Stimulation of lignification in neoformed calli induced by Agrobacterium rhizogenes on bean hypocotyls

Abstract Histological and biochemical modifications following the inoculation of bean hypocotyls with Agrobacterium rhizogenes have been studied with special emphasis on lignification. Infection with A. rhizogenes led to a swelling of the inoculated region (2 days after inoculation), giving rise to a nodular callus (4–5 days) from which transformed roots arose after 10–12 days. Time-course histochemical studies performed during callus formation have shown an increased differentiation of lignified elements concerning both direct differentiation of tracheids from cortical cells and, to a greater extent, tracheid redifferentiation in the neoformed calli. These observations have been confirmed by determinations of lignin contents and measurements of related enzyme activities. Cinnamoyl CoA reductase (E.C.1.2.1.44) and cinnamyl alcohol dehydrogenase (E.C.1.1.1.-), two specific enzymes of lignification, are rapidly stimulated with a maximum activity 8 days after inoculation in correlation with the rapid increase in amount of lignins. Hybridization experiments (Northern) demonstrate an accumulation of mRNAs corresponding to cinnamyl alcohol dehydrogenase during the callus formation. Comparative infections performed with different bacterial strains support the involvement of auxins both in tumor formation and in lignification stimulation.