A Germin-Like Protein with Superoxide Dismutase Activity
in Pea Nodules

The formation of the nitrogen fixing symbiotic root nodule in legumes involves considerable developmental changes. These require the remodelling of plant cell walls and hydrogen peroxide is thought to be involved in this process. In the search for enzymes capable of producing hydrogen peroxide, we have identified a germin-like protein (PsGER1) that converts superoxide to hydrogen peroxide and dioxygen. Few germin-like proteins are known to possess catalytic activity but PsGer1 joins the small number that have superoxide dismutase activity. Its properties are also consistent with it being manganese-dependent, like the related oxalate-degrading enzymes of the cupin superfamily. The true germins are associated with oxalate oxidase activity, but PsGER1 does not possess this activity. PsGER1 possesses a predicted signal sequence for export to the cell wall, consistent with its possible role. It was surprising to find that the N-terminus of the mature PsGER1 protein sequence was identical to that of a putative rhicadhesin receptor, which was thought to be required in the initial bacterial attachment to root hairs. However, the location of PsGER1 expression was not predominantly epidermal, casting doubt on the link between this protein and such a receptor. This was a collaboration with Nick Brewin.