Expanding nematode-induced syncytia: the role of expansins

Parasitism of the cyst nematode Heterodera schachtii is characterised by the formation of syncytial feeding structures within the host root. Syncytia are formed by the fusion of root cells accompanied by local cell wall degradation, fusion of protoplasts and hypertrophy. Expansins are cell wall-loosening proteins involved in growth and cell wall disassembly. In this study we analysed whether members of the expansin gene family are specifically and developmentally regulated during syncytium formation in roots of Arabidopsis thaliana. By screening of a cDNA library of syncytia for expansin transcripts several AtEXPAs and one AtEXPB were found. In a semi-quantitative RT-PCR and a Genechip analysis AtEXPA3, AtEXPA6, AtEXPA8, AtEXPA10 and AtEXPA16 were found to be up-regulated specifically in syncytia but not to be transcribed in surrounding root tissue. Histological analyses including analysis of promoter::GUS lines and in situ RT-PCR supported the specific expression pattern. Among the specifically expressed genes, AtEXPA3 and AtEXPA16 turned out to be of special interest as they are shoot-specific in un-infected plants. We conclude that a number of expansins take part in cell growth and cell wall disassembly during syncytium formation.