Nano zinc oxide activates MdCDF2 to promote DNA replication and cell proliferation in apple calli

Apple (Malus domestica) is grown worldwide. The yield and quality of apple depend on cultivation techniques and nutrition. Nano zinc oxide (ZnO NPs) has been widely applied in agricultural production, commonly used in fertilizers to help crops increase yield and enhance abiotic stress tolerance. However, there are few studies on the effects of ZnO NPs on apple growth. This study found that 500 ppm ZnO NPs treatment can promote the proliferation of apple calli cells. Using Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) technology, it was confirmed that ZnO NPs can enter apple calli cells more efficiently than ZnO bulk particles (ZnO BPs) and ZnSO4. Through scanning electron microscopy (SEM) and transmission electron microscopy (TEM), it was observed that the ZnO NPs were mainly aggregated in the cytoplasm and nucleus. Transcriptome analysis showed that the expression levels of some zinc finger-containing transcription factors (TFs) were up-regulated after 500 ppm ZnO NPs treatment. Kyoto Encyclopedia of Genes and Genomes Enrichment Analysis (KEGG) and Gene Ontology Analysis (GO) found that the expression levels of some DNA replication-related genes were significantly changed. DNA pull-down experiments proved that MdCDF2 (a zinc finger-containing TF) can bind to the promoter regions of three DNA replication-related genes (MdClpB1, MD01G1182200, MD12G1082300). These data indicated that ZnO NPs may promote DNA replication and cell proliferation in apple calli cells by increasing the expression level of MdCDF2. This study provides new insights into the molecular mechanisms by which ZnO NPs enhance plant growth efficiency and crop yield.