Transcriptome analysis of mistletoe (Viscum album) haustorium development

Abstract

The haustorium is a specific organ for penetration of host tissue in parasitic plants. To better understand the molecular mechanisms of the establishment of host-parasite relationships using gene expression profiles, early stage haustorium germination from mature mistletoe seeds was examined. We have generated a cDNA library that is an excellent source of expressed sequence tags (ESTs) specifically related to haustorium development and host penetration. We analyzed 4,771 ESTs derived from a mistletoe (Viscum album) haustorium cDNA library. Cluster analysis identified 727 contigs (23.88%) and 2,317 singletons (76.12%), yielding a total of 3,044 unique genes. Among these genes, 1,760 clones (57.8%) showed significant similarity to known genes; the remaining 1,284 clones were novel. Annotation of genes with Gene Ontology indicated that the most abundant ESTs reveals that defense against biotic / abiotic stresses, primary metabolic processes, cell wall loosening and modification is critical for haustorium development and establishment of the host-parasite connection. Especially, it is likely that xyloglucan endotransglycosylases (XETs), glucanase, expansins and other cell wall hydrolases cooperate in cell wall modification during the stages of host-parasite connection. Functional expression of full-length forms corresponding to target ESTs was applied to characterize haustorium genes. This EST information will help gene discovery and characterization and provide promising targets for genetic engineering. The EST analysis presented here represents the first reported transcriptome data for V. album var. coloratum. Although the number of ESTs analyzed may not be sufficient to completely elucidate the mechanism of the host-parasite connection, the integrated approaches reported here represent an essential step toward understanding haustorium development in parasitic plants and provide a valuable resource for defining molecular mechanisms in the host-parasite connection.