Symbiosome functionality in Medicago truncatula nodules requires continuous clearing of pectins from the symbiosome space

Central to the legume-rhizobium symbiosis is the formation of organelle-like symbiosomes where nitrogen-fixing bacteroids are enclosed by a host-derived symbiosome membrane. This creates the symbiosome space, which topologically resembles an apoplastic compartment within the cell. While the apoplast of plant cells is largely occupied by the cell wall, symbiosomes are devoid of cell wall polymers. Here, we describe a mechanism that functions to protect and maintain effective nitrogen fixation through the action of cell-wall-degrading enzymes that prevent accumulation of un-esterified pectin within symbiosomes. We identify two symbiotically-induced polygalacturonase (PG) genes in Medicago truncatula, SyPG1 and SyPG2, that are secreted into the symbiosome space. Silencing the expression of SyPG1/2 or editing SyPG1/2 via CRISPR-Cas9 both lead to nodule senescence and trigger excessive accumulation of un-esterified pectin in symbiosome containing cells. Additionally, we show that un-esterified pectins inhibit rhizobial growth both in vivo and in vitro. Together, our results provide evidence for a host-controlled cell wall clearance mechanism that is essential for symbiosome maintenance.