Host reaction of Aloe vera infected by Meloidogyne incognita and M. javanica in Crete Island (Greece)

Abstract

Root-knot nematodes (Meloidogyne incognita and M. javanica) were found infecting cultivated aloe (Aloe vera) plants on the island of Crete, Greece. The infected plants showed reddish brownish leaf discolouration associated with poor plant growth, and roots with dark necrotic areas and irregular thickening but without the typical root-knot nematode galls. Females and egg masses were embedded deeply inside the root. Nematodes were identified using PCR assays either of females, juveniles and males directly obtained from aloe roots and surrounding soil, or of females generated by culturing the nematodes from the aloe on potted tomato plants. Histopathological studies showed female and juvenile bodies and feeding sites surrounded by darkened cells, suggesting the presence of phenolic contents at high levels. Transverse and longitudinal sections confirmed that this reaction was not related to a hypersensitive response because the feeding site was established, but that giant cell formation was accompanied by phenolic compound accumulation. Egg masses isolated from aloe roots were placed in water but juveniles did not hatch. Inspection under the light microscope showed different levels of darkening, with the majority of eggs undeveloped or degenerated. Additionally, PCR reactions which were successful for females failed in these egg masses, suggesting DNA degradation or presence of inhibitory compounds. These results indicate a possible plant defence mechanism impairing viability and development of eggs which are laid inside the aloe roots. It is suggested that the females either produced detoxification enzymes or expressed higher tolerance levels to toxic compounds and thus their development was not impaired.