A Pectin Methylesterase Is Expressed in Silks and Maps to that Locus in Maize ( L.).

The locus of maize confers unilateral cross incompatibility, preventing cross pollination between females carrying the incompatible allele and males not carrying a corresponding compatible allele. To characterize this system at the molecular level, we carried out a transcript profiling experiment in which silks from near isogenic lines carrying the and alleles were compared. While several differentially expressed genes were identified, only one mapped to the known location of . This gene is a pectin methylesterase (PME), which we designated as , and is present and expressed only in - genotypes. While a functional ZmPME3 is not present in the genotypes examined, a pectin methylesterase gene cluster is found in genotypes. The gene cluster in W22 contains 58 tandem full-length or partial PME pseudo genes. These data combined with a wealth of previously published data on the involvement of PMEs in pollen tube growth suggest a role for cell wall modification enzymes in the pollen exclusion component of gametophytic incompatibility. Consistent with this role, a third allele which lacks the female function of , has a mutationally inactivated version of .