Protein Metabolism in Cultured Plant Tissues

Rates of protein synthesis in normal callus tissues (either tight or loose morphological form), in crown gall callus tissues and in cultured pith cells were measured for both the lower surface cells (those in contact with the original growth medium) and upper surface cells (those never in contact with the growth medium until labeling). Cells of both surfaces of loose and crown gall callus and the upper‐surface cells of tight callus had similar rates of protein synthesis, 29–31 mg of protein synthesized × (g protein) −1 × h −1 . The lower surface cells of tight callus had a 35% lower rate of synthesis, 20 mg × g −1 × h −1 . Pulse‐chase experiments suggested that rates of protein degradation for all tissues were the same, 21–23 mg protein × (g protein) −1 × h −1 . Thus, there probably was no accumulation of protein in the lower surface cells of tight callus tissue, but the other tissues had rates of accumulation equaling 10 mg × (g protein) −1 × h −1 . Autoradiography and electron‐microscopic examination of cells in tight callus labeled with 3 H‐leucine show that: (a) the lower‐surface cells were more degenerate than cells within the callus or on the upper surface; and (b) the first few cell layers nearest the medium were preferentially labeled. Pulse‐chase experiments were also used to quantitate the nonprecursor pool (defined as that tritium in the soluble amino acid pool that does not equilibrate with protein during a pulse‐chase experiment). The nonprecursor pool increased linearly with time at the same rate as incorporation of 3 H‐leucine into protein. Furthermore, the nonprecursor pool copurified with leucine and was probably either D‐ or L‐leucine.