Sieve-element biology provides leads for research on phytoplasma lifestyle in plant hosts

Phytoplasmas reside exclusively in sieve tubes, tubular arrays of sieve element-companion cell complexes. Hence, the cell biology of sieve elements may reveal (ultra)structural and functional conditions that may be of significance for survival, propagation, colonization and effector spread of phytoplasmas. Electron-microscopic pictures suggest that sieve elements offer facilities for mobile and stationary stages in phytoplasma movement. Stationary stages may enable phytoplasmas to closely interact with diverse sieve-element compartments. The unique, reduced sieve-element outfit requires permanent support by companion cells. This notion implies a future focus on the molecular biology of companion cells to understand the sieve-element/phytoplasma interrelationship. Supply of macromolecules by companion cells is channelled via specialized symplasmic connections. Ca2+-mediated gating of symplasmic corridors is decisive for the communication within and beyond the sieve element-companion cell complex and for the dissemination of phytoplasma effectors. Thus, Ca2+ homeostasis, which affects sieve-element Ca2+ signatures and induces a range of modifications, is a key issue during phytoplasma infection. The exceptional physical and chemical environment in sieve elements seems an essential, though not the only factor for phytoplasma survival.