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

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Abstract
This study was conducted to examine protein synthesis and L-[35S] methionnine incoporation into the endosperm of Z. 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 .mu.Ci L-[35S] methionine per milliliter (13 mCi 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 mg compared to 175 mg for field-grown controls. Field and cultured kernels had similar concentration (microgram per milligram endosperm) 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.