Euphorbia marginata Alleviate Heavy Metal Ni-Cu Combined Stress by Regulating the Synthesis of Signaling Factors and Flavonoid Organisms

It is of great importance to explore how plants respond to excess accumulation of Cu and Ni in soil, yet the mechanisms by which <i>Euphorbia marginata</i>, a common ornamental plant in China, responds to heavy metal stress remain unclear. In this study, <i>E. marginata</i> seedlings were subjected to CK, Ni 500 mg/kg, and Cu 900 mg/kg, with Ni-Cu combined stress, and their growth, physiological indexes, heavy metal accumulation, and their corresponding gene expression were evaluated after 45 d. The results showed that the two heavy metals mainly accumulated in plant roots and severely inhibited root growth, while the combined stress promoted the accumulation of heavy metals to a small extent. Either Cu or Ni stresses inhibit photosynthetic pigment synthesis as well as activate antioxidant and osmoregulatory systems, but there are differences in their effects. Combined stress has a synergistic stress effect, severely damaging the cell membrane structure and leading to dysregulation of antioxidant and osmoregulatory systems. The expression of CDPK, CaMCML, MEKK3/6 signaling factors, UFGT, and COMT was severely suppressed under the combined stresses of Cu and Ni compared to the single stress of both. These results provide evidence of a specific defense response to heavy metal stress in <i>E. marginata</i>, which could help guide new research efforts and support the development of strategies for phytoremediation using <i>E. marginata</i>.