Downregulation of parallel spindle1 (atps1) orthologs impairs pollen development in potato

Parallel Spindle1 of Arabidopsis thaliana (AtPS1) is implicated in the formation of 2n pollen generated by parallel orientation of the spindles at 2nd meiotic division. Since decades, 2n pollen is known in potato, its wild relatives and interspecific hybrids, as a noteworthy reproductive anomaly with important breeding applications. For this reason, we investigated AtPS1 orthologs in potato. In our previous work, sequence-homology-based strategy combined with a bioinformatic approach revealed three Parallel Spindle Like loci (PSL1-3) in potato genome thus showing a greater PSL complexity respect to Arabidopsis. PSL1 and PSL3 encode predicted proteins characterized by both FHA and PINc domains such as found in Arabidopsis. FHA domain is implicated in protein-protein interactions while PINc domain has RNA-binding properties often associated with RNAse activity. In this work, to establish the function of PSLs, their silencing was performed in Solanum tuberosum and S. commersonii through short interfering RNA target sequence common to PSLs. Down-regulated expression of PSL1 and PSL3 was found in several independent transformants. Interestingly, a strong reduction of pollen viability, estimated by Alexander staining, was observed in lines with the lowest PSL expression, whereas the expected 2n pollen formation was lacking. Microsporogenesis and microgametogenesis analysis pointed out irregularities during pollen development at the first mitotic division after meiosis. These results, together with the evidence that PSL3 failed to functionally complement Atps1 mutation, suggest that a functional diversification of PSLs occur in potato respect to Arabidopsis.