A self-fertile mutant of Phalaris produces an S protein with reduced thioredoxin activity

Gametophytic self-incompatibility in the Phalaris coerulescens is controlled by two unlinked genes, S and Z. Isolation of the S gene from the pollen of this grass species indicated that the C terminus has significant hemology with thioredoxin H proteins. The protein from the C terminus, expressed in Escherichia coli, exhibits thioredoxin-like activity. This paper demonstrates that the C terminus of the S protein from an S complete mutant shows significant reduction in thioredoxin activity when compared with the wild-type form. Both pollen and stigma have lost self-incompatibility in this mutant. Close examination of the lesions, which were found only in the C terminus of the mutant gene suggests that the substitution of a serine by an arginine is responsible for the reduced enzymatic activity. The association between reduced activity and the loss of the self-incompatibility provides evidence for a role of thioredoxin activity in the self-incompatibility reaction of this species.