Physiological and molecular responses of poplar to salt stress and functional analysis of <i>PagGRXC9</i> to salt tolerance

Soil salinization is increasingly recognized as a critical environmental challenge that significantly threatens plant survival and agricultural productivity. To elucidate the mechanism of salt resistance in poplar, physiological and transcriptomic analyses were conducted on 84K poplar (Populus alba × Populus glandulosa) under varying salt concentrations (0, 100, 200 and 300 mM NaCl). As salt levels increased, observable damage to poplar progressively intensified. Differentially expressed genes under salt stress were primarily enriched in photosynthesis, redox activity and glutathione metabolism pathways. Salt stress reduced chlorophyll content and net photosynthetic rate, accompanied by the downregulation of photosynthesis-related genes. NaCl (300 mM) significantly inhibited the photochemical activity of photosystems. The higher photochemical activity under 100 and 200 mM NaCl was attributed to the activated PGR5-cyclic electron flow photoprotective mechanism. However, the NAD(P)H dehydrogenase-like (NDH)-cyclic electron flow was inhibited under all salt levels. Salt stress led to reactive oxygen species accumulation, activating the ASA-GSH cycle and antioxidant enzymes to mitigate oxidative damage. Weighted gene co-expression network analysis showed that five photosynthesis-related hub genes (e.g., FNR and TPI) were down-regulated and nine antioxidant-related hub genes (e.g., GRX, GPX and GST) were up-regulated under salt stress conditions. PagGRXC9 encodes glutaredoxin and was found to be differentially expressed during the salt stress condition. Functional studies showed that overexpressing PagGRXC9 enhanced salt tolerance in yeast, and in poplar, it improved growth, FV/FM, non-photochemical quenching values and resistance to H2O2-induced oxidative stress under salt stress. This study constructed the photosynthetic and antioxidant response network for salt stress in poplar, revealing that PagGRXC9 enhances salt tolerance by reducing photoinhibition and increasing antioxidant capacity. These findings provide valuable insights for breeding salt-tolerant forest trees.