Root specific elicitation and exudation of fluorescent β-carbolines in transformed root cultures of Oxalis tuberosa

Abstract

Stable transformation was achieved in oca ( Oxalis tuberosa L.) using an Agrobacterium rhizogenes-mediated system. Transformation frequencies varied with the use of different types of strains of A. rhizogenes and the age of explants. The transfer of rol A gene into the oca genome was confirmed by PCR analysis. In vitro transformed root cultures of oca grown in sterile liquid media induced purplish-blue fluorescence of the culture flask medium when irradiated with UV light. We have previously observed a similar phenomenon, the exudation of fluorescent compounds by the roots of in vitro and field-grown oca plants. Hairy root cultures of O. tuberosa transformed with A. rhizogenes (ATCC-15834) exuded constitutive levels of harmine (7-methoxy-1-methyl-β-carboline) and harmaline (3,4-dihydroharmine), the main fluorescent compounds detected from oca’s root exudates. Transformed roots showed better growth and exudation of harmine and harmaline compared to the untransformed normal roots. Upon elicitation with fungal cell wall elicitors from Phytophthora cinnamoni, the production and exudation of harmine/harmaline was enhanced in both transformed and non-transformed roots. Harmine and harmaline showed a wide range of antimicrobial activity against soil-borne microorganisms. Biologically, these findings suggest that in nature β-carbolines are constitutive antimicrobial compounds released into the rhizosphere upon microbial challenge. Transformed root cultures of oca make a simple, reliable and well-defined model system to investigate the molecular and metabolic exudation of fluorescent β-carboline biosynthesis, and to evaluate the biological significance of the phenomenon of root exudation of fluorescent metabolites.