The ScFRK2 MAP kinase kinase kinase from Solanum chacoense affects pollen development and viability.

We have previously described the FERTILIZATION-RELATED KINASE 2 (ScFRK2), a MAP kinase kinase kinase from Solanum chacoense that is predominantly expressed in reproductive tissues. Overexpression of the ScFRK2 gene modifies the cell fate of ovule initials and induces homeotic transformation of ovules into carpelloid structures. Since the ScFRK2 gene is normally expressed also in anthers, we now further our observations on the effect of ScFRK2 overexpression in male reproductive structures. Although ScFRK2 mRNA levels detected by RNA blot were relatively constant during early anther development, there was a dramatic change in tissue distribution of ScFRK2 mRNA when detected by in situ RNA hybridization. In the young anther, ScFRK2 mRNA accumulated mainly in microsporocytes and tapetum. By the time of anthesis, ScFRK2 mRNA was no longer found in degenerating tapetum or pollen grains but instead found abundantly on the anther wall, including epidermis and endothecium. Overexpression of ScFRK2 transcripts strongly disturbed pollen development. At maturity, almost two-thirds of pollen grains were severely affected and non-viable, while the remaining pollen grains were significantly smaller than wild type pollen. Cross with pollen from a ScFRK2 overexpression line into a wild type female plant produced an F1 population with 44% of the progeny having the transgene, suggesting that the pollen defect is caused by a sporophytic dysfunction, leading to major structural defects and incomplete pollen development.