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Endosperm Protein Synthesis and L-[³⁵S]Methionine Incorporation in Maize Kernels Cultured In Vitro¹

Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, Minnesota 55108, Department of Genetics and Cell Biology, University of Minnesota, St. Paul, Minnesota 55108

This study was conducted to examine protein synthesis and L-[³⁵S] methionine incorporation into the endosperm of Zea mays L. kernels developing in vitro. Two-day-old kernels of the inbred line W64A were placed in culture on a defined medium containing 10 microCuries L-[³⁵S] methionine per milliliter (13 milliCuries per millimole) and harvested at 10, 15, 20, 25, 30, 35, and 40 days after pollination. Cultured kernels attained a final endosperm mass of 120 milligrams compared to 175 milligrams for field-grown controls. Field and cultured kernels had similar concentrations (microgram per milligram endospern) for total protein, albumin plus globulin, zein, and glutelin fractions at most kernel ages.

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and isoelectric focusing patterns for endosperm proteins were similar for field and cultured kernels throughout development. By 15 days, over 70% of the L-[³⁵S]methionine taken up was present in endosperm proteins. Label incorporation visualized by fluorography generally followed the protein intensity of the stained gels. The high methionine content, low molecular weight zeins (i.e. 15 and 9 kilodaltons) were highly labeled. All of the radioactivity in hydrolyzed zein samples was recovered in the methionine peak indicating minimal conversion to L-[³⁵S]cysteine. The procedure described here is suitable for long term culture and labeling experiments in which continued kernel development is required.

³ Present address: Department of Biochemistry and Biophysics, Texas A & M University, College Station, TX 77840.

¹ Supported by the United States Department of Agriculture under Grant No. 5901-0410-8-0176 and 59-2271-0-1501 from the Competitive Research Grants office and by the Minnesota Agriculture Experiment Station. Paper No. 13519 Scientific Journal series.

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