Maize embryo maturation proteins comprise 9 to 11 polypeptide species when separated by 2D IEF-SDS PAGE, resolving over a range in pH of 5.0-8.0 and in molecular weight of 10,000-100,000. Individual species (A-K) have approximate pHs and molecular weights, respectively: A (7.7, 66,000); B (7.2, 66,000); C (6.8, 52,000); D (5-5, 39,000); E (6.4, 29,000); F (6.6/6.5, 8 1,000); G (6.3, 34,000); H (7-7, 73,000); J (7.4, 73,000), and K (7.2, 73,000). Although the precise age at which synthesis is initiated varies among the different polypeptides, synthesis of all species occurs within a 5 to 10 day period beginning 20-25 days after pollination (DAP) under field conditions. Maturation protein synthesis has been observed as early as 26 DAP in embryos from greenhouse-grown plants.
Following their initial detection, the maturation proteins rapidly become the most prominently synthesized species of embryo polypeptides, and high levels of synthesis relative to other polypeptides are maintained until maturity at 40-50 DAP In studies involving 3 inbred cultivars (Oh43, M14, W23) and hybrids derived from their reciprocal crosses, developing embryos of all genotypes exhibited a similar pattern of maturation protein synthesis. Isoelectric variation between cultivar was detected for only 1 polypeptide, whose assignment to the maturation protein group is tentative.
Although a number of polypeptides are common to both embryo and seedling tissues, the maturation proteins are embryo specific. They are not observed on fluorograms from radicles, mesocotyls or plumules of 5 day old seedlings or from germinating embryos between 4 and 6 hours of imbibition. Maturation proteins appear to be present in tissues of both the scutellum and embryonic axis since they are observed in excised scutella and are not noticeably reduced in embryonic axes from which most of the scutellum has been removed.
With the exception of 1 species (E), maturation proteins cannot be identified among the products obtained from the in vitro translation of either total cellular RNA or intact polysomes when separated under conditions similar to those employed for in situ extracts. These results suggest that translational or post-translational controls may be involved in maturation protein expression. However, the in vitro translation of total cellular RNA in the presence of a dog pancreas microsomal membrane post-translational modification system does not result in the expression of additional maturation proteins.
J.G. Boothe* and D.B. Walden
*Current address: Plant Research Institute, Saskatoon, Sask.
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