The relevance of tomatinase activity in pathogens of tomato: disruption of the β2-tomatinase gene inColletotrichum coccodes and Septoria lycopersici and heterologous expression of the Septoria lycopersiciβ2-tomatinase in Nectria haematococca, a pathogen of tomato fruit

Abstract

Many fungal pathogens of tomato have the ability to enzymatically detoxify the toxic steroidal glycoalkaloid α-tomatine, synthesized by Lycopersicon and some Solanum species. However, it is unclear whether detoxification of this phytoanticipin is involved in the ability of these fungi to parasitize tomato. We have taken two approaches to address this question. For one approach, we have reduced or eliminated this enzymatic function by transformation-mediated disruption of the gene encoding β2-tomatinase in Colletotrichum coccodes andSeptoria lycopersici . Gene disruption of the S. lycopersiciβ2-tomatinase gene in an albino strain resulted in both a loss of β2-tomatinase activity and a loss of tolerance to α-tomatine. The results of pathogenicity tests with this mutant were inconclusive due to the lack of pathogenicity of the albino mutant. Disruption of the β2-tomatinase gene in C. coccodes resulted in a loss of β2-tomatinase activity but these mutants retained their tolerance to α-tomatine and their abilities to degrade α-tomatine via other enzyme(s). The gene-disrupted mutants were as pathogenic as wild type isolates on green tomato fruit, an organ containing high levels of α-tomatine. For our other approach, we have expressed the S. lycopersiciβ2-tomatinase gene in a tomatine tolerant mutant of Nectria haematococca MPVI, a fungus that normally can infect ripe but not green tomato fruit. Expression of β2-tomatinase in this N. haematococca mutant enabled it to detoxify α-tomatine and resulted in its ability to parasitize green tomato fruit.