The vacuolar Ca 2+ transporter CATION EXCHANGER 2 regulates cytosolic calcium homeostasis, hypoxic signaling, and response to flooding in Arabidopsis thaliana.

Flooding represents a major threat to global agricultural productivity and food security, but plants are capable of deploying a suite of adaptive responses that can lead to short- or longer-term survival to this stress. One cellular pathway thought to help coordinate these responses is via flooding-triggered Ca 2+ signaling. We have mined publicly available transcriptomic data from Arabidopsis subjected to flooding or low oxygen stress to identify rapidly upregulated, Ca 2+ -related transcripts. We then focused on transporters likely to modulate Ca 2+ signals. Candidates emerging from this analysis included AUTOINHIBITED Ca 2+ ATPASE 1 and CATION EXCHANGER 2. We therefore assayed mutants in these genes for flooding sensitivity at levels from growth to patterns of gene expression and the kinetics of flooding-related Ca 2+ changes. Knockout mutants in CAX2 especially showed enhanced survival to soil waterlogging coupled with suppressed induction of many marker genes for hypoxic response and constitutive activation of others. CAX2 mutants also generated larger and more sustained Ca 2+ signals in response to both flooding and hypoxic challenges. CAX2 is a Ca 2+ transporter located on the tonoplast, and so these results are consistent with an important role for vacuolar Ca 2+ transport in the signaling systems that trigger flooding response.