Figure 7. Model summarizing the evidence emerging from immunolocalization and functional experiments.
Gastrodermal cell A depicts an aposymbiotic host cell in the process of symbiont acquisition. Results indicate that the addition of soluble TSR proteins promotes and enhances symbiont colonization. We suggest that secreted host TSR proteins may interact with MAMPs and/or secondary proteins to promote tolerance and initiate phagocytosis. Peptide experiments provide evidence against the hypothesis that membrane-linked host TSRs are serving as PRRs to promote inter-partner recognition; we hypothesize that host TSR proteins are secreted rather than membrane-anchored (see discussion text for further explanation). Gastrodermal cell B depicts a symbiotic host cell. Fluorescence microscopy suggests that TSR proteins are expressed within the host-derived symbiosome membrane complex and are concentrated around the symbionts within host gastrodermal tissue.