Populus euphratica Glycine-Rich RNA-Binding Protein 2 Interacts with Target mRNAs to Negatively Regulate Salt Tolerance in Poplar

Transcription of glycine-rich RNA-binding protein 2 (PeGRP2) transiently increased in root and shoot of Populus euphratica (salt-resistant poplar) upon initial salt exposure and tended to decrease after long-term NaCl stress (100 mM, 12 days). PeGRP2 overexpression in the hybrid Populus tremula × P. alba ‘717-1B4’ (P. × canescens) increased salt sensitivity, which was reflected in plant growth and photosynthesis. PeGRP2 contains a conserved RNA recognition motif domain at the N-terminus, and RNA affinity purification (RAP) sequencing was developed to enrich target mRNAs that physically interact with PeGRP2 in P. × canescens. RAP sequencing combined with RT-qPCR revealed that NaCl decreased the transcripts of PeGRP2-interacting mRNAs encoding photosynthetic proteins, antioxidative enzymes, ATPases, and Na+/H+ antiporters in transgenic poplar. Particularly, PeGRP2 negatively affected the stability of target mRNAs encoding photosynthetic proteins (PETC and RBCMT), antioxidant enzymes (SOD[Mn], CDSP32 and CYB1-2), ATPases (AHA11, ACA8 and ACA9), and Na+/H+ antiporter (NHA1). This resulted in (i) a greater reduction in Fv/Fm, YII, ETR and Pn, (ii) a less pronounced salt activation of antioxidative enzymes, and (iii) a reduced ability to maintain Na+ homeostasis in transgenic poplars during long-term salt stress, leading to a lowered ability to tolerate salinity stress.