Pathology of Meloidogyne — Galling, Giant Cell Formation, Effects on Host Physiology

Gall Formation. The hypothesis that the localized tissue reaction to invasion by infective larvae results from the introduction of growth regulators by the nematodes is put forward. Critical evidence in support of the hypothesis is lacking, but there is some evidence that auxins are present in nematodes. It is suggested that Meloidogyne respond to a signal, from the plant, to release the galling stimulus. Giant Cell Formation. The organization of a localized source of proteins in the giant cells under the direction of the developing, female nematode is the outstanding characteristic of Meloidogyne infections. A number of possible mechanisms are considered. A. The nematode may remove important end‐products such as amino acids or peptides. B. The developing larva may introduce specific regulators of protein synthesis, such as nucleic acids. C. Growth regulators are known to control protein synthesis. D. The nematode may affect the permeability of the membranes, increasing the flow of important ions into giant cells. E. The carbohydrases and proteolytic enzymes of Meloidogyne may affect the patterns of wall formation and nuclear division. F. A specific inhibitor of cell wall formation could account for many of the observations. Effects on Host Physiology. An increased ratio of root/shoot weights is characteristic of Meloidogyne infections. The nematode affects top growth by altering the traffic patterns of export and import between root and stem. In the field, this is manifest as increased susceptibility to water stress, reduced top growth, and especially, increased damage to roots by other micro‐organisms.