The Potassium Transporter SlHAK10 Is Involved in Mycorrhizal Potassium Uptake

Most terrestrial plants form a root symbiosis with arbuscular mycorrhizal (AM) fungi, which receive fixed carbon from the plant and enhance the plant's uptake of mineral nutrients. AM symbiosis improves the phosphorous and nitrogen nutrition of host plants; however, little is known about the role of AM symbiosis in potassium (K⁺) nutrition. Here, we report that inoculation with the AM fungus Rhizophagus irregularis improved tomato (Solanum lycopersicum) plant growth and K⁺ acquisition and that K⁺ deficiency has a negative effect on root growth and AM colonization. Based on its homology to a Lotus japonicus AM-induced K⁺ transporter, we identified a mycorrhiza-specific tomato K⁺ transporter, SlHAK10 (Solanum lycopersicum High-affinity Potassium Transporter10), that was exclusively expressed in arbuscule-containing cells. SlHAK10 could restore a yeast K⁺ uptake-defective mutant in the low-affinity concentration range. Loss of function of SlHAK10 led to a significant decrease in mycorrhizal K⁺ uptake and AM colonization rate under low-K⁺ conditions but did not affect arbuscule development. Overexpressing SlHAK10 from the constitutive cauliflower mosaic virus 35S promoter or the AM-specific Solanum melongena Phosphate Transporter4 not only improved plant growth and K⁺ uptake but also increased AM colonization efficiency and soluble sugar content in roots supplied with low K⁺ Our results indicate that tomato plants have a SlHAK10-mediated mycorrhizal K⁺ uptake pathway and that improved plant K⁺ nutrition could increase carbohydrate accumulation in roots, which facilitates AM fungal colonization.