"Microbe" is short for "microorganism" and denotes living things too small to be seen with the naked eye. They include bacteria, microscopic fungi, algae and protozoa (single-celled animals). Viruses are usually not counted among the microbes, since they are not independent organisms. Microbes are mostly smaller than 0.1 millimetres. Only a microscope makes their fascinating world visible. And they are everywhere: in soils and waters, in the air, on plants and animals, and in the human body. Microbes perform central functions in ecosystems and material cycles and influence, among other things, the health of humans, animals and plants.

For plants, microbes can be both useful and harmful: they promote growth and nutrient supply, for example, but they can also cause disease. In this exhibition we present two beneficial and two harmful interactions between plants and microbes as examples.

Plant diseases: when microbes make plants ill. Plant diseases caused by microorganisms can severely impair growth and yield and lead to crop losses. Typical symptoms are leaf yellowing, wilting, necrosis or fruit rot, which can damage entire stands. Fungi and bacteria are among the most important pathogens. Two examples are powdery mildew fungi such as Blumeria hordei, which weaken many crop plants (more at station 2a), and bacteria such as Pseudomonas syringae, which enter plant tissue and trigger defence responses (more at station 3a).

Plants too have an immune system that protects them against pathogens. Scientists are currently investigating how plants recognise pathogens, defend themselves and can be protected in a targeted way. The aim is to develop more resistant varieties and to make crop protection more sustainable. Resistant plants and more precise protective measures allow the use of pesticides and synthetic fertilisers to be reduced, which spares soils, waters and biodiversity.

Plant nutrition: improving yields with the help of microbes. Plant nutrition describes how plants take up and use nutrients in order to grow, develop and form fruit. Unlike humans or animals, plants do not "eat": they generate their own energy through photosynthesis from light, water and carbon dioxide. In addition, they take up essential minerals such as nitrogen, phosphorus, potassium, magnesium and trace elements from the soil through their roots. Excessive use of fertiliser can damage soils, pollute waters and affect the climate.

The term "symbiosis" denotes a close interaction between organisms of different species: it can be beneficial for both partners (mutualistic), useful for only one, or even harmful for one of them. Mutualistic symbioses with microbes can improve plant nutrition by making nutrients more available and easier to take up. Examples are the arbuscular mycorrhizal symbiosis with tiny fungi (more at station 2b) and the root nodule symbiosis with nitrogen-fixing bacteria (more at station 3b). Yields can rise while less mineral fertiliser is needed.

"Microbe" is short for "microorganism" and denotes living things too small to be seen with the naked eye. They include bacteria, microscopic fungi, algae and protozoa. Viruses are usually not counted among them, because they are not independent organisms. Microbes are generally smaller than 0.1 millimetres and occur in soils and waters, in the air, on plants and animals, and in the human body.

All the microorganisms of a habitat, together with their metabolites and the surrounding conditions, are referred to as the microbiome. In plants, a distinction is made by site of colonisation: the phyllosphere on above-ground surfaces, the rhizosphere in the root zone, and the endosphere inside the tissue. Microbes that colonise plant tissue without causing symptoms of disease are called endophytes.

Whether a microbe benefits or harms a plant is not a fixed property of the species. According to the disease triangle model, three conditions must coincide for a disease to develop: a susceptible host plant, a virulent microbial strain, and environmental conditions under which the disease can take hold. Strains of the same species may behave differently depending on host and surroundings; such a range has been described for Pseudomonas syringae and Fusarium oxysporum.

Which molecular conditions decide whether a colonisation turns out beneficial or harmful for the plant has not yet been resolved. This is precisely what plant–microbe research is working on. This exhibition shows four examples: two harmful interactions at stations 2a and 3a, and two beneficial ones at stations 2b and 3b.