Agrobacterium-mediated transformation of Ceratocystis albifundus

Functional association between genomic loci and specific biological traits remains lacking in many fungi, including the African tree pathogen Ceratocystis albifundus. This is mainly because of the absence of suitable transformation systems for allowing genetic manipulation of this and other fungi. Here, we present an optimized protocol for Agrobacterium tumefaciens-mediated transformation of C. albifundus. Strain AGL-1 of A. tumefaciens and four binary T-DNA vectors (conferring hygromycin B or geneticin resistance and/or expressing the green fluorescent protein [GFP]) were used for transforming germinated conidia of three isolates of C. albifundus. Stable expression of these T-DNA-encoded traits was confirmed through sequential sub-culturing of fungal transformants on selective and non-selective media and by using PCR and sequence analysis. Single-copy integration of the respective T-DNAs into the genomes of these fungi was confirmed using Southern hybridization analysis. The range of experimental parameters determined and optimised included: (i) concentrations of hygromycin B and geneticin required for inhibiting growth of the wild type fungus and (ii) the dependence of transformation on acetosyringone for inducing the bacterium's virulence genes, as well as (iii) the duration of fungus-bacterium co-cultivation periods and (iv) the concentrations of fungal conidia and bacterial cells used for the latter. The system developed in this study is stable with a high-efficiency, yielding up to 400 transformants per 10⁶ conidia. This is the first report of a transformation protocol for C. albifundus and its availability will be invaluable for functional studies in this important fungus.