Analysis of Cell Wall Synthesis Genes in Feeding Cells Formed by Root-Knot Nematodes.

Root-knot nematodes (Meloidogyne sp.) are sedentary endoparasites that infect roots of a wide range of plant species and cause considerable economic loss to many crops. Root-knot nematodes (RKN) transform selected root vascular cells into enlarged, multinucleate feeding sites called giant-cells that arise from repeated karyokinesis without cytokinesis. Giant-cells undergo extensive modifications of the cell wall architecture including cell wall thickening and the formation of ingrowths that act to increase the surface area of the plasma membrane to facilitate solute uptake by the nematode. Extensive cell division is stimulated around the giant-cells to give rise to the root gall that is characteristic of RKN infection. Extensive cell wall modifications taking place in feeding cells are hypothesized to be mediated by both cell wall-loosening and cell wall biosynthetic enzymes of plant origin based on evidence that nematodes alter gene expression in plants during formation of feeding cells. Ten members of the cellulose synthase (CesA) gene family of Arabidopsis thaliana were analyzed to monitor cell wall deposition in RKN infection sites. CesA gene promoter:: GUS constructs and developmental quantitative RT-PCR indicated that CesA genes responsible for both primary and secondary cell wall synthesis were temporally and quantitatively expressed in the same pattern, with peak activity in RKN infection sites at five days post-inoculation. Sections of RKN infection sites in CesA promoter:: GUS roots indicated that upregulated secondary cell wall CesA genes were localized within the central giant-cells and primary cell wall CesA genes were primarily localized to the surrounding dividing cells (gall tissue) of the infection site. The number of galls and RKN female development were decreased in Arabidopsis mutants in eight of the CesA genes, and complementation studies with the constitutive 35S promoter restored the mutant phenotypes of CESA4, CESA5, and CESA7 (involved in secondary cell wall synthesis) and also restored normal RKN infection levels. Mutant complementation of the CESA4, CESA5, and CESA7 genes with the giant-cell-inducible NtCel7 promoter had limited effects on mutant plant phenotype and RKN infection rates, but the development of successful infective RKN females was increased dramatically. The combined data support a critical role for plant CESA gene activity working in consort to generate the proper root morphology to promote nematode infection and for the development of feeding cells to support nematode growth and reproductio