Biosynthesis of Nonspecific Lipid Transfer Proteins in Germinating Castor Bean Seeds

The biosynthesis of nonspecific lipid transfer proteins (ns-LTPs) in germinating castor bean (Ricinus communis L.) seeds were investigated. Lipid transfer activities of ns-LTPs in the cotyledons, axis, and endosperm increased with growth after germination. The activity increases were accompanied by increased amounts of ns-LTPs in each tissue, as measured by immunoblot using anti-ns-LTP serum. These results suggest that the ns-LTPs are synthesized de novo in each tissue after germination and not activated from inactive proteins synthesized before germination. Comparison of the immunoblot products in each tissue from 4-day-old seedlings indicate the occurrence of tissue-specific isoforms of ns-LTPs; 9 kilodaltons (major) and 7 kilodaltons (minor) in the cotyledons, and 7 kilodaltons (major) and 9 kilodaltons (minor) in the axis, whereas only the 8-kilodalton ns-LTP is present in the endosperm. In vitro translation from poly(A)(+) RNAs from three tissues of castor bean seedlings and the detection of immunoprecipitated products indicate that translatable mRNAs for ns-LTPs exist in the three tissues a day before the synthesis of ns-LTPs; the translation products, which are 3.5 to 4.0 kilodaltons larger than ns-LTPs, were processed to the mature ns-LTPs. The production of mature ns-LTPs from translatable mRNAs without any delay suggests that gene expression of ns-LTPs in castor bean seedlings is controlled at a step before the formation of translatable mRNAs.