Some effects of analogues of uracil on cell elongation and wall metabolism in excised pea root segments.

Excised root segments of Pisum sativum (L.) cut from the region 2-4 mm behind the root tip were cultured in a 2% sucrose medium containing analogues of uracil and proline. Of several uracil analogues tested only those containing a thiol group (2-thiouracil and 2-thio-6-azauracil) markedly stimulated the growth rate and prolonged the duration of growth of the segments, whereas other uracil analogues which enhanced growth affected only its duration. Uracil had no effect on cell elongation and did not completely prevent the effects of 2-thiouracil and 2-thio-6-azauracil; it did, however, prevent the stimulation by those analogues without a thiol group. Two analogues of proline, thioproline and hydroxyproline, also enhanced cell elongation but whereas the effect of hydroxyproline was completely prevented by proline, the stimulation produced by thioproline was not.During cell elongation, [(14)C]thiouracil was incorporated into RNA, where it replaced uracil, and into a non-nucleotide fraction of the cell wall from which it could not be removed with perchloric acid, sodium hydroxide, ribonuclease or pronase. Experiments using labelled thiouracil, orotic acid, leucine, proline and hydroxyproline strongly suggest that 2-thiouracil stimulates the growth rate of segments by becoming attached to cysteine in cell-wall proteins and delaying an increase in wall rigidity caused by the formation of disulphide bridges between proteins.