Phosphomannose isomerase (PMI) 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 as a carbon source. Maize, wheat and barley plants, genetically modified to express the Escherichia coli manA gene (pmi) under the control of a plant promoter, were able to survive selection on mannose-containing medium. Transformation frequencies averaged 45% for maize transformation via Biolisticse, 35% for maize Agrobacterium-mediated transformation, 20% for wheat, 3% for barley, and 2% for watermelon transformation. Moreover, the frequencies exceeded those obtained for maize and wheat using the pat or bar gene with Basta w selection. A preliminary safety assessment has been conducted for PMI. Purified PMI protein demonstrates no adverse effects in an acute mouse toxicity test. Purified PMI protein was readily digested in simulated mammalian gastric and intestinal fluids. Plants derived from sugar beet and corn cells that had been genetically modified to express the E. coli manA gene were evaluated for biochemical changes in mannose-associated pathways. No detectable changes in glycoprotein profiles were detected in PMI-transformed plants as compared to nontransgenic controls. 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. The inherent safety of a system based on simple sugar metabolism coupled with high transformation frequencies for monocots make pmi an ideal selectable marker for plant transformation.
BACKGROUND: Declines due to Papaya ringspot virus (PRSV) are the primary constraint limiting production of papaya (Carica papaya L.) in tropical and subtropical regions. Given the lack of suitable sources of resistance in the genus, transgenic papayas carrying the coat protein gene of a local strain of PRSV were recently developed to manage the disease in Jamaica. For assurance of food safety, a 90 day subchronic whole-food feeding study in rats was conducted with transgenic and non-transgenic papayas.
Transformation of agricultural crops with novel genes has significantly advanced disease-resistance breeding, including virus resistance through the expression of virus sequences. In this study, the effects of long-term, repeated exposure to transgenic papayas carrying the coat protein gene of Papaya ringspot virus and conventional non-transgenic papaya on the histology and selected biochemical parameters of the intestinal tract were compared. For 3 months, male and female Wistar rats received diets containing transgenic or non-transgenic papaya at twice the equivalent of the average daily consumption of fresh papayas. Gross and macroscopic appearance of intestinal tissues, as well as stomach tissues, was comparable (P < 0.05) as were total intestinal bacterial counts and activities of beta-glucuronidase. Activities of disaccharidases were not affected, neither were those of amylase (P < 0.05). Although significant differences were noted in the activity of Ca(2+) and Na(+)/K(+) ATPase brush border enzymes, no morphological alteration in the integrity of the intestinal mucosa was found. Overall, negligible effects on feed intake, body weight, and fecal output were observed (P < 0.05). Taken together, long-term exposure to diets formulated with transgenic papaya did not result in biologically important unintended effects.
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