PATHOGENESIS BY COCHLIOBOLUS-HETEROSTROPHUS TRANSFORMANTS EXPRESSING A CUTINASE-ENCODING GENE FROM NECTRIA-HAEMATOCOCCA

Nectria haematococca, a pathogen of pea stems and roots, produces cutinase (pH optimum 9.5), an enzyme that degrades cutin found in the plant epidermis. A cutinase encoding gene (CUT-T8) from N. haematococca was overexpressed in transformants of Cochliobolns heterostrophus (a pathogen of corn foliage) with or without simultaneous over-expression of PDA-T9, a gene from N. haematococca encoding pisatin demethylase (PDA), which detoxifies the pea phytoalexin pisatin. Transformants were compared with each other and with wild type for ability to attack roots, stems, or leaves of pea, and roots or leaves of corn. Overexpression of PDA alone caused increased virulence to pea of several independently generated C. heterostrophus transformants, confirming a previous report, which was based on analysis of one transformant. N. haematococca cutinase alone had no detectable effect on virulence of C. heterostrophus to pea, but in combination with PDA caused distinctively dark-colored lesions on nonwounded pea stems, suggesting cutinase (in addition to PDA) as a possible factor in this heterologous plant/fungus interaction. All transformants caused normal symptoms (indistinguishable from those of wild-type C. hererostrophus) on corn leaves, but no detectable damage to roots of corn or pea. Thus, over-expression of heterologous cutinase did not change the organ specificity of C. heterostrophus on either its own host (corn) or a nonhost (pea).