Populus euphratica PeNAC029 suppresses phospholipase D and phosphatidic acid signaling under salt stress

Populus euphratica phospholipase Dδ (PePLDδ) was previously shown to mediate Na⁺ and reactive oxygen species (ROS) homeostasis during salt stress. To explore the transcriptional regulation of PLDs in salt response, DNA pull-down and mass spectrometry was used to identify the potential transcription factor for binding the promoter of PePLDδ. The target transcription factor PeNAC029 contains a conserved NAC transcription repression domain (NARD-like domain) with LVFY motif in the N-terminal from 102 to 136 amino acid residues. The PeNAC029-regulated expression of PLDs were investigated in Arabidopsis and P. euphratica, the two contrasting species differing in salt tolerance. PeNAC029 overexpression increased the sensitivity to salt stress in the two species, attributed to downregulated expression of PLD family genes, diminished PLD activity and lower phosphatidic acid (PA) levels in PeNAC029-transgenic Arabidopsis and P. euphratica. PeNAC029 was discovered to bind to a particular cis-element (ACGT motif) in the promoters and suppress the expression of AtPLDδ, PePLDα3, and PePLDβ2 under saline conditions. The PeNAC029 repression of PLD-PA signaling adversely affects the plant capacity to maintain Na⁺ homeostasis by inhibiting the expression of Na⁺/H⁺ antiporter genes AtSOS1 and PeSOS1 under salt conditions. Furthermore, PeNAC029-repressed PLD-PA signaling also hinders the ability to maintain ROS homeostasis by inhibiting the transcription and activities of antioxidant enzymes during salt stress. Notably, PeNAC029 expression was downregulated in response to prolonged NaCl exposure, potentially allowing the salt-resistant poplar to mitigate the detrimental effects of salinity. We propose a transcriptional regulatory network to highlight the salt response of PeNAC029-transgenic Arabidopsis and P. euphratica.