Populus euphratica GRP2 Interacts with Target mRNAs to Negatively Regulate Salt Tolerance by Interfering with Photosynthesis, Na + , and ROS Homeostasis.

The transcription of glycine-rich RNA-binding protein 2 ( PeGRP2 ) transiently increased in the roots and shoots of Populus euphratica (a 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 its salt sensitivity, which was reflected in the plant's 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 the target mRNAs that physically interacted 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 this transgenic poplar. Specifically, PeGRP2 negatively affected the stability of the target mRNAs encoding the photosynthetic proteins PETC and RBCMT ; antioxidant enzymes SOD[Mn] , CDSP32 , and CYB1-2 ; ATPases AHA11 , ACA8 , and ACA9 ; and the Na + /H + antiporter NHA1 . This resulted in (i) a greater reduction in Fv/Fm, YII, ETR, and Pn; (ii) less pronounced activation of antioxidative enzymes; and (iii) a reduced ability to maintain Na + homeostasis in the transgenic poplars during long-term salt stress, leading to their lowered ability to tolerate salinity stress.