Protein synthesis and leucine uptake during fusicoccin-stimulated growth of oat coleoptile tissue

Abstract

Avena coleoptile sections were incubated with or without cycloheximide (CHI) in solutions containing indoleacetic acid (IAA), fusicoccin (FC), or IAA and FC. Resulting growth, incorporation of [3H]leucine into protein, and uptake of [3H]leucine into an ethanol-soluble fraction were determined. FC-stimulated growth was greater and less sensitive to CHI treatment than IAA dependant growth which was virtually eliminated by CHI. IAA alone had little or no influence on [3H]leucine utilization, whereas FC treatment stimulated [3H]leucine incorporation into protein by 36% and uptake into the ethanol-soluble fraction by 75%. CHI inhibited the incorporation of [3H]leucine label into protein such that no significant difference was observed in radioactivity in protein from control, FC-, or IAA-treated tissue. However, CHI did not inhibit the FC-stimulated uptake of [3H]leucine into the ethanol soluble fraction. Total uptake of [3H]leucine obtained from the sum of radioactivities in the protein and ethanol-soluble fractions was stimulated by FC approximately 54% in the absence of CHI and 92% in the presence of CHI. IAA in combination with FC stimulated a further increase in leucine uptake. It is proposed that IAA and FC stimulate growth through processes which differ in their dependency on protein synthesis, and that FC-stimulated incorporation of label into protein results from FC-stimulated leucine uptake, not FC-stimulated protein synthesis.