Lipid kinases PIP5K7 and PIP5K9 are required for polyamine‐triggered K(+) efflux in Arabidopsis roots

Polyamines, such as putrescine, spermidine and spermine (Spm), are low‐molecular‐weight polycationic molecules present in all living organisms. Despite their implication in plant cellular processes, little is known about their molecular mode of action. Here, we demonstrate that polyamines trigger a rapid increase in the regulatory membrane lipid phosphatidylinositol 4,5‐bisphosphate (PIP(2)), and that this increase is required for polyamine effects on K(+) efflux in Arabidopsis roots. Using in vivo (32)P(i)‐labelling of Arabidopsis seedlings, low physiological (μm) concentrations of Spm were found to promote a rapid PIP(2) increase in roots that was time‐ and dose‐dependent. Confocal imaging of a genetically encoded PIP(2) biosensor revealed that this increase was triggered at the plasma membrane. Differential (32)P(i)‐labelling suggested that the increase in PIP(2) was generated through activation of phosphatidylinositol 4‐phosphate 5‐kinase (PIP5K) activity rather than inhibition of a phospholipase C or PIP(2) 5‐phosphatase activity. Systematic analysis of transfer DNA insertion mutants identified PIP5K7 and PIP5K9 as the main candidates involved in the Spm‐induced PIP(2) response. Using non‐invasive microelectrode ion flux estimation, we discovered that the Spm‐triggered K(+) efflux response was strongly reduced in pip5k7 pip5k9 seedlings. Together, our results provide biochemical and genetic evidence for a physiological role of PIP(2) in polyamine‐mediated signalling controlling K(+) flux in plants.