Phosphomannose Isomerase, A Novel Selectable Plant Selection System: Mode of Action and Safety Assessment

Phosphomannose isomerase (PMI), an enzyme not present in many plants, catalyzes the reversible interconversion of mannose 6-phosphate and fructose 6-phosphate. Plant cells lacking this enzyme are incapable of surviving on synthetic medium containing mannose. Plants derived from sugar beet and corn cells that had been genetically modified to express the E. coli manA gene encoding PMI were evaluated for biochemical changes in mannose-associated pathways. No detectable changes in glycoprotein profiles were detected in PMI-transformed plants as compared to non-transgenic controls. Purified PMI protein was readily digestible in a simulated gastric environment. Purified PMI protein demonstrated no adverse effects in an acute mouse oral toxicity study. The yield and nutritional composition of grain from PMI-transformed corn plants compared to their non-transformed isogenic counterparts were also determined and no statistically significant differences were found. These results indicate that PMI is an ideal selectable marker for plant transformation.