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 day s after nematode inoculation. Pathotype Ha(22) selected as a feeding s ite the stelar parenchyma cells of wheat roots, which responded with e xtensive hypertrophy, increase in cellular organelles and formation of elaborate ingrowths of the cell walls adjacent to xylem tracheal elem ents 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 cal lose-like and osmiophilic granule deposits on their walls, leading to the degradation of the induced syncytium within 5 days. Feeding of bot h pathotypes on oat roots induced a hypersensitive host reaction, char acterized by extensive necrosis of the endodermis and pericycle. Patho type Ha(22) induced small syncytia in the stele where cellular respons e was expressed through involution of the cytoplasmic content, indicat ing a lack of nutrients from plant to nematode. Pathotype Ha(71) did n ot form syncytia in the stelar tissue and fed only on cortical cells, which were transformed into secretory transfer-like cells.