SlCIPK9 regulates pollen tube elongation in tomato plants via a K+-independent mechanism

Potassium (K⁺) is an essential macronutrient which contributes to osmotic- and turgor-related processes in plants. Calcineurin-B like Interacting Protein Kinases (CIPKs) play crucial roles in plants under low-K⁺ supply since they activate root K⁺ uptake transport systems such as AKT1 and AtHAK5. In Arabidopsis, AtCIPK9 is important for low-K⁺ tolerance since atcipk9 plants exhibited poor growth and leaf chlorosis when K⁺ was scarce. Part of these phenotypes could be ascribed to the activation of AtHAK5 by AtCIPK9. It has been reported that important differences exist between Arabidopsis and other plant species such as tomato with respect to the regulation of K⁺ uptake systems. Thus, our aim was to evaluate the contribution of SlCIPK9, the homologous protein of AtCIPK9 in tomato, to K⁺ nutrition. Unexpectedly, phenotyping experiments carried out with slcipk9 loss-of-function mutants revealed that SlCIPK9 did not play a clear role in tomato K⁺ homeostasis. By contrast, it was found that SlCIPK9 contributed to pollen tube elongation, but not to pollen germination, via a K⁺-independent mechanism. Therefore, our results highlight the remarkable differences that exist in Ca²⁺ signaling pathways between plant species and encourage the realization of more comparative studies as the one presented here.