Regulation of pentosan biosynthesis in barley aleurone tissue by gibberellic acid

Summary Treatment of isolated barley aleurone layers with gibberellic acid (GA₃) resulted in a progressive inhibition of cell-wall synthesis after a 4-h lag period. The incorporation of both [¹⁴C]arabinose and [¹⁴C]glucose into the cell wall was inhibited by GA₃, but analysis of the labelled sugars in the polymerized product showed that the process most affected by the hormone treatment was pentosan biosynthesis. Labelling kinetics and pulse-chase analysis indicated that the pentosans were synthesized in the cytoplasm and subsequently transferred to the cell wall; GA₃ did not significantly affect the latter step. The GA₃-inhibited labelling of the cell-wall pentosans cannot be explained on the basis of an effect on uptake of radioactive cell-wall precursor, expansion of the free pentose pool, or degradation of newly-formed pentosan. GA₃ inhibited the activity of a membrane-bound arabinosyl transferase present in the aleurone layers. This inhibition may explain the inhibition of cell-wall pentosan synthesis by GA₃.Abbreviations GA gibberellin - GA₃ gibberellic acid