Systematic Mutagenesis of Serine Hydroxymethyltransferase Reveals an Essential Role in Nematode Resistance

Abstract

<i>Rhg4</i> is a major genetic locus that contributes to soybean cyst nematode (SCN) resistance in the Peking-type resistance of soybean (<i>Glycine max</i>), which also requires the <i>rhg1</i> gene. By map-based cloning and functional genomic approaches, we previously showed that the <i>Rhg4</i> gene encodes a predicted cytosolic serine hydroxymethyltransferase (GmSHMT08); however, the novel gain of function of <i>GmSHMT08</i> in SCN resistance remains to be characterized. Using a forward genetic screen, we identified an allelic series of <i>GmSHMT08</i> mutants that shed new light on the mechanistic aspects of <i>GmSHMT08</i>-mediated resistance. The new mutants provide compelling genetic evidence that Peking-type <i>rhg1</i> resistance in cv Forrest is fully dependent on the <i>GmSHMT08</i> gene and demonstrates that this resistance is mechanistically different from the PI 88788-type of resistance that only requires <i>rhg1</i> We also demonstrated that <i>rhg1-a</i> from cv Forrest, although required, does not exert selection pressure on the nematode to shift from HG type 7, which further validates the bigenic nature of this resistance. Mapping of the identified mutations onto the SHMT structural model uncovered key residues for structural stability, ligand binding, enzyme activity, and protein interactions, suggesting that GmSHMT08 has additional functions aside from its main enzymatic role in SCN resistance. Lastly, we demonstrate the functionality of the <i>GmSHMT08</i> SCN resistance gene in a transgenic soybean plant.