The CaM1-CBP60b-MYB77 Transcriptional Cascade Regulates K⁺ Homeostasis and Salt Tolerance in Barley

Soil salinity is a major abiotic stress that threatens global crop productivity. Calcium (Ca²⁺) is a pivotal second messenger in plant stress signalling, but how Ca²⁺ sensors such as calmodulin (CaM) transcriptionally control ion homeostasis and salt tolerance remains largely understood in crops. Here, we identified a novel calmodulin-binding protein HvCBP60b, which is a negative regulator of salt resilience in salt-tolerant barley. We showed that HvCBP60b interacts with Ca²⁺ sensor HvCaM1 via its C-terminal domain in a Ca²⁺-dependent manner. Loss of either HvCBP60b or HvCaM1 promoted the uptake of K⁺, increased the K⁺/Na⁺ ratio, and markedly enhanced both growth performance and grain yield under salinity stress. Furthermore, HvCBP60b binds to the promoter of R2R3-type MYB transcription factor HvMYB77 through its N-terminal domain. HvMYB77 then transcriptionally reduces the expression of HvHAK5, a high-affinity K⁺ transporter essential for maintaining K⁺ homeostasis under salinity. Mutations in HvMYB77 or HvHAK5 compromise salt tolerance, whereas the salt-sensitive phenotype of hvhak5 is rescued in hvcbp60b-hvhak5 double mutants. Therefore, we uncover a novel Ca²⁺-responsive regulatory module CaM1-CBP60b-MYB77-HAK5 essential for K⁺ homeostasis and salt tolerance in barley, highlighting new targets of Ca²⁺ signalling for improving salt resilience in cereal crops.