Abstract
Aucuba japonica Thunb. is an evergreen understory shrub that grows naturally at a mine site. The mine soil contains high concentrations of heavy metals, and A. japonica appears to maintain detoxification mechanisms against heavy metals in the study site’s understory. This study aimed to investigate the heavy metal tolerance mechanisms in A. japonica, considering the possible roles of arbuscular mycorrhizal and root-endophytic fungi. We conducted fieldwork in summer (canopy-foliation season) and winter (canopy-defoliation season) to measure the heavy metal concentrations in leaves, branches, and roots and analyze possible detoxicants in the roots. The infection rates of arbuscular mycorrhizal and root-endophytic fungi were evaluated via microscopic observation, and heavy metal (Zn) localization in A. japonica roots was observed using confocal laser scanning microscopy. Field analysis showed that A. japonica accumulated excessive Zn and produced aucubin and citric acid in the roots in both summer and winter. Zn localization observations clarified that Zn was distributed in thickened epidermal and cortical cell walls, suggesting that the cell walls functioned as Zn deposition sites, reducing Zn toxicity. It was further clarified that Zn was contained within cortical cells, indicating that Zn might be detoxified by aucubin and citric acid. Arbuscular mycorrhizal and root-endophytic fungi within cortical cells adsorbed Zn on fungal cell walls, indicating that these fungi would reduce Zn content within root cells and might alleviate Zn toxicity. Our results indicated that A. japonica would maintain Zn tolerance in both summer and winter via Zn immobilization in the cell walls and production of aucubin and citric acid, and that arbuscular mycorrhizal and root-endophytic fungi might play important roles in the Zn tolerance of A. japonica.
Highlights
Heavy metals, which refer to metals with densities of more than 5 g/cm3 [1], naturally occur in rocks and can be found around mine sites, agricultural lands, and urban industrial areas due to industrial activities [2]
By observing Zn localization, it was confirmed that Zn accumulated in the thickened epidermal and cortical cell walls of A. japonica roots (Fig 2A, 2C and 2E), indicating that the cell walls function as Zn deposition sites in the roots
Few studies have focused on Zn tolerance and cell wall functions in wild tree species, but it has been shown that Populus tremula sapling, grown in soil with added Cd, Cu, Pb, and Zn, exhibited metal tolerance by accumulating large amounts of Cu and Zn in the fine root cell walls [42]
Summary
Heavy metals, which refer to metals with densities of more than 5 g/cm3 [1], naturally occur in rocks and can be found around mine sites, agricultural lands, and urban industrial areas due to industrial activities [2] Some heavy metals, such as Cu and Zn, are essential for plant growth, but excess heavy metals (both essential and non-essential) in plant tissues can cause a range of toxicity symptoms such as chlorosis, root-tip browning, and plant growth retardation [3,4,5]. Plants produce various types of primary and secondary metabolites with antioxidant activity or high affinity to metals These detoxicants directly scavenge reactive oxygen species or prevent heavy metals from binding to biomolecules [11, 12]. The detoxification mechanisms described above are considered necessary for plants to survive under heavy metal stress
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