The PsnWRKY70 transcription factor (TF) was reported to play an important role in the salt stress response mechanism of Populus simonii × Populus nigra in our previous research, and we also produced several PsnWRKY70 overexpression (OEXs) and RNAi suppression (REXs) P. simonii × P. nigra lines. In order to further compare the photosynthetic and physiological characteristics of NT (non-transgenic line) and transgenic lines under salt stress, the dynamic phenotypic change, Na⁺ and K⁺ content in leaf and root tissues, superoxide dismutase (SOD) and peroxidase (POD) activity, malondialdehyde (MDA) content, chlorophyll content (Chl), photosynthesis parameters (net photosynthetic rate, P_n; stomatal conductance, Gs; intercellular CO₂ concentration, C_i; transpiration rate, T_r), chlorophyll fluorescence parameters (electron transport rate, ETR; maximum photochemical efficiency of photosystem II (PSII), F_v/F_m; actual efficiency of PSII, Φ_PSII; photochemical quenching coefficient, q_P; non-photochemical quenching, NPQ; the photosynthetic light-response curves of Φ_PSII and ETR) and RNA-seq of NT, OEX and REX lines were detected and analyzed. The phenotypic observation, MDA content and Chl detection results indicate that the stress damage of REXs was less severe than that of NT and OEX lines under salt stress. Photosynthesis parameter (P_n, Gs, T_r and C_i) and chlorophyll fluorescence parameter (ETR, F_v/F_m, Φ_PSII q_P and NPQ) detection results indicate that the REX lines exhibited much better photosynthetic adaptability than NT and OEX lines during salt stress. The photosynthetic light-response curves of Φ_PSII and ETR of NT, OEX and REX lines indicate that REXs exhibited better ability to activate the photosynthetic protection mechanism and adapt to a certain degree of strong light than NT and OEX lines under salt stress. RNA-seq analysis indicates that the DEGs between OEX1 vs. NT and REX1 vs. NT in different tissues (apical bud and fifth functional leaf) were all different in category and change trend. The expression of PsnWRKY70 was significantly up-regulated in both the apical bud and fifth functional leaf of OEX1, and showed no significant change (namely maintained low expression level) in both the apical bud and fifth functional leaf of REX1, thus indicating the negative regulation role of PsnWRKY70 in P. simonii × P. nigra under salt stress. Additionally, there were a lot of stress response-related TF genes (such as bHLH, WRKY, MYB, NAM and AP2/EREBP) and photosynthesis-related genes among all the DEGs. In REX1, the expression of three Photosystem I P700 chlorophyll a apoprotein A1 genes (Potri.003G065200, Potri.013G141800 and Potri.019G028100) and a Photosystem II protein D1 gene (Potri.013G138300) were significantly up-regulated after 6 days of salt stress. In OEX1, the Heterodimeric geranylgeranyl pyrophosphate synthase small subunit gene (Potri.015G043400) and Phospho-2-dehydro-3-deoxyheptonate aldolase 1 gene (Potri.007G095700) were significantly down-regulated after 6 days of salt stress. These photosynthesis-related genes are probably regulated by PsnWRKY70 TF in response to salt stress. In conclusion, the REX lines suffered less severe salt damage and exhibited better photosynthetic adaptability than NT and OEXs under salt stress. The differences among the DEGs between OEX1 vs. NT and REX1 vs. NT in apical bud and fifth functional leaf, and the significantly differentially expressed photosynthesis-related genes are probably the key clues for discovering the photosynthesis adaptability mechanism of PsnWRKY70 transgenic P. simonii × P. nigra under salt stress.

Paulownia species not only have significant economic benefits but also show great potential in ecological conservation. However, they are highly susceptible to phytoplasma infections, causing Paulownia witches’ broom (PaWB), which severely restricts the development of the Paulownia industry. Salicylic acid (SA) plays a crucial role in plant disease resistance. However, there have been no reports on the effect of SA on PaWB. Due to the properties of SA, it may have potential in controlling PaWB. Based on the above speculation, the prevention and therapeutic effect of SA on PaWB and its effect on the PaWB-infected Paulownia transcriptome and proteome were studied in this work. The results indicated that 0.1 mmol/L was the optimal SA concentration for inhibiting the germination of Paulownia axillary buds. In terms of resistance physiological indicators, SA treatment significantly affected both Paulownia tomentosa infected (PTI) seedlings and Paulownia fortunei infected (PFI) seedlings, where the activities of peroxidase (POD) and superoxide dismutase (SOD) were enhanced. Malondialdehyde (MDA), O₂⁻, and H₂O₂, however, were significantly reduced. Specifically, after SA treatment, SOD activity increased by 28% in PFI and 25% in PTI, and POD activity significantly increased by 61% in PFI and 58% in PTI. Moreover, the MDA content decreased by 30% in PFI and 23% in PTI, the H₂O₂ content decreased by 26% in PFI and 19% in PTI, and the O₂⁻ content decreased by 21% in PFI and 19% in PTI. Transcriptomic analysis showed that there were significant upregulations of MYB, NAC, and bHLH and other transcription factors after SA treatment. Moreover, genes involved in PaWB-related defense responses such as RAX2 also showed significant differences. Furthermore, proteomic analysis indicated that after SA treatment, proteins involved in signal transduction, protein synthesis modification, and disease defense were differentially expressed. This work provides a research foundation for the prevention and treatment of PaWB and offers references for exploring anti-PaWB methods.