Physiological, anatomical, and transcriptional responses of mulberry (Morus alba L.) to Cd stress in contaminated soil

Mulberry has been widely studied for its capacity to tolerate heavy metals. However, the anatomical and molecular response mechanisms of Cd detoxification and transportation in mulberry have not been fully elucidated. In this study, the anatomical characteristics, Cd and mineral element uptake and transport, and transcriptome profiling of mulberry were studied under Cd stress. The results showed that mulberry possessed strong detoxification and self-protection abilities against Cd stress. The growth and photosynthetic pigment contents of mulberry were only slightly affected when the soil Cd content was less than 37.0 mg/kg, while the Ca and Mg contents in the mulberry roots were clearly (p < 0.05) increased by 37.85%-40.87% and 36.63%-53.06% in 37.0-55.4 mg/kg Cd-contaminated soil. Meanwhile, the relationships between antioxidant enzyme activities, such as pemxidase, catalase, and ascorbate peroxidase, and Cd content in plants were positive. Furthermore, the structures of leaf cells, root and stem tissues were largely intact; simultaneously, the increase in osmiophilic particles and the dissolution of starch granules in mulberry leaves significantly responded to Cd stress. Clusters of Orthologous Groups of proteins (COG) and Gene Ontology (GO) classification analysis indicated that mulberry can enhance the catalytic activity, regulate the transport and metabolism of inorganic ions, and strengthen its antioxidant enzyme activity and defense mechanism to decrease Cd intoxication. Large numbers of differentially expressed genes associated with cell wall biosynthesis, antioxidant enzyme activities, glutathione metabolism, chelation, plant hormone signal transduction, and the mitogen-activated protein kinase (MAPK) signaling pathway were upregulated under Cd stress. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis indicated that plant hormone signal transduction was significantly (p < 0.05) enriched in mots, stems, and leaves of mulberry, and abscisic acid and ethylene can mediate MAPK signaling pathways to increase plant tolerance to Cd stress. The results suggested that the physiological, cellular and tissue, and transcriptional regulation of mulberry can facilitate its stress adaptation in Cd-contaminated soil.