(100-97) The vacuolar Ca2+ transporter CATION EXCHANGER 2 modulates hypoxic signaling and response to flooding in Arabidposis thaliana

Body of Abstract: Flooding represents a major threat to global agricultural productivity and food security. Such stresses encountered by plants involve stimulus-specific cellular transport of Ca²⁺ ions, mediated by an array of channels and pumps, leading to physiological changes. As a result of flooding, plants also experience limited oxygen availability (hypoxia), resulting in altered Ca²⁺ signaling. However, the biological and molecular roles of pumps regulating this flooding triggered Ca²⁺ signaling are still relatively unknown.

To help define components of this regulatory network, we therefore mined the publicly available transcriptomes of Arabidopsis plants subjected to flooding or hypoxic stress for rapidly upregulated, Ca²⁺-related transcripts. Candidates emerging from this study included two Ca²⁺ transporters, AUTOINHIBITED CA²⁺ ATPASE 1 (ACA1) and CATION EXCHANGER 2 (CAX2), which are predicted to affect the dynamics of Ca²⁺ signals most directly associated with flooding/hypoxia stresses and hence, further used them to test their role in regulating flooding-related responses.

Using a combination of physiological, biochemical, and molecular-based assays, current findings revealed that knockout mutants in CAX2 but not ACA1 showed enhanced survival to both short and long-term flooding. In addition, we also discovered that knocking out CAX2 leads to slightly elevated resting Ca²⁺ levels and larger flooding-induced Ca²⁺ signals. The elevated basal levels also triggered some flooding-responsive genes to be constitutively activated in the cax2 mutant plants, preadapting them to flooding stress. These effects are consistent with a role for this vacuolar pump, exporting Ca²⁺ ions from the cytosol whereas, when these pumps are inactive, it attenuates the Ca²⁺ signals, which in turn, triggers adaptation to flooding. These observations suggest that CAX2 is an important component regulating the activity of the flooding response network, likely through its effects on the flooding-induced Ca²⁺-signaling network, highlighting the key role of the vacuole in modulating flooding-related signaling systems.