Integrated transcriptomic and metabolomic profiling and functional characterization of the MaXTH23 gene in boron stress adaptation in mulberry (Morus alba L.)

Plants have evolved intricate and sophisticated mechanisms to sense and respond to boron (B) stresses. Alterations to the cell wall and other molecular pathways are strategies that help plants adapt to B stresses by cross-linking with rhamnogalacturonan II (RG-II) to form borate-dimers. However, the molecular mechanism by which cell wall components and organization respond to B stresses is not fully understood in mulberry plants. This study, via conjoint transcriptomics-metabolomics and virus-induced gene silencing analyses, aimed to explore the diverse B stress response mechanisms and functionally characterize the role of MaXTH23 in cell wall remodeling in mulberry leaves subjected to different levels of B, ranging from deficiency (0 mM; T1), sufficiency (0.1 mM; control, CK), moderate deficiency (0.02 mM; T2), toxicity (0.5 and 1.0 mM as T3 and T4, respectively) and cultivated under greenhouse conditions. The analyses identified a total of 6114 and 441 differentially expressed genes (DEGs) and metabolites (DEMs), respectively, in the different KEGG pathways in the separate omics analysis for all treatments. However, our conjoint analysis identified 1120 DEGs associated with 78 DEMs and were significantly co-enriched in 96 different KEGG pathways. Meanwhile, the functional characterization via silencing of MaXTH23 did not nullify its function in cell wall modification and remodeling but concomitantly caused significant increases in total pectin and water-soluble pectin contents, quintessentially promoting pectin cross-linking in the cell wall. This study highlights a novel perspective for identifying and characterizing the regulatory functions of MaXTH23 and the B-induced pathways and tolerance mechanisms employed by mulberry plants.