TiO2 nanoparticles improves cadmium toxicity tolerance in Hemerocallis citrina Baroni by modulating photosynthetic and antioxidative profile.

Key message: TiO₂ nanoparticles mitigates the toxicity of Cd to Hemerocallis citrina Baroni (daylily) by modulating the photosynthetic and antioxidative system, as revealed by physiological and transcriptomic analysis. Cadmium (Cd) is a common heavy metal pollutant exerting toxicity to plants. The unique physiochemical properties of titanium dioxide nanoparticles (TiO₂ NPs) suggest their potential applications in agriculture. The molecular and physiological responses of Hemerocallis citrina Baroni (daylily) to Cd stress and the ameliorative effect of TiO₂ NPs were investigated. KEGG enrichment analysis on differentially expressed genes (DEGs) revealed pronounced enrichment of pathways related to photosynthesis. GO enrichment analysis showed that chlorophyll metabolism and redox process were also notably enriched. Furthermore, weighted gene co-expression network analysis (WGCNA) demonstrated remarkable responses of photosynthetic characteristics and antioxidative system, and identified MYB, NAC, and WRKY transcription factors which played key roles in the Cd-stress response and regulation by TiO₂ NPs. Under 5 mmol·L^-1 Cd stress, daylily growth was severely inhibited, and cell membrane permeability and osmolytes significantly increased. Additionally, Cd stress pronouncedly impaired photosynthesis, increased the accumulation of reactive oxygen species in leaves, and inhibited the activities of most antioxidants. However, foliar spraying of 200 mg·L^-1 TiO₂ NPs promoted plant growth and increased osmolytes. The inhibition on leaf photosynthetic antenna proteins, photosystem reaction center activity, electron transfer rate, chlorophyll synthesis, and Calvin cycle process was markedly alleviated by upregulating corresponding gene expression as revealed by photosynthesis-related traits and DEG analysis. The activities of key enzymes in ascorbate-glutathione (AsA-GSH) cycle and thioredoxin-peroxiredoxin (Trx-Prx) pathway were enhanced, and the regeneration of AsA and GSH was promoted. Overall, TiO₂ NPs mitigated Cd-induced inhibition of photosynthesis and antioxidative system, and enhanced Cd tolerance of daylily.