cDNA cloning of a sorghum pathogenesis-related protein (PR-10) and differential expression of defense-related genes following inoculation with Cochliobolus heterostrophus or Colletotrichum sublineolum

A sorghum cDNA clone was isolated by differential screening of a cDNA libra ry prepared from mesocotyls (cultivar DK18) inoculated with fungal pathogen s. The deduced translation product shows sequence similarity to a family of intracellular pathogenesis-related proteins (PR-10) with a potential ribon uclease function. We studied the accumulation of PR-10 and chalcone synthas e (CHS) transcripts in mesocotyls following inoculation with Cochliobolus h eterostrophus or Colletotrichum sublineolum. CHS is involved in phytoalexin synthesis in sorghum. Coordinate expression of PR-10 and CHS genes was loc alized in the area of inoculation along with the accumulation of phytoalexi ns. C. heterostrophus is a nonpathogen of sorghum and cytological studies i ndicated that cultivar DK18 is resistant to C. sublineolum, a sorghum patho gen, We demonstrated that the two fungi triggered different time courses of plant defense reactions. Inoculation with C. heterostrophus resulted in ra pid accumulation of PR-10 and CHS transcripts after appressoria had become mature. Accumulation of these transcripts was delayed in plants inoculated with C. sublineolum until penetration of host tissue had been completed and infection vesicles had formed. Results suggest that different recognition events are involved in the expression of resistance to the two fungi used o r that C. sublineolum suppresses the nonspecific induction of defense respo nses.