AKA, SYMbiosis-Receptor-Kinase gene.

About 80% of all land plants have a symbiotic relationship with fungi of the phylum Glomeromycota. The fungus penetrates cells in the plant’s roots, and provides the plant with phosphates and other nutrients from the soil. This kind of symbiosis is called an arbuscular mycorrhiza, and evolved more than 400 million years ago. Professor Martin Parniske and colleagues started their study by looking at genes known to be involved in arbuscular mycorrhiza, to see whether they could find evidence of any specific genetic differences in plants that form symbioses also with nitrogen-fixing bacteria. . .

It had already been speculated that genes involved in the arbuscular mycorrhiza symbiosis might have been recruited for nodulation, as these symbioses both involve intracellular relationships. One clue was that several genes, including the so-called “symbiosis-receptor-kinase-gene” (SYMRK), are involved in a genetic program that links arbuscular mycorrhiza and one form of bacterial nodule symbiosis. And the analysis of SYMRK in several species of plant provided the striking evidence that Parniske and his colleagues had been hoping for.

“Our results reveal that an expansion of the functions of SYMRK constituted an important step in the evolution of intracellular nodule symbiosis,” reports Parniske. Most plants have a short version of SMYRK, which is required for AM symbiosis. A longer variant of SMYRK was found only in plants involved in the symbiotic relationships with nitrogen-fixing bacteria. Importantly, the longer version was found in both legumes (which form symbioses with rhizobia – the textbook nitrogen-fixing symbiosis) and in actinorhiza (such as alder) which form symbiotic relationships with frankia bacteria . . .

Interesting, but so what? . . . you might ask. It's a clue that might help discover ways to make some of those 80% of all land plants that link up with arbuscular mycorrhiza also link up with rhizobia or frankia. Then they'd have improved sources of nitrogen as well as phosphorous.