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

Crude extracts from carrot roots induce the picA chromosomal locus of Agrobacterium tumefaciens. The inducer is a complex pectic polysaccharide whose activity can be destroyed by incubation with a highly purified cloned pectate lyase (PelE protein of Erwinia chrysanthemi EC16). Oligogalacturonates of degree of polymerization (dp) 6-17 induce the picA locus, with peak activity at dp 8. However, the inducing compound partially purified from carrot root extracts is about 100-fold more active (per uronic acid content) than is the most active oligogalacturonate in inducing the picA locus. Chemical linkage analysis of a QAE Sephadex fraction containing peak inducing activity from the carrot extract indicates that it contains a complex mixture of acidic and neutral sugars. The inducer is not simply a rhamnogalacturonan but requires both arabinose and galacturonic acid for activity. De-esterification by alkali treatment of the carrot inducer also increases activity. Partially purified inducing compound from carrot root extracts can induce the picA locus at galacturonate concentrations of 5-10 mu M, suggesting that it may act as a signal molecule from the plant. Compounds from carrot root extracts also induce the pgl locus, which encodes a predicted protein with homology to known polygalacturonases.