SlCIPK26 enhances tomato fertility by activating the K ⁺ transporter SlHAK5 in reproductive tissues

In tomato plants, the potassium (K⁺) transporter SlHAK5 is integral to root K⁺ uptake and overall plant fertility. Under K⁺ deficiency, SlHAK5 expression is induced in roots and the encoded transporter is activated via the Ca²⁺-sensing CIPK/CBL complex SlCIPK23/SlCBL1-9. In Arabidopsis, multiple CIPK/CBL complexes can activate AtHAK5, providing alternative regulatory pathways that enhance K⁺ uptake. However, the architecture of CIPK/CBL signaling networks has diverged among plant species, necessitating species-specific identification of novel regulatory components. Accordingly, we screened additional tomato CIPK proteins for their capacity to modulate SlHAK5 activity in yeast. SlCIPK15 and SlCIPK26 emerged as potent activators of SlHAK5, acting in concert with SlCBL9. Functional characterization of slcipk15 and slcipk26 mutants revealed that neither contributed significantly to SlHAK5-mediated K⁺ uptake in roots. Conversely, both mutants exhibited impaired pollen tube elongation, correlating with reduced K⁺ content in pollen relative to wild type. Notably, slcipk26 mutants displayed more severe pollen defects, phenocopying the slhak5 mutant. Further analyses demonstrated that slcipk26 plants suffered compromised seed set and pistil function, paralleling the reproductive deficiencies observed in slhak5 mutants. These findings implicate SlCIPK26 as the principal regulator of SlHAK5 in reproductive tissues. Collectively, our data underscore the role of CIPK paralogs in orchestrating tissue-specific regulation of target proteins, thereby enabling fine-tuned modulation of K⁺ transport essential for both vegetative and reproductive development.