Functions of PRK1, a receptor-like kinase of Petunia inflata: Phenotypes of the transgenic plants expressing the antisense PRK1 gene

We previously identified and characterized a predominantly pollen- expressed gene of Petunia inflata that encodes a receptor-like kinase named PRK1. The extracellular domain of PRK1 contained leucine-rich repeats, and the cytoplasmic kinase domain was found to autophosphorylate on serine and tyrosine. To investigate the function of PRK1 in pollen development, we transformed P. inflata plants with a construct containing the promoter of a predominantly pollen-expressed gene of tomato, LAT52, fused loan antisense PRK1 cDNA. Three transgenic plants produced approximately equal amounts of normal and aborted pollen. Progeny analysis revealed that the transgene cosegregated with the pollen abortion phenotype. The mutant microspores of the transgenic plants developed normally until the uninucleate stage, but were not able to carry out microspore mitosis and eventually disintegrated. Northern blot analysis suggested that the pollen abortion phenotype most likely resulted from down-regulation of the PRK1 gene by the antisense PRK1 transgene. Progeny analysis in two of the transgenic plants, ASRK-13 arid ASRK-20, showed that the transmission of the antisense PRK1 gene was also greatly reduced-through the female. Light microscopic examination revealed that embryo sac development of approximately half of the ovules of ASRK-13 was abnormal. The majority of the affected ovules were able to progress through most of megagametogenesis to produce a seven-cell embryo sac, but they failed to complete the maturation stages of the embryo sac development when cell expansion, nuclear migration, and differentiation take place. These ovules could not achieve fertilization and subsequently degenerated, resulting in abortion of half of the seed. RNA gel blot results showed that the PRK1 gene is expressed in the ovary. These results suggest that PRK1 plays an essential role in a signal transduction pathway that mediates post-meiotic development of microspores as well as embryo sac.