The B″-family subunits of protein phosphatase 2A are necessary for in-vitro dephosphorylation of the Arabidopsis mechanosensory transcription factor VIP1

Characterizing the role of PP2A B'' family subunits in mechanical stress response and plant development through calcium and ABA signaling in Arabidopsis thaliana

Protein phosphatase 2AB'' (PP2A B'') family subunits have calcium-binding EF-hand motifs, facilitating interaction with PP2A substrates. In Arabidopsis thaliana, the PP2A B'' 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 PP2A B'' 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 AtB''α, AtB''β, AtB''γ, and AtB''δ was generated. atb''αβγδ 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 CYP707A3, a gene potentially targeted by VIP1 that regulates abscisic acid (ABA) catabolism, in the atb''αβγδ mutant compared to wild type leaves. However, other genes, including XTH23, EXLA1, and CYP707A1, also VIP1 targets, exhibited similar induction in both brushed atb''αβγδ 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 atb''αβγδ leaves compared to brushed wild type leaves. These findings suggest that PP2A B'' 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 atb''αβγδ seedlings was retarded when compared to wild type and single knockout mutants, atb''γ and atb''δ, indicating a crucial role in plant development by modulating calcium or ABA signaling.

Dephosphorylation of bZIP59 by PP2A ensures appropriate shade avoidance response in Arabidopsis

Changes in light quality and quantity experienced by many shade-intolerant plants grown in close proximity lead to transcriptional reprogramming and shade avoidance syndrome (SAS). Despite the importance of phosphorylation-dependent signaling in cellular physiology, phosphorylation events during SAS are largely unknown. Here, we examined shade-regulated phosphorylation events in Arabidopsis using quantitative phosphoproteomics. We confirmed shade-induced dephosphorylation of bZIP59, a basic region/leucine zipper motif (bZIP) transcription factor. Shade treatment promotes the nuclear localization of bZIP59, which can be mimicked by mutation of the phosphorylation sites on bZIP59. Phenotypic analysis identified that bZIP59 negatively regulated shade-induced hypocotyl elongation. bZIP59 repressed the shade-induced activation of certain growth-related genes, while shade increased the DNA binding of bZIP59. Furthermore, the protein phosphatase 2A (PP2A) mediated dephosphorylation of bZIP59. Our study characterized a previously unidentified mechanism by which the phytochrome B (phyB)-PP2A-bZIP59 regulatory module integrates shade signals and transcriptomes, broadening our knowledge of phosphorylation strategies for rapid adaptation to shade.

VIP1 and its close homologs confer mechanical stress tolerance in Arabidopsis leaves

VIP1, an Arabidopsis thaliana basic leucine zipper transcription factor, and its close homologs are imported from the cytoplasm to the nucleus when cells are exposed to mechanical stress. They bind to AGCTG (G/T) and regulate mechanical stress responses in roots. However, their role in leaves is unclear. To clarify this, mutant lines (QM1 and QM2) that lack the functions of VIP1 and its close homologs (bZIP29, bZIP30 and PosF21) were generated. Brushing more severely damaged QM1 and QM2 leaves than wild-type leaves. Genes regulating stress responses and cell wall properties were downregulated in brushed QM2 leaves and upregulated in brushed VIP1-GFP-overexpressing (VIP1-GFPox) leaves compared to wild-type leaves in a transcriptome analysis. The VIP1-binding sequence AGCTG (G/T) was enriched in the promoters of genes downregulated in brushed QM2 leaves compared to wild-type leaves and in those upregulated in brushed VIP1-GFPox leaves. Calmodulin-binding transcription activators (CAMTAs) are known regulators of mechanical stress responses, and the CAMTA-binding sequence CGCGT was enriched in the promoters of genes upregulated in the brushed QM2 leaves and in those downregulated in the brushed VIP1-GFPox leaves. These findings suggest that VIP1 and its homologs upregulate genes via AGCTG (G/T) and influence CAMTA-dependent gene expression to enhance mechanical stress tolerance in leaves.

"B" Regulatory Subunits of PP2A: Their Roles in Plant Development and Stress Reactions

Protein phosphatase PP2A is an enzyme complex consisting of C (catalytic), A (scaffold) and B (regulatory) subunits. B subunits are a large family of proteins that regulate activity, substrate specificity and subcellular localization of the holoenzyme. Knowledge on the molecular functions of PP2A in plants is less than for protein kinases, but it is rapidly increasing. B subunits are responsible for the large diversity of PP2A functioning. This paper intends to give a survey on their multiple regulatory mechanisms. Firstly, we give a short description on our current knowledge in terms of "B"-mediated regulation of metabolic pathways. Next, we present their subcellular localizations, which extend from the nucleus to the cytosol and membrane compartments. The next sections show how B subunits regulate cellular processes from mitotic division to signal transduction pathways, including hormone signaling, and then the emerging evidence for their regulatory (mostly modulatory) roles in both abiotic and biotic stress responses in plants. Knowledge on these issues should be increased in the near future, since it contributes to a better understanding of how plant cells work, it may have agricultural applications, and it may have new insights into how vascular plants including crops face diverse environmental challenges.

Genomic Analysis of Soybean PP2A-B ' ' Family and Its Effects on Drought and Salt Tolerance

Abiotic stresses induce the accumulation of reactive oxygen species (ROS) and significantly affect plant growth. Protein phosphatase 2A (PP2A) plays an important role in controlling intracellular and extracellular ROS signals. However, the interaction between PP2A, ROS, and stress tolerance remains largely unclear. In this study, we found that the B ' ' subunit of PP2A (PP2A-B ' ' ) can be significantly induced and was analyzed using drought- and salt-induced soybean transcriptome data. Eighty-three soybean PP2A-B ' ' genes were identified from the soybean genome via homologous sequence alignment, which was distributed across 20 soybean chromosomes. Among soybean PP2A-B ' ' family genes, 26 GmPP2A-B ' ' members were found to be responsive to drought and salt stresses in soybean transcriptome data. Quantitative PCR (qPCR) analysis demonstrated that GmPP2A-B ' ' 71 had the highest expression levels under salt and drought stresses. Functional analysis demonstrated that overexpression of GmPP2A-B ' ' 71 in soybeans can improve plant tolerance to drought and salt stresses; however, the interference of GmPP2A-B ' ' 71 in soybean increased the sensibility to drought and salt stresses. Further analysis demonstrated that overexpression of GmPP2A-B ' ' 71 in soybean could enhance the expression levels of stress-responsive genes, particularly genes associated with ROS elimination. These results indicate that PP2A-B ' ' can promote plant stress tolerance by regulating the ROS signaling, which will contribute to improving the drought resistance of crops.