Life cycle of the Root-Knot Nematode Meloidogyne javanica in papaya and histological analyses of root infection and gall development.

Papaya ( Carica papaya L.) is susceptible to attacks by root-knot nematodes (RKN), which lead to significant production losses. Understanding the life cycle of RKN in papaya is essential for developing effective control strategies and screening for natural resistance in papaya cultivars. In this study, the development of the RKN Meloidogyne javanica was assessed in the susceptible papaya variety Solo8 over a period of 35 days, using microscopic observation of inoculated roots stained with fuchsin. By 9 days after inoculation (DAI), second-stage juveniles (J2s) were observed migrating through the root cortex and feeding within the vascular cylinder. At 16 DAI, galls containing enlarged J2s had formed within the stele, where feeding sites were established. Young females began appearing within the galls at 26 DAI, and mature pear-shaped females with egg masses were present by 35 DAI. Remarkably, some egg masses were deposited within the cortex, where a new generation of J2s hatched inside the root. Histological cross-sections of galls revealed that feeding sites initiated around the nematode head within the stele by 9 DAI, with four to eight multinucleated giant cells (GCs) present at these sites. These GCs, located within xylem and protophloem elements, were progressively encased by additional cell layers from adjacent vascular and cortical tissues as the nematode developed within the gall at 16, 26, and 35 DAI. The GCs reached maximum size by 16 DAI. Cytoplasmic analysis showed that GCs were rich in proteins, as evidenced by Naphtol Blue Black staining, and that their cell walls were strongly stained for polysaccharides using PAS. This study offers comprehensive histological insights into nematode development within papaya roots, underscoring that screening papaya genotypes for RKN resistance should consider egg mass production within the root tissue.