Biotic Stress. Plant Disease Caused by Pathogens

Plants are constantly under attack. They are surrounded by a wide range of potential enemies— organisms that strive to exploit them as substrates. Spores of fungi and oomycetes land on plant leaves all the time and germinate when conditions are right. Insects seek to make the nearest plant their next meal. The soil abounds with bacteria and nematodes, which roots are exposed to. In light of this, plant disease is amazingly rare in wild populations.

If it occurs at all, usually only a few plant individuals are affected and the extent of damage is limited to parts of a leaf or a few leaves at most. Most plants remain healthy because they possess an array of preformed (constitutive) and inducible defence mechanisms that restrict the success of potential pathogens or herbivores. Having said this, there are nonetheless instances in which herbivores or pathogens cause devastating damage to one or several species across large areas. One current example is the mountain pine beetle outbreak, destroying large areas of pine forest in British Columbia, Canada.

Another one is sudden oak death, a newly discovered disease caused by the oomycete Phytophthora ramorum. Wide areas of California and Oregon in the USA have been affected by the pathogen, which apparently was recently introduced and can colonise not only oaks but approximately 40 other species as well.

As we will see, there are—despite the fundamentally different strategies of, for example, phytopathogenic fungi and chewing insects— many similarities between the strategies plants use to fend off microbial pathogens or herbivores. Both defences can be divided into mechanical/ physical and chemical strategies. Both involve recognition events and signalling cascades that result in the induction of defence mechanisms upon the perception of a potential foe or the damage caused by it. There are even common motifs when comparing plant and animal (including human) defence systems. It has in recent years become increasingly clear that they share an innate immunity based on the ability to detect the presence of cells that are foreign to the organism.

The observation and analysis of plant disease are among the oldest questions in biology. Unlike wild plants in their natural habitats, cultivated crops are far more likely to suffer disease.

Because of the yield losses caused by pathogens and herbivores, the ancient Roman writer Columella examined plant disease. Since the nineteenth century, phytopathology has been an active scientific discipline motivated by the goal of reducing susceptibility of cultivated plants to pathogens.

Moreover, plant-pathogen relations serve as an excellent model for the interaction between organisms and their co-evolution. Biotic interactions represent a major driver of biodiversity, and the “warfare” between plant hosts and their potential pathogens explains why.

This chapter first describes the interaction of plants with predominantly microbial and fungal pathogens that can cause disease in plants. We will look at the types of pathogens and their weapons (i.e. pathogenicity determinants), at preformed plant defences and, more prominently, at the inducible plant immune system. The latter encompasses mechanisms involved in the recognition of potential pathogens or their activities, the ensuing activation of defence responses and the apparent co-evolution between hosts and pathogens.

The discussion of defence against herbivores (often referred to as plant pests) will be focused on insect herbivores. Finally, the chapter covers damage caused by parasitic plants and the hostile interaction between plants (i.e. allelopathy). Parasitic plants and allelopathic interactions are not nearly as intensively investigated as the plant immune system and herbivore defence. Thus, those parts will present much less molecular insight.

Plant Disease Caused by Pathogens. Heterotrophic microorganisms have three principal ways of utilising plant biomass as a substrate. The majority are restricted to a saprophytic lifestyle—that is, the degradation of dead plant material—because the plant defence system effectively prevents colonisation. Only a limited number of microorganisms have evolved the ability to either establish a symbiosis with a plant or to overcome the plant immune system and thus gain access to the resources of a living plant.

Fig. 8.1. Symptoms of disease caused by plant pathogens. “Cherry spot hole disease” is a generic term used for bacterial leaf spot caused by the bacterium Xanthomonas pruni and cherry leaf spot caused by the fungus Blumeriella jaapii

The latter group of microorganisms is collectively called pathogens. Their ability to complete at least part of their life cycle within a plant causes disease (Fig. 8.1). Pathogens other than viruses can be divided into biotrophic, hemibiotrophic and necrotrophic pathogens. Biotrophic pathogens keep their host alive while exploiting its resources for growth and reproduction, whereas necrotrophic pathogens kill their host before establishing themselves on it. Hemibiotrophic pathogens represent an intermediate form, as they eventually kill their host at later stages of the colonization.