Regulation of pentosan biosynthesis in barley aleurone tissue by gibberellic acid

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 [(14)C]arabinose and [(14)C]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.