Abstract
To develop elite crops with low cadmium (Cd), a fundamental understanding of the mechanism of Cd uptake by crop roots is necessary. Here, a new mechanism for Cd2+ entry into rice root cells was investigated. The results showed that Cd2+ influx in rice roots exhibited spatially and temporally dynamic patterns. There was a clear longitudinal variation in Cd uptake along rice roots, with the root tip showing much higher Cd2+ influx and concentration than the root mature zone, which might be due to the much higher expression of the well-known Cd transporter genes OsIRT1, OsNRAMP1, OsNRAMP5, and OsZIP1 in the root tip. Both the net Cd2+ influx and the uptake of Cd in rice roots were highly inhibited by ion channel blockers Gd3+ and TEA+, supplementation of Ca2+ and K+, and the plasma membrane H+-ATPase inhibitor vanadate, with Gd3+ and Ca2+ showing the most inhibitory effects. Furthermore, Ca2+- or Gd3+-induced reduction in Cd2+ influx and Cd uptake did not coincide with the expression of Cd transporter genes, but with that of two Ca channel genes, OsAAN4 and OsGLR3.4. These results indicate that Cd transporters are in part responsible for Cd2+ entry into rice root, and provide a new perspective that the Ca channels OsAAN4 and OsGLR3.4 might play an important role in rice root Cd uptake.
Highlights
As a result of atmospheric deposition, wastewater irrigation, use of metal-containing fertilizers and pesticides, and many other industrial processes, cadmium (Cd) has become one of the most toxic and widespread environmental pollutants in agricultural soil (Rizwan et al, 2016)
In agreement with the previous microelectrode measurements in other plant species (Piñeros et al, 1998; He et al, 2011; Sun et al, 2013), the present study found that the Cd2+ influx in roots of rice exposed to 20 μM CdCl2 was much greater in the root tip (0–2 mm from the root cap) than in the mature zone (MZ) (10–20 mm from root cap) (Figs 1, 2), which was further evidenced by the measurements of Cd concentration in root segments and the fluorescent labeling of Cd ions in the root tip, whether with or without pre-treatments (Figs 5, 6)
No significant difference in the development of Casparian bands and suberin lamellae and cell lignification was observed between the root tip (0–2 mm from root cap) and the MZ (10–15 mm from root cap) after 3 d of 20 μM CdCl2, indicating an inability of the apoplastic barrier to explain the occurrence of longitudinal variation in Cd2+ influx in this study
Summary
As a result of atmospheric deposition, wastewater irrigation, use of metal-containing fertilizers and pesticides, and many other industrial processes, cadmium (Cd) has become one of the most toxic and widespread environmental pollutants in agricultural soil (Rizwan et al, 2016). To ensure food safety, breeding ‘low-Cd’ crops has become one of the most important strategies to reduce Cd in crops, for which a fundamental understanding of the Cd uptake mechanism in plant roots would be a critical issue. A recent study by Takahashi et al (2014) revealed that the knock-down of OsNRAMP5 triggered only ~20% reduction in root Cd content but a significant increase in shoot Cd content in both hydroponic and field trials These results indicated that there might be other pathways for Cd entry into root cells, apart from the poor selectivity of transition ion transporters. More effort is still needed to elucidate thoroughly the mechanism by which Cd is taken up into plant roots and cells
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