Ultrastructural response of coffee roots to root-knot nematodes, Meloidogyne exigua and M-megadora

Histological and ultrastructural changes induced in Coffea arabica cv. Catuai amarelo and Catimor (Coffea arabica cv. Caturra x Timer Hybrid) by two populations of Meloidogyne exigua and one of M. megadora are reported. Al second-stage juveniles of both species penetrated through the rhizodermis of the root directly behind the root cap and reached the stele near the vascular elements. In Catuai amarelo roots, cells acting as feeding sites increased their synthetic activity and became multinucleate cells. Nuclear activity and dense cytoplasm observed in cells of roots inoculated with M. exigua indicated a susceptible response of the coffee plants. Seedling cells fed upon by M. megadora showed retraction and roughening of the inner wall surface, great presence of paramural bodies and processes of autophagy within the vacuoles, all suggesting an intermediate response between susceptibility and resistance. In Catimor coffee, all Meloidogyne populations tested induced necrosis of injured root cells. Following minor cytoplasmatic changes, the nurse cells showed condensation of chromatin, altered carbohydrate metabolism development of lysosomes and autophagic vacuoles. This response seems to be a hypersensitive-like response that leads to cell death six days after nematode infection.