ULTRASTRUCTURE OF INITIAL RESPONSE OF GRAMINEOUS ROOTS TO INFECTION BY HETERODERA-AVENAE

Ultrastructural changes induced by two pathotypes, Ha(22) and Ha(71) (Spanish populations), of the cereal cyst nematode Heterodera avenae on wheat cv. Anza and on oat cv. Cometa are described from 24 h to 5 days after nematode inoculation. Pathotype Ha(22) selected as a feeding site the stelar parenchyma cells of wheat roots, which responded with extensive hypertrophy, increase in cellular organelles and formation of elaborate ingrowths of the cell walls adjacent to xylem tracheal elements as seen in a 5 day-old syncytium. On the same host, pathotype Ha(71) fed upon xylem parenchyma cells that reacted early (24 h) with callose-like and osmiophilic granule deposits on their walls, leading to the degradation of the induced syncytium within 5 days. Feeding of both pathotypes on oat roots induced a hypersensitive host reaction, characterized by extensive necrosis of the endodermis and pericycle. Pathotype Ha(22) induced small syncytia in the stele where cellular response was expressed through involution of the cytoplasmic content, indicating a lack of nutrients from plant to nematode. Pathotype Ha(71) did not form syncytia in the stelar tissue and fed only on cortical cells, which were transformed into secretory transfer-like cells.