Comparative gene expression analysis of differentiated terminal and lateral haustoria of the obligate root parasitic plantPhelipanche ramosa(Orobanchaceae)

A bstract Branched broomrape ( Phelipanche ramosa , Orobanchaceae) is the most important parasitic weed of oilseed and vegetable crops in Europe. Deciphering the parasite’s basic biology and genetics underpinning adaptations to parasitism are essential for effective weed management. Here, we compared the gene expression profiles of differentiated terminal haustoria, formed immediately after the germination of P. ramosa with those of the parasite’s adventitious roots and lateral haustoria, which develop on the latter as it grows. Principal component analysis and functional annotation of differentially expressed genes indicate a greater transcriptional similarity between adventitious roots and lateral haustoria compared to terminal haustoria. Genes involved in hydrogen peroxide catabolic processes and mucilage metabolic processes were more abundant in lateral haustoria compared to adventitious roots, indicating that secondary host attachment involves secretion of adhesive compounds, cell wall modification, and a termination of the developmental program associated with root growth. In terminal haustoria, phytohormonal signaling pathways for post-germination growth and meristem development were the most prominent expression profile difference to lateral haustoria. Besides this, wide similarity of expressed genes between lateral and terminal haustoria suggest overlapping pathways underlying haustorium differentiation and a conserved functional relevance of both haustoria types at maturity. Together, our study provides first insights into the transcriptional landscapes of the primary organs securing P . ramosa ’s nutritional success. However, more research is needed to investigate whether the parasite’s various haustorium types differ in their sensitivities to environmental stimuli and whether transcriptional divergence in terminal and lateral haustoria of P. ramosa reflect differences regarding their developmental origin.