Gene Expression and Synthesis of Phytohemagglutinin in the Embryonic Axes of Developing Phaseolus vulgaris Seeds

Abstract

Phytohemagglutinin (PHA), the major seed lectin of the common bean (P. vulgaris), is found largely in the cotyledons, but is also present in the embryonic axis. At mid-maturation, the percentage of total protein synthesis which is directed towards making PHA is 5-10 times greater in the cotyledons than in the axes. This lower rate of synthesis in the axes is correlated with a lower abundance of mRNA for PHA, as determined by dot blot hybridization using a c[complementary]DNA clone for PHA. Manen and Pusztai (1982) claimed on the basis of immunocytochemical evidence that in the axis PHA is found in the cytosol although it is present in protein bodies in the cotyledons. In the cotyledons, PHA is synthesized on rough endoplasmic reticulum, and its transport to the protein bodies via the Golgi complex is associated with specific post-translational processing steps (Vitale and Chrispeels, 1984). A cytosolic localization of axis PHA would be an indication of a different site of synthesis and transport pathway. The results presented here indicate that the site of synthesis of PHA and the posttranslational modification of PHA are the same in the axes as in the cotyledons. Since in the cotyledons these modifications take place in the endoplasmic reticulum, the Golgi, and the protein bodies, the transport pathway and the site of accumulation of PHA in the axes apparently is similar to that in the cotyledons.