Abstract
The ga1 locus of maize confers unilateral cross incompatibility, preventing cross pollination between females carrying the incompatible Ga1-s 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 Ga1-s and ga1 alleles were compared. While several differentially expressed genes were identified, only one mapped to the known location of ga1. This gene is a pectin methylesterase (PME), which we designated as ZmPme3, and is present and expressed only in Ga1-s genotypes. While a functional ZmPME3 is not present in the ga1 genotypes examined, a pectin methylesterase gene cluster is found in ga1 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 Ga1-s gametophytic incompatibility. Consistent with this role, a third allele which lacks the female function of Ga1-s, Ga1-m, has a mutationally inactivated version of ZmPme3.
Highlights
Maize is a wind-pollinated plant, where pollen grains released from anthers on the tassel are dispersed to the female silks on the ear
Since genetic experiments suggest that ga1 may be a null allele, it could be missing from ga1 genotypes such as W22 and B73 and mapping reads to these genotypes would not be expected to identify transcripts from the Gametophyte factor1-s (Ga1-s) gene
We have identified a pectin methylesterase gene designated as ZmPme3 that is present in the silks of Ga1-s plants and maps to the ga1 locus
Summary
Maize is a wind-pollinated plant, where pollen grains released from anthers on the tassel are dispersed to the female silks on the ear. One of the first genetic studies in maize following the rediscovery of Mendel’s laws was a study of cross incompatibility between maize and popcorn (Correns, 1901). The genetics of this and other cross incompatibility systems in maize have been studied intensely since and have been reviewed (Nelson, 1994). One of these systems was designated Gametophyte factor (Mangelsdorf and Jones, 1926) or Ga ( called Ga1) because of the involvement of the gamete in cross incompatibility. In the Ga1 system, cross incompatibility is conferred by a factor in silks of plants
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