Nematode-induced syncytium--a multinucleate transfer cell.

The formation and structure of asyncytium induced by the potato cyst-nematode {Heterodera rostockiensis Woll.) in potato roots is described. At the permanent feeding site of the nematode larva, usually in the root cortex, the larva pierces a cell with its mouth stylet and injects saliva. Cell wall dissolution occurs to incorporate neighbouring cells into a syncytium. A column of cells is incorporated towards the vascular tissue. Centripetal advance is limited by the lignified xylem, then syncytial spread continues laterally along xylem parenchyma and pericycle cells. Wall protuberances form on syncytial walls adjacent to conducting elements. This indicates the syncytium is a multinucleate transfer cell, and by ingesting syncytial contents the larva is the nutrient sink. As syncytial expansion occurs, sieve elements are crushed and probably cease to function, hence protuberance development continues only against xylem elements. Cell alterations on incorporation into the syncytium involve expansion, loss of cell vacuole, nuclear hypertrophy and a proliferation of cytoplasmic organelles free to move through wall gaps into the communal cytoplasm. 'Boundary formations' and microtubules are associated with the growing ends of protuberances, and appear to be involved in their synthesis. Fibrillar material, possibly cellulose microfibrils, occurs between the plasmalemma and the membrane of the 'boundary formation', and the forming protuberance. To induce the formation of the syncytium, the larva controls the differentiation of unspecialized cells to cells with a specific physiological function. The occurrence of wall protuberances suggests that transfer cells form as a response to solute flow.