Figure 3.

The phytoplasma life cycle involves replication in plants and insects. (a) Schematic illustration of the different stages of phytoplasma movement through the plant and leafhopper hosts. Phytoplasmas are indicated as dark red dots and the movement of the phytoplasmas is indicated with dark red arrows. Leafhoppers acquire phytoplasmas from the plant phloem (ph). Phytoplasmas that are ingested with plant sap move through the stylet's food canal and the intestinal tract, and invade epithelial and muscle cells of the oesophagus (Es), anterior midgut (Amg), mid midgut (Mmg), filter chamber (Fc), Malpighian tubules (Mt) and hindgut (Hg). Similarly to spiroplasmas (Özbek et al., 2003), phytoplasmas probably cross the basal lamina to enter the haemolymph (He), from where they can move to the salivary glands (Sg). They multiply in secretory salivary gland cells and then are transported along with the saliva to the salivary duct (Sd). Phytoplasmas are introduced back into the phloem tissue of host plants during feeding and salivation of leafhoppers. (b–e) Electron micrographs of phytoplasmas (arrowheads) in the plant phloem and in various leafhopper tissues as indicated in (a). (b) AY‐WB in adjacent sieve elements (se1) close to the nucleus (n) of an aster leaf; the arrow indicates a sieve pore between the sieve plates (sp), and asterisks indicate possibly dividing phytoplasma cells. (c) AY‐WB (arrowheads) in a midgut epithelial cell of the leafhopper vector M. quadrilineatus; asterisks indicate less dense areas of the cytoplasm. (d) Accumulations of maize bushy stunt phytoplasma (MBSP, arrowheads) in the cytoplasm of a cell in the muscle layer around the midgut of the leafhopper vector D. maidis; note that phytoplasmas are located close to the nucleus (n). (e) MBSP (arrowheads) in the cytoplasm of a salivary gland secretory cell in D. maidis; arrow indicates phytoplasma close to a secretory vesicle (sv). Other abbreviations: bl, basal lamina; Br, brain; Cng, compound nerve ganglion; mv, microvilli; pm, plasma membrane; sm, secretory material; Sp, salivary pump; St, stylet; Xy, xylem. Scale bars, 1 µm.