Exogenous coumarin improves cell wall and plasma membrane stability and function by maintaining copper and calcium homeostasis in citrus roots under copper excess.

Most citrus trees are planted in acidic soil with high availability of copper (Cu). Little is known about the mechanisms by which coumarin (COU) reduces Cu excess in plants. 'Xuegan' (Citrus sinensis) seedlings were treated with 0.5 (Cu0.5) or 400 (Cu excess or Cu400) CuCl and 0 (COU0) or 100 (COU100) μM COU for 24 weeks. COU100 alleviated Cu400-induced alterations in gene expression and metabolite profiles, cell wall (CW) materials (CWMs), CW components (CWCs), and Fourier transform infrared (FTIR) spectra of CWMs in roots; increase in Cu concentration in roots, root CWMs (RCWMs), root CWCs (RCWCs), Cu and Ca fractions in RCWMs, and Cu fraction in CW pectin; and decrease in Ca concentrations in roots, RCWMs, and RCWCs. In addition, COU100 mitigated Cu400-induced increase in electrolyte leakage and concentrations of total coumarins, total phenolics, total falvonoids, and nonstructural carbohydrates (NCs) and decrease in total free amino acid concentration in roots, as well as impairment in root system architecture (RSA) and root growth. Our results corroborated the hypothesis that the alleviation of root Cu excess by COU was caused by the combination of following several aspects: (a) reduced impairment to root growth and RSA; (b) upregulated ability to maintain CW and plasma membrane stability and function by maintaining Cu and calcium homeostasis; (c) elevated adaptability of primary metabolism to Cu excess; and (d) upregulated biosynthesis and catabolism (turnover) of secondary metabolites (SMs) and less upregulation of SMs. COU0-treated roots underwent some physiological and molecular adaptations to Cu excess.