reminiscent of immune responses in (in)vertebrates. Resistance genes against viruses, bacteria, fungi and nematodes show previously unsuspected homologies which might be exploited to create hybrid resistance genes that can be used in molecular natural resistance breeding.

Not much is known yet about signal transduction routes that are followed in plants after pathogens are recognized. Unraveling signal transduction pathways in plants induced by various pathogen-derived signals have a high priority in future research programmes. A common defense response occurring in many plants against viruses, bacteria, fungi and even nematodes and endophytic insects is the hypersensitive response (HR). HR is a special type of programmed cell death, which is presently actively studied in the animal field. Spontaneous lesion simulating disease resistance mutants have been described in many plant species. Unraveling the molecular basis of HR should enable us to develop new strategies for disease resistant plants. HR-based defense responses initiated in transgenic plants containing a resistance gene and its complementary avirulence gene are being studied. Alternatively, resistance mechanisms are known that are not associated with HR and may be more durable. Research is devoted towards histological and molecular genetic characterization of these, often polygenic, alternative resistance mechanisms.

Biological control of herbivorous insects through employment of carnivorous insects is successful in many crops. In addition, breeding for host-plant resistance is a second environmentally benign method of insect control. These two methods have been developed and employed separately. Integration of the two methods is not by definition synergistic, but needs fundamental research. Plant characteristics affect herbivores as well as carnivores and chemical signals play an important role.

For instance, herbivory can induce plant volatiles that attract carnivorous insects, while plant volatiles also affect the response of herbivores to pheromones. Understanding the mechanisms of chemically-mediated interactions in plant-insect systems is important to find alternatives for chemical control of insect pests.

The reseach in the area of molecular plant-pathogen interactions has grown rapidly in recent years. EPS can not cover the entire field. The research is focused on four main areas.

Subtheme 2a:

Pathogenicity Factors

Pathogens and pest organisms that parasitize crop plants are diverse, ranging from viruses, bacteria, fungi to nematodes and insects. Due to this diversity they utilize different mechanisms to recognize, penetrate, colonize, parasitize and/or multiply on the expenses of the plant. Viruses have been shown to contain different genes which are essential for uptake and transmission in vectors and for multiplication and dissemination within host plants. The small size of plant virus genomes permits a study at the molecular level of pathogenicity factors in the context of their function in the virus replication cycle. Special attention will be given to the 'cell-to-cell' movement proteins encoded by viruses, and which are essentially involved in establishing systemic infection through the plant, by modifying plasmodesmata. In addition, RNA-dependent RNA-polymerases are pathogenicity factors unique to these pathogens. Also for the other pathogens, which include agrobacteria, identification and study of pathogenicity factors will be addressed. Special attention will be given to the role of saliva proteins of cyst nematodes in inducing giant cells. For fungi various strategies to isolate in planta expressed genes are used, followed by targeted gene disruption or gene displacement in order to determine their role in pathogenicity. The role of fungal avirulence genes as potential pathogenicity factors will also be studied. One of the long term aims is to interfere with crucial pathogenicity factors in transgenic plants.

Subtheme 2b:

Resistance

Various ways of resistance breeding have generated durable crop protection. However, due to high selection pressures highly uniform crop plants become susceptible to new variants of pathogens. It is important to understand the molecular basis of this adaptation in order to breed for more durable resistant plants. Therefore pathogen's avirulence genes and their complementary natural plant resistance genes or gene clusters are isolated and studied in detail. Also signal transduction pathways initiated by activation of resistance genes after interaction with avirulent pathogens are being studied. They include locally and systemically induced defense response mechanisms and programmed cell death. In some cases molecular interactions between well characterized elicitors and their plant receptors will be studied. In addition resistance mechanisms are studied that are not associated with HR, such as pre-haustorial and non-host resistance and avoidance. Molecular genetic analyses of these resistances give strong evidence that the genes, involved in these resistances, are different from those involved in HR. Detailed studies will give further insight into the function of these genes and clues as to why they are more durable. In addition to exploring natural forms of host plant resistance, also artificial resistance genes are being developed. This resistance is based on pathogen derived sequences, co-suppression and / or plantibodies.

Subtheme 2c:

Biocontrol Bacteria

Plant roots provide beneficial soil organisms with a suitable environment. The use of biocontrol bacteria is an alternative for the use of synthetic pesticides. Several non-pathogenic root- colonizing bacteria are able to decrease pathogen survival, reduce build-up of pathogen populations, or counteract infection of susceptible plants. The mechanisms involved comprise competition for nutrients, particularly iron and carbon, production of antibiotics, and induction of disease resistance in the plant. Practical applications of biocontrol bacteria are often hampered by limited effectiveness and consistency, that appear to be caused by

1. insufficient antifungal activity,
2. poor root colonization, and
3. limited knowledge of the regulation of disease-suppressive mechanisms and their expression in the natural soil environment.

Subtheme 2d:

Chemical Ecology

Chemical signals play an important role in the interaction between plants and insects. Herbivorous insects select suitable host plants on the basis of plant chemicals. Signals from conspecific herbivores and signals from their enemies can play a role as well. The response of insects to each of these signals may be modified by the other signals. For instance, the response to insect pheromones may be affected by plant chemicals. Insect feeding often results in induced resistance and induced production of carnivore attractants, i.e. chemicals attracting predators and parasitoids that feed on the herbivores.

Signal transduction pathways in plant-insect interactions are becoming of increasing interest in future research projects. Major research themes comprise chemical analysis of the signals involved and development of new analytical-chemical techniques, behavioural and electrophysiological analysis of insect responses and elicitation of induction mechanisms in plants.