Dissecting the temporal dynamics of leaf Zn concentration following foliar Zn application in soybean through Zn quantification and transcriptomic analysis

Foliar Zn fertilization is an important agronomic practice for correcting Zn deficiency and achieving Zn biofortification in crops. However, the long-term temporal dynamics of leaf Zn concentration following foliar Zn application and the associated leaf gene expression patterns remain poorly understood. In the present study, ZnSO 4 was foliar-applied to soybean ( Glycine max (L.) Merr.) at different growth stages, and Zn concentrations and transcriptome profiles of the Zn-applied leaves were tracked to characterize the temporal changes in leaf Zn concentration and investigate the associated gene expression patterns. Regardless of the Zn application time, the Zn concentration in the Zn-applied leaves decreased after the onset of reproductive growth. This suggests a temporal association between the decline in leaf Zn concentration and the transition from vegetative to reproductive growth. Transcriptomic profiling and weighted gene co-expression network analysis revealed a gene module that was significantly correlated with the observed decline in leaf Zn concentration. The MAPK cascades and calcium signaling pathways were enriched in this module and may be involved in the regulation of this change in leaf Zn concentration. Additionally, candidate Zn transporters ( Gm ZIP12, Gm ZIP13, Gm ZIP23, Gm HMA7, Gm HMA11, Gm HMA12, Gm MTP2.1, Gm MTP5.2, and Gm YSL3Lb) and transcription factors (from families such as bZIP, NAC, bHLH, MYB, and ERF) potentially involved in the observed decline in leaf Zn concentration were also identified. The present study found that the Zn concentration in Zn-applied soybean leaves mainly declined after the onset of reproductive growth. In parallel, potential signaling pathways and candidate genes that may be involved in the observed decline in leaf Zn concentration were identified. The temporal leaf Zn concentration dataset and the associated transcriptomic profiles provide foundational information for future investigations into the translocation of foliar-applied Zn and the underlying molecular mechanisms in soybean.