DEFOLIATION-INDUCED CHANGES IN ABUNDANCE AND IMMUNO-LOCALIZATION OF VEGETATIVE STORAGE PROTEINS IN TAPROOTS OF MEDICAGO-SATIVA

Herbage regrowth after defoliation of forage species involves the mobi lization of nitrogen from source organs (i.e. roots and crown) to regr owing shoot tissues. Changes in soluble protein content during regrowt h were followed in alfalfa (Medicago sativa L., cv Europe) taproots, b oth biochemically and microscopically at the tissue and subcellular le vels. After 6 and 10 days of shoot regrowth, 33 and 38% respectively o f soluble proteins initially present in taproots were hydrolyzed. SDS- PAGE analysis of proteins showed the presence of three prominent polyp eptide of 15, 19 and 32 kDa molecular mass. These polypeptides represe nted 28% of total soluble proteins extracted from roots on the day of cutting. They underwent rapid and extensive mobilization between days 0 and 6 of regrowth followed by reaccumulating-to 33% of total taproot soluble protein content by day 30. The production of anti -15, -19 an d -32 kDa protein antibodies and their utilization for immunoblotting confirm the results of SDS-PAGE analysis. Ln addition, light microscop y observations, coupled with antibody detection, revealed the same def oliation-induced pattern of protein mobilization/reaccumulation. These results support the view that these 3 polypeptides (15, 19 and 32 kDa ) serve as vegetative storage proteins (VSPs) during alfalfa shoot reg rowth. Immunolocalization revealed the presence of VSPs in parenchyma cells of wood rays, and to a lesser extent, in cells of bark parenchym a of alfalfa taproots. At the ultrastructural level, these 3 VSPs were found specifically in vacuoles as an electron-dense, fibrillar materi al, and on the periphery of starch granules.