Pectin Remodeling and Involvement of AtPME3 in the Parasitic Plant–Plant Interaction, Phelipanche ramosa–Arabidospis thaliana

<i>Phelipanche ramosa</i> is a root parasitic plant fully dependent on host plants for nutrition and development. Upon germination, the parasitic seedling develops inside the infected roots a specific organ, the haustorium, thanks to the cell wall-degrading enzymes of haustorial intrusive cells, and induces modifications in the host's cell walls. The model plant <i>Arabidopsis thaliana</i> is susceptible to <i>P. ramosa</i>; thus, mutants in cell wall metabolism, particularly those involved in pectin remodeling, like <i>Atpme3-1</i>, are of interest in studying the involvement of cell wall-degrading enzymes in the establishment of plant-plant interactions. Host-parasite co-cultures in mini-rhizotron systems revealed that parasite attachments are twice as numerous and tubercle growth is quicker on <i>Atpme3-1</i> roots than on WT roots. Compared to WT, the increased susceptibility in <i>AtPME3-1</i> is associated with reduced PME activity in the roots and a lower degree of pectin methylesterification at the host-parasite interface, as detected immunohistochemically in infected roots. In addition, both WT and <i>Atpme3-1</i> roots responded to infestation by modulating the expression of PAE- and PME-encoding genes, as well as related global enzyme activities in the roots before and after parasite attachment. However, these modulations differed between WT and <i>Atpme3-1</i>, which may contribute to different pectin remodeling in the roots and contrasting susceptibility to <i>P. ramosa</i>. With this integrative study, we aim to define a model of cell wall response to this specific biotic stress and indicate, for the first time, the role of PME3 in this parasitic plant-plant interaction.