Cyst nematodes invade the roots of their host plants as second stage juveniles and induce a syncytium which is their source of nutrients throughout their life. A transcriptome analysis of syncytia induced by the beet cyst nematode Heterodera schachtii in Arabidopsis roots has shown that gene expression in the syncytium is different from that of the root with thousands of genes upregulated or downregulated. Among the downregulated genes are many which code for defense-related proteins. One gene which is strongly downregulated codes for the ethylene response transcription factor RAP2.6. The genome of Arabidopsis contains 122 ERF transcription factor genes which are involved in a variety of developmental and stress responses.
Expression of RAP2.6 was studied with RT-PCR and a promoter::GUS line. During normal growth conditions the gene was expressed especially in roots and stems. It was inducible by Pseudomonas syringae but downregulated in syncytia from a very early time point on. Overexpression of the gene enhanced the resistance against H. schachtii which was seen by a lower number of nematodes developing on these plants as well as smaller syncytia and smaller female nematodes. A T-DNA mutant had a reduced RAP2.6 transcript level but this did not further increase the susceptibility against H. schachtii. Neither overexpression lines nor mutants had an effect on P. syringae. Overexpression of RAP2.6 led to an elevated expression of JA-responsive genes during early time points after infection by H. schachtii. Syncytia developing on overexpression lines showed enhanced deposition of callose.
Our results showed that H. schachtii infection is accompanied by a downregulation of RAP2.6. It seems likely that the nematodes use effectors to actively downregulate the expression of this and other defense-related genes to avoid resistance responses of the host plant. Enhanced resistance of RAP2.6 overexpression lines seemed to be due to enhanced callose deposition at syncytia which might interfere with nutrient import into syncytia.