<b>Characterizing the role of PP2AB'' family subunits in mechanical stress response and plant development through calcium and ABA signaling in </b><b><i>Arabidopsis thaliana</i></b>

Protein phosphatase 2AB'' (PP2AB'') family subunits have calcium-binding EF-hand motifs, facilitating interaction with PP2A substrates. In <i>Arabidopsis thaliana</i>, the PP2AB'' family subunits consist of six members, AtB''α-ε and FASS. These subunits can interact with a basic leucine zipper transcription factor, VIP1, and its close homologs. Mechanical stress triggers PP2A-mediated dephosphorylation of VIP1 and its close homologs, leading to nuclear localization and gene upregulation to alleviate touch-induced root bending and leaf damage. However, the physiological roles of PP2AB'' family subunits in the mechanical stress response in Arabidopsis remain unclear. This study aims to characterize such roles. A quadruple knockout mutant with T-DNA insertions in <i>AtB''α</i>, <i>AtB''β</i>, <i>AtB''γ</i>, and <i>AtB''δ</i> was generated. <i>atb''αβγδ</i> mutants exhibited no significant damage upon brushing or touch-induced root bending compared to the wild type. Transcriptome analysis showed a significant decrease in the expression of <i>CYP707A3</i>, a gene potentially targeted by VIP1 that regulates abscisic acid (ABA) catabolism, in the <i>atb''αβγδ</i> mutant compared to wild type leaves. However, other genes, including <i>XTH23</i>, <i>EXLA1</i>, and <i>CYP707A1</i>, also VIP1 targets, exhibited similar induction in both brushed <i>atb''αβγδ</i> mutants and wild type leaves. We observed an enrichment of the CAMTA motif, CGCG(C/T) in the promoters of genes showing downregulated expression levels in brushed <i>atb''αβγδ</i> leaves compared to brushed wild type leaves. These findings suggest that PP2AB'' family subunits exhibit functional redundancy in the VIP1-dependent pathway but influence CAMTA-dependent gene expression under mechanical stress. Under calcium-deficient and ABA-supplemented conditions, growth of <i>atb''αβγδ</i> seedlings was retarded when compared to wild type and single knockout mutants, <i>atb''γ </i>and<i> atb''δ</i>, indicating a crucial role in plant development by modulating calcium or ABA signaling.