Abstract
Ammonium () alleviates manganese (Mn) toxicity in various plant species, but the underlying mechanisms are still unclear. In this study, we compared the effects of and nitrate () on rice (Oryza sativa L.) growth, accumulation and distribution of Mn, accumulation of iron (Fe), zinc (Zn) and copper (Cu), root cell wall components, and expression of Mn and Fe transporter genes. After rice seedlings were grown in non-pH-buffered nutrient solution for 2 days, the pH of growth medium changed from an initial value of 4.5 to 3.5 and to 5.5 in the presence of and in the presence of , respectively. Compared with , ammonium decreased nutrient-solution pH and alleviated Mn toxicity and accumulation in rice under non-pH-buffered conditions. This alleviation disappeared when 5 mM Homo-PIPES pH buffer was added. Regardless of N form, roots, shoots, root cell sap, and xylem sap accumulated much lower Mn at pH 3.5 than at pH 5.5, whereas Mn distribution in different leaves and Mn accumulation in root cell walls was affected by neither N form nor pH. Ammonium decreased the expression of the Mn influx transporter gene OsNramp5 in roots under non-pH-buffered conditions, but not under pH-buffered ones. OsNramp5 expression was down-regulated at pH 3.5 compared with pH 5.5. Another efflux Mn transporter gene, OsMTP9, was not regulated by either N form or pH. High pH (5.5) enhanced the expression of the Fe transporter gene OsIRT1 and increased the accumulation of Zn but not Fe or Cu in shoots compared with pH 3.5. Taken together, our results indicate that alleviates Mn toxicity and accumulation in rice through the down-regulatory effects of rhizosphere acidification on the Mn influx transporter gene OsNramp5. In addition, the up-regulation of OsIRT1 expression may contribute to the increased Zn uptake by rice at high pH of nutrient solution.
Highlights
Manganese (Mn), an essential microelement required by plants, is involved in the regulation of several metabolic processes, such as photosynthesis, respiration, and antioxidant activity
In order to investigate whether the effects of pH on Mn uptake and transporter gene expression are specific, we further examined the expression of the Fe transporter gene OsIRT1 in roots and the accumulation of Fe, Zn, and Cu in rice
In agreement with previous studies of other plant species (Arnon, 1937; Vlamis and Williams, 1962; McGrath and Rorison, 1982; Elamin and Wilcox, 1986; Reddy and Mills, 1991; Langheinrich et al, 1992), we found that NH4+ alleviated Mn toxicity and decreased Mn accumulation in rice
Summary
Manganese (Mn), an essential microelement required by plants, is involved in the regulation of several metabolic processes, such as photosynthesis, respiration, and antioxidant activity. Mn plays an important role in activating approximately. Excess Mn can be toxic to plant growth, especially in acid soils where. Mn toxicity may be the second most important limiting factor, after aluminum (Al) toxicity, for plants in acid soils (Foy, 1984; Millaleo et al, 2010). 30% of the world’s ice-free land and 50% of arable and potentially arable lands are composed of acid soils (Von Uexküll and Mutert, 1995). The threshold of Mn accumulation before Mn toxicity occurs generally varies from 200 to 3,500 mg Mn·kg−1 DW (Krämer, 2010), but some Mn hyperaccumulators, such as Proteaceae, Phytolacca, and Gossia species, can accumulate more than 10,000 mg Mn·kg−1 DW in their aerial parts without detrimental effects (Brooks et al, 1981; Xue et al, 2004; Fernando et al, 2009; Liu et al, 2010)
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