Ethylene signaling pathway modulates attractiveness of host roots to the root-knot nematode Meloidogyne hapla.

Infective juveniles of the root-knot nematode Meloidogyne hapla are attracted to the zone of elongation of roots where they invade the host but little is known about what directs the nematode to this region of the root. We found that Arabidopsis roots exposed to an ethylene (ET)-synthesis inhibitor attracted significantly more nematodes than control roots and that ET-overproducing mutants were less attractive. Arabidopsis seedlings with ET-insensitive mutations were generally more attractive whereas mutations resulting in constitutive signaling were less attractive. Roots of the ET-insensitive tomato mutant Never ripe (Nr) were also more attractive, indicating that ET signaling also modulated attraction of root-knot nematodes to this host. ET-insensitive mutants have longer roots due to reduced basipetal auxin transport. However, assessments of Arabidopsis mutants that differ in various aspects of the ET response suggest that components of the ET-signaling pathway directly affecting root length are not responsible for modulating root attractiveness and that other components of downstream signaling result in changes in levels of attractants or repellents for M. hapla. These signals may aid in directing this pathogen to an appropriate host and invasion site for completing its life cycle.

A sequence-anchored linkage map of the plant-parasitic nematode Meloidogyne hapla reveals exceptionally high genome-wide recombination.

Root-knot nematodes (Meloidogyne spp.) cause major yield losses to many of the world's crops, but efforts to understand how these pests recognize and interact with their hosts have been hampered by a lack of genetic resources. Starting with progeny of a cross between inbred strains (VW8 and VW9) of Meloidogyne hapla that differed in host range and behavioral traits, we exploited the novel, facultative meiotic parthenogenic reproductive mode of this species to produce a genetic linkage map. Molecular markers were derived from SNPs identified between the sequenced and annotated VW9 genome and de novo sequence of VW8. Genotypes were assessed in 183 F2 lines. The colinearity of the genetic and physical maps supported the veracity of both. Analysis of local crossover intervals revealed that the average recombination rate is exceptionally high compared with that in other metazoans. In addition, F2 lines are largely homozygous for markers flanking crossover points, and thus resemble recombinant inbred lines. We suggest that the unusually high recombination rate may be an adaptation to generate within-population genetic diversity in this organism. This work presents the most comprehensive linkage map of a parasitic nematode to date and, together with genomic and transcript sequence resources, empowers M. hapla as a tractable model. Alongside the molecular map, these progeny lines can be used for analyses of genome organization and the inheritance of phenotypic traits that have key functions in modulating parasitism, behavior, and survival and for the eventual identification of the responsible genes.