Developmental biochemistry of cottonseed embryogenesis and germination : XVII. Developmental expression of genes for the principal storage proteins.

The developmental time period and the magnitude of expression of the genes for the principal cottonseed storage proteins have been measured by several means. RNA was extracted from cotton cotyledons at stages during embryogenesis and the relative amounts of the mRNAS for these proteins were determined by cell-free translation in the wheat germ system and by dot and northern hybridization of the RNA with cloned cDNA probes representing the three subfamilies of the major storage protein genes. The rates of reassociation in solution of some of the RNAs with one of the cDNA clones were also determined. Data from all four procedures show that the storage protein mRNAs are demonstrable in very small embryo cotyledons, rapidly reach a high abundance level that is maintained during most of embryo growth, and then fall precipitously in amount in the last days of embryogenesis. The expression of all three gene subfamilies appears coordinate.Further, cDNA reverse transcribed from the poly(A)(+) mRNA from a stage of maximum storage protein synthesis was hybridized to saturation with cDNA clones representing each of the subfamilies. These data indicate that the mRNAs for two of the families reach the same relative level in the total mRNA population which is about 15% of the total mRNA mass. The mRNA of the third subfamily comprises only 5% of the total mRNA mass at this stage. This apparent 3∶3∶1 ratio of mRNAs does not change during the period of storage protein synthesis. Based on the amounts of the storage protein species in the mature seed, the mRNAs of each subfamily appear to be translated to the same extent during embryogenesis.