Annals of Botany

The gall of the Lotus eaters

One of the lesser-known aspects of plant pathology is the parasitism of plants by nematodes, estimated to cause annual yield losses worth $100 000 000. Interactions between parasite and host cause various modifications to plant morphology/anatomy; here we focus on parasitism by “root-knot nematodes”. In this type of plant-nematode interaction, infection leads to the formation of giant cells adjacent to the vascular cylinder, surrounded by a gall or root-knot, as described by Poch et al. (Harpenden, UK, pp. 1223–1229). Formation of the giant cells triggers, in some way, the development of the nematodes from juvenile to adult female forms, which feed on nutrients from the giant cells. Intriguingly, the cells are only maintained if the nematode continues feeding. Thus, there is a two-way interaction between the plant and its parasite. The authors are interested in the extent to which these interactions vary in relation to the success of the parasite; they have investigated this in a range of ecotypes, 60 in all, of the legume Lotus japonicus. Plants were inoculated with equal numbers of stage-2 juveniles of Meloidogyne incognita, all hatched from the same egg mass. Plants were scored for gall (root-knot) formation and for nematode egg production 6 weeks after inoculation. There was an over 100-fold range in susceptibility, with some ecotypes sustaining very few galls and others several hundred. Further, the number of nematode egg masses was highly correlated with the number of galls formed (r=0.794), indicating the necessity of gall formation for nematode reproduction (although in four ecotypes the two processes were not tightly linked). Interestingly, neither gall formation nor egg production showed a plateau when ecotypes were arranged in rank order. This suggests that there may be even more susceptible ecotypes. At the other end of the scale, the highly resistant ecotypes are likely to provide a genetic resource for plant breeding.