Ca2+‐regulated and diurnal rhythm‐regulated Na+/Ca2+ exchanger AtNCL affects flowering time and auxin signalling in Arabidopsis

Calcium (Ca²⁺) is vital for plant growth, development, hormone response and adaptation to environmental stresses, yet the mechanisms regulating plant cytosolic Ca²⁺ homeostasis are not fully understood. Here, we characterize an Arabidopsis Ca²⁺‐regulated Na⁺/Ca²⁺ exchanger AtNCL that regulates Ca²⁺ and multiple physiological processes. AtNCL was localized to the tonoplast in yeast and plant cells. AtNCL appeared to mediate sodium (Na⁺) vacuolar sequestration and meanwhile Ca²⁺ release. The EF‐hand domains within AtNCL regulated Ca²⁺ binding and transport of Ca²⁺ and Na⁺. Plants with diminished AtNCL expression were more tolerant to high CaCl₂ but more sensitive to both NaCl and auxin; heightened expression of AtNCL rendered plants more sensitive to CaCl₂ but tolerant to NaCl. AtNCL expression appeared to be regulated by the diurnal rhythm and suppressed by auxin. DR5::GUS expression and root responses to auxin were altered in AtNCL mutants. The auxin‐induced suppression of AtNCL was attenuated in SLR/IAA14 and ARF6/8 mutants. The mutants with altered AtNCL expression also altered flowering time and FT and CO expression; FT may mediate AtNCL‐regulated flowering time change. Therefore, AtNCL is a vacuolar Ca²⁺‐regulated Na⁺/Ca²⁺ exchanger that regulates auxin responses and flowering time.