Enzyme activities of Arabidopsis inositol polyphosphate kinases AtIPK2 and AtIPK2 are involved in pollen development, pollen tube guidance and embryogenesis

Inositol polyphosphate kinase (IPK2) is a key component of inositol polyphosphate signaling. There are two highly homologous inositol polyphosphate kinases (AtIPK2 and AtIPK2) in Arabidopsis. Previous studies that overexpressed or reduced the expression of AtIPK2 and AtIPK2 revealed their roles in auxiliary shoot branching, abiotic stress responses and root growth. Here, we report that AtIPK2 and AtIPK2 act redundantly during pollen development, pollen tube guidance and embryogenesis. Single knock-out mutants of atipk2 and atipk2 were indistinguishable from the wild type, whereas the atipk2atipk2 double mutant could not be obtained. Detailed genetic and cytological investigations showed that the mutation of AtIPK2 and AtIPK2 resulted in severely reduced transmission of male gametophyte as a result of abnormal pollen development and defective pollen tube guidance. In addition, the early embryo development of the atipk2atipk2 double mutant was also aborted. Expressing either catalytically inactive or substrate specificity-altered variants of AtIPK2 could not rescue the male gametophyte and embryogenesis defects of the atipk2atipk2 double mutant, implying that the kinase activity of AtIPK2 is required for pollen development, pollen tube guidance and embryogenesis. Taken together, our results provide genetic evidence for the requirement of inositol polyphosphate signaling in plant sexual reproduction.