SOLUBLE CELL-WALL COMPOUNDS FROM CARROT ROOTS INDUCE THE PICA AND PGLLOCI OF AGROBACTERIUM-TUMEFACIENS

Crude extracts from carrot roots induce the picA chromosomal locus of Agrobacterium tumefaciens. The inducer is a complex pectic polysacchar ide whose activity can be destroyed by incubation with a highly purifi ed cloned pectate lyase (PelE protein of Erwinia chrysanthemi EC16). O ligogalacturonates of degree of polymerization (dp) 6-17 induce the pi cA locus, with peak activity at dp 8. However, the inducing compound p artially purified from carrot root extracts is about 100-fold more act ive (per uronic acid content) than is the most active oligogalacturona te in inducing the picA locus. Chemical linkage analysis of a QAE Seph adex fraction containing peak inducing activity from the carrot extrac t indicates that it contains a complex mixture of acidic and neutral s ugars. The inducer is not simply a rhamnogalacturonan but requires bot h arabinose and galacturonic acid for activity. De-esterification by a lkali treatment of the carrot inducer also increases activity. Partial ly purified inducing compound from carrot root extracts can induce the picA locus at galacturonate concentrations of 5-10 mu M, suggesting t hat it may act as a signal molecule from the plant. Compounds from car rot root extracts also induce the pgl locus, which encodes a predicted protein with homology to known polygalacturonases.