Phytoplasmas have two hosts: insects and plants

All phytoplasmas are transmitted to plants by phloem-feeding insects in a persistent propagative manner (Fig. 3). In insects, phytoplasmas move from the gut lumen into epithelial cells. After migrating through gut epithelial cells, they are released into the space between the basal plasmalemma and the basal lamina and, from there, they move to the hemolymph, where they circulate. They replicate frequently at high levels in various insect tissues, including cells of muscle (Fig. 4A) salivary glands (Fig. 4B). Phytoplasmas move into salivary ducts, and are introduced into plant hosts with insect saliva during feeding. The latent period in the insect is at least two weeks.

Fig. 3. A schematic illustration of the phytoplasma infection route in a leafhopper. Leafhoppers acquire phytoplasmas while feeding. Phytoplasmas move from the gut lumen across the git epithelial cell layer into the hemolymph. They infect various leafhopper tissues including nerve and muscle cells. They enter the saliva via the salivary gland cells, and are introduced into plants with the saliva during insect feeding.

Fig. 4. Maize bushy stunt phytoplasmas (MBSP) in cells of the leafhopper vector Dalbulus maidis; A. Large accumulations of MBSP (mbs) in muscle cells around the midgut (mg); B. MBSP (arrows) in a secretory cell of the principle salivary gland. Other abbreviations in figure: mf, myofibrils; n, nucleus; sg, secretory granule. Scale bars, A-B, 1 mm.

The aster leafhopper, Macrosteles quadrilineatus, is the most important vector of aster yellows phytoplasmas in the United States (Fig. 5), and the leafhopper Dalbulus maidis is the most important vector of maize bushy stunt phytoplasma in southern United States, and Central and South America.

Fig. 5. Macrosteles quadrilineatus, the most important vector of aster yellows phytoplasma in the United States, feeding on aster.

Plant-pathogenic mollicutes can be a mutually beneficial symbiont of leafhoppers

The interactions of maize bushy stunt phytoplasma with different leafhoppers of the genus Dalbulus vary (Fig. 6). In efficient vector systems, infections with these mollicutes improve leafhopper fitness, whereas in inefficient vector systems, infections result in virulent pathogenicity. For example, infection of D. elimatus by maize bushy stunt phytoplasma enhances resistance to cold temperatures of the leafhopper and increases its ability to survive maize free winter periods, thereby benefiting both the plant pathogen and the insect. The differences in association can be in part explained ecologically; more ancient associations of phytoplasmas with their host organisms have less detrimental effects than relatively recent ones.

Fig. 6. Schematic representation of the interaction of phytoplasmas, leafhoppers and plants. Phytoplasmas do not affect adapted host plants and insect vectors but cause adverse effects on introduced or foreign plants (serrated leaves) and vectors (spotted leafhopper).