Identification of Protein-Protein Interactions Among ABA Receptors, PP2C Phosphatases, and MAP Kinases in the Populus trichocarpa stress Response Network

In the core abscisic acid (ABA) signaling pathway, previously described for Arabidopsis, protein phosphatases of the 2C family (PP2C) maintain sucrose non-fermenting related kinases (SnRK) in a dephosphorylated inactive state under normal cellular conditions. ABA perception by a family of soluble cytoplasmic receptors called PYRABACTIN RESISTANCE1 (PYR1)/PYR1-LIKE (PYL)/REGULATORY COMPONENTS OF ABA RECEPTORS (RCAR) promotes sequestration of PP2C, releasing SnRK2/OPEN STOMATA 1(OST1)-related kinase clade members from inhibition. These kinases then proceed to modulate gene expression and protein activity as it relates to osmotic stress response. In our work we have shown that the core ABA signaling pathway from receptor through SnRK2 kinase is conserved between Arabidopsis and Populus trichocarpa. We selected poplar orthologs of the Arabidopsis PP2C protein phosphatases based on protein homology assessment and expression profiling of drought- and salt-stressed tissues. To date we have documented 13 yeast two-hybrid (Y2H) interactions involving 9 PP2C phosphatases and 2 poplar PYR1/PYL/RCAR receptors in the presence and/or absence of ABA.  Additionally, the poplar PP2C PtHAB1.2 interacts with two orthologs of SnRK2/OST1. Other poplar PP2C phosphatases HAI1-related and 2 AHG3-related homologs interact with ABA-inducible mitogen-activated protein (MAP) kinases. These MAP kinases also have interactions with an upstream MAP kinase kinase 3 in poplar, whose ortholog in Arabidopsis plays an important role in drought stress response. The poplar stress signaling modules identified in this work represent attractive starting points for functional characterization in support of future efforts to improve poplar growth and development during abiotic stresses caused by drought and salinity.  Supported by DOE-BER DE-SC0008570.