Protein degradation and nitrogen supply during germination and seedling growth of vetch ( Vicia sativa L.)

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Published reports on storage protein degradation during and after legume seed germination have nearly exclusively been focussed on the cotyledons as storage organs. There, concomitantly with an increase in proteolytic activity, globulin degradation starts no earlier than day 2 to 3 after the beginning of imbibition (dai). Proteinases that intiate storage protein degradation in legume cotyledons seem to be de novo synthesized. Since protein biosynthesis begins as early as during the first hour of imbibition (dai) legume seeds should initially utilize amino acid sources other than storage protein degradation in the cotyledons to support protein formation in the axis and cotyledons. Using germinating vetch ( Vicia sativa L.) we started a thorough analysis of the temporal and histochemical pattern of storage protein degradation and cysteine proteinase activity in axis and cotyledons during germination and early seedling growth. The amount of protein does not significantly change in the embryonic axis and cotyledons during the two initial days of germination. Afterwards, total protein increases in the axis but decreases in the cotyledons. Similar to the cotyledons, the axis contains storage globulins in protein bodies. In the axis globulin degradation rapidly commences after the beginning of imbibition and it is nearly finished by 2 dai, whereas no significant globulin degradation occurs in the cotyledons. During this time there are no conductive tissue connections between the axis and the cotyledons. By the time protein reserves in the axis are exhausted after 2 dai conductive connections have been formed, and now globulin breakdown commences in the cotyledons. Independent of when globulin breakdown starts, vicilin degradation predominates in the beginning, and at first this globulin is locally degraded in those tissues in the regions of the embryo where growth and differentiation start. Molecular and biochemical analysis of five different cysteine proteinases confirmed that in cotyledons these enzymes are formed de novo at a time when globulin degradation becomes visible. Stored proteinases could also play a role in the rapid commencement of protein breakdown in the embryonic axis. The results suggest that during the first 2 dai protein biosynthesis in the axis is mainly supported by the degradation of internal protein reserves and that only after these have been exhausted globulin breakdown in the cotyledons takes over to provide amino acids to the growing axis.