Understanding how plants balance between enabling microbial symbionts and fending off pathogens has direct implications both for basic plant biology and optimal use of crop plants in agriculture. The degree to which the processes associated with these two types of interactions overlap is poorly known. Recent studies revealed that symbiotic and pathogenic filamentous microbes require common plant genetic elements to establish colonisation (Wang et al., 2012; Rey et al., 2013), supporting the long-held view that plants have evolved the ability to accommodate microbes (Parniske, 2000) and that pathogens have exploited these pathways. However, the extent to which plant genes implicated in fungal or bacterial symbioses are involved in interactions with biotrophic pathogens is unknown and research has been hampered by the lack of suitable common host experimental systems. P. palmivora, a root-infecting oomycete, is an aggressive biotrophic pathogen of M. truncatula, a model legume plant that is widely used in symbiosis research. Expression of fluorescent proteins in P. palmivora permits visualisation of infection structures in M. truncatula roots. During its initial biotrophic colonisation of M. truncatula roots which lasts until about 48 hours post infection (hpi), P. palmivora zoospores expressing tdtomato red fluorescent protein (isolate AJ-td) germinate and form appressoria to penetrate the epidermis followed by rapid colonisation of the root cortex apoplast and projection of short specialised hyphae, termed haustoria into plant cells (Fig. 1a). P. palmivora infection is accompanied by visible disease development in M. truncatula seedlings consisting of translucent tissue at the root tip at two days post inoculation (dpi) and tissue browning in upper parts of the root at three dpi. (Fig. 1b, see also supplementary material). Concomitant with a switch to a nectrophic lifestyle, the pathogen enters the vasculature (Fig. 1a).