Tonoplast-localized Ca²⁺ pumps regulate Ca²⁺ signals during pattern-triggered immunity in Arabidopsis thaliana

One of the major events of early plant immune responses is a rapid influx of Ca²⁺ into the cytosol following pathogen recognition. Indeed, changes in cytosolic Ca²⁺ are recognized as ubiquitous elements of cellular signaling networks and are thought to encode stimulus-specific information in their duration, amplitude, and frequency. Despite the wealth of observations showing that the bacterial elicitor peptide flg22 triggers Ca²⁺ transients, there remain limited data defining the molecular identities of Ca²⁺ transporters involved in shaping the cellular Ca²⁺ dynamics during the triggering of the defense response network. However, the autoinhibited Ca²⁺-ATPase (ACA) pumps that act to expel Ca²⁺ from the cytosol have been linked to these events, with knockouts in the vacuolar members of this family showing hypersensitive lesion-mimic phenotypes. We have therefore explored how the two tonoplast-localized pumps, ACA4 and ACA11, impact flg22-dependent Ca²⁺ signaling and related defense responses. The double-knockout aca4/11 exhibited increased basal Ca²⁺ levels and Ca²⁺ signals of higher amplitude than wild-type plants. Both the aberrant Ca²⁺ dynamics and associated defense-related phenotypes could be suppressed by growing the aca4/11 seedlings at elevated temperatures. Relocalization of ACA8 from its normal cellular locale of the plasma membrane to the tonoplast also suppressed the aca4/11 phenotypes but not when a catalytically inactive mutant was used. These observations indicate that regulation of vacuolar Ca²⁺ sequestration is an integral component of plant immune signaling, but also that the action of tonoplast-localized Ca²⁺ pumps does not require specific regulatory elements not found in plasma membrane-localized pumps.