How Pathogens Affect Root Structure

Several groups of soil microorganisms cause diseases to the roots. These “soilborne” plant pathogens are particularly challenging since they often survive in soil for many years and each crop may be susceptible to several species. They belong to various systematic groups. Major groups are oomycetes, fungi, bacteria, protists, and nematodes. Few soilborne viruses affect vegetable crops. The impact of these pathogens on the growth of plants can be massive. They may lead to the complete destruction of the roots and subsequently of the plants. Soilborne pathogens cause three major types of disease: (i) damping off caused by oomycetes such as Phytophthora, Pythium, and by fungi such as Rhizoctonia species, when seeds or seedlings are killed before or after germination; (ii) vascular wilt when the pathogens (e.g., Fusarium or soilborne Ralstonia bacteria) penetrate the roots and infect the xylem vessels, which become plugged; and (iii) root and crown rots caused by species of Phytophthora, Rhizoctonia, Fusarium, Gaeumannomyces, and Sclerotinia (Katan 1996). In addition, plant-parasitic nematodes and biotrophic protists (Plasmodiophora) are obligate parasites able to induce dramatic morphological and physiological changes in host roots. Infected roots undergo a developmental switch that results in the formation of aberrant root structures (clubs or root-knots) (Figure 10.1). The most intriguing pathogens in this group are sedentary endoparasitic nematodes. Two of the most economically damaging groups are the rootknot nematodes (RKN, Meloidogyne spp.) and the cyst nematodes (CN). CN are mainly represented by the two genera Globodera and Heterodera. These microscopic roundworms invade the roots and engage in sophisticated interactions with their host plants inducing the formation of permanent feeding sites. These nematodes only feed after they have reached their destination in the host roots and have become sedentary by selecting a feeding site. These feeding sites consist of cells within the vascular cylinder, which become completely reorganized, hypertrophied, and metabolically highly active and serve as food sources throughout the nematode life cycle. It remains unclear how these pathogens cause such root alterations, but increasing evidence showed that nematode parasitism proteins, secreted from the esophageal gland cells through a hollow protrusive stylet, have direct effects on plant metabolic and developmental pathways to reprogram cells for feeding cell ontogenesis (Davis et al. 2008; Bird et al. 2009; Gheysen and Mitchum 2011). This chapter reviews biology and molecular insights into mechanisms and plant developmental pathways manipulated by RKN, CN, and Plasmodiophora brassicae, elucidated in the last years