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.