ANATOMICAL MODIFICATIONS IN RICE (ORYZA SATIVA) ROOTS CAUSED BY THREE ENDOPARASITIC NEMATODE SPECIES
Istituto di Nematologia Agraria, CNR, Via Amendola 165/a, 70126 Bari, Italy
Histopathology and associated damage to rice (Oryza sativa) root tissues infected by Heterodera sacchari, Meloidogyne javanica and Hirschmanniella oryzae are described and illustrated. Syncytia formation was observed in rice roots infected with infective juveniles and adults of H. sacchari. They were initiated by second-stage juveniles, 3-5 days after root penetration, and increased in size when adult females reached maturity. Cross-sections of infected roots showed that the nematode established its permanent feeding site in a large stelar syncytium. Within the stele, the expansion of syncytium occurred longitudinally on both sides of the initial site, reaching 2-5 mm in length. Syncytia formation and disorder of root stelar structures were the main anatomical changes induced by this nematode species, but no evidence of root distortion was observed. On the other hand, small root galls (localized swellings) were observed in rice roots infected by M. javanica. Cross- and longitudinal sections also showed the giant cells (specialized nurse cells) in the stele. Usually four to six large, multinucleate giant cells surrounded the anterior body portion of the adult females. As observed in H. sacchari infection, the pathological reaction of the roots to M. javanica infection included abnormal xylem around giant cells as a result of nematode feeding and giant cell expansion. Rice roots infected by H. oryzae developed necrotic lesions (3-10 mm long) observed on the root surface. Nematodes were commonly found in the aerenchyma root tissues, and in many cases cortical and lacunal cells were collapsed. Sometimes the walls of several cortical cells were broken by the nematode movement (endoparasitic migratory species) and feeding. Hypertrophied nuclei, thickened cell walls and necrosis were common around the nematode feeding sites.