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Influence of Auxin and Incubation on the Relative Level of Polyribosomes in Excised Soybean Hypocotyl ¹

^a Department of Botany, University of Georgia, Athens, Georgia 30602

The influence of incubation and auxin (2,4-D) on polyribosome level in soybean hypocotyl was studied.

A marked drop in the relative level of polyribosomes in excised apical or meristematic tissue (0 to 5 millimeters below the cotyledons) occurred during incubation. The addition of auxin to the incubation medium did not affect polyribosome level. A similar decrease in polyribosome level occurred in excised elongating tissue (5 to 15 millimeters below cotyledons) during incubation. Auxin, however, caused a small but highly reproducible stabilization of polyribosomes in this tissue. There was a rapid, but small, auxin-independent increase in polyribosomes of basal or nongrowing hypocotyl (from 20 to 40 millimeters below cotyledons) during incubation, followed by a larger auxin-dependent increase in polyribosomes. While auxin is known to cause an increase in total ribosomes during incubation of the excised basal hypocotyl, the observed transformation from monoribosomes to polyribosomes was not dependent on new ribosome synthesis.

Protein synthetic activity (poly U-directed phenylalanine incorporation) of the 80S monoribosomes at low Mg²⁺ levels increased during incubation of the excised basal hypocotyl. The increase in ribosome activity was biphasic (an initial auxin-independent phase followed by an auxin-dependent increase in activity) correlating with the biphasic increase in polyribosomes. The enhanced activity of 80S monoribosomes was related, at least in part, to an increase in the level of peptidyl-tRNA associated with the ribosome population. Removal of peptidyl-tRNA from the ribosomes reversed the auxin effect.

The hypothesis is advanced that the increase in polyribosomes in response to incubation and to auxin is preceded by and dependent upon the activation of 80S monoribosomes. This activation is in addition to a requirement for continued RNA synthesis, at least in part mRNA, for the transition from monoribosomes to polyribosomes.