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

Nectria haematococca, a pathogen of pea stems and roots, produces cuti nase (pH optimum 9.5), an enzyme that degrades cutin found in the plan t epidermis. A cutinase encoding gene (CUT-T8) from N. haematococca wa s overexpressed in transformants of Cochliobolns heterostrophus (a pat hogen 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 co mpared 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 independentl y generated C. heterostrophus transformants, confirming a previous rep ort, which was based on analysis of one transformant. N. haematococca cutinase alone had no detectable effect on virulence of C. heterostrop hus to pea, but in combination with PDA caused distinctively dark-colo red lesions on nonwounded pea stems, suggesting cutinase (in addition to PDA) as a possible factor in this heterologous plant/fungus interac tion. All transformants caused normal symptoms (indistinguishable from those of wild-type C. hererostrophus) on corn leaves, but no detectab le damage to roots of corn or pea. Thus, over-expression of heterologo us cutinase did not change the organ specificity of C. heterostrophus on either its own host (corn) or a nonhost (pea).