Abstract
Knowledge of arsenic (As) accumulation in rice (Oryza sativa L.) is important for minimizing As transfer to the food chain. The aim of this study was to investigate the role of rice nodes in As storage and distribution. Synchrotron μX-ray fluorescence (μ-XRF) was used to map As distribution in the top node and internode of a lsi2 mutant defective in silicon/arsenite efflux carrier and its wild-type (WT) grown in soil. Lsi2 expression in different tissues during grain filling was investigated by quantitative RT-PCR. Arsenite or dimethylarsinic acid (DMA) was supplied to excised panicles to investigate the roles of Lsi2 and phytochelatins (PC) in As distribution. μ-XRF mapping revealed As storage in the phloem of different vascular bundles in the top node and internode. Soil-grown plants of lsi2 had markedly decreased As accumulation in the phloem compared with the WT. Lsi2 was strongly expressed, not only in the roots but also in the nodes. When excised panicles were exposed to As(III), the lsi2 mutant distributed more As to the node and flag leaf but less As to the grain compared with the WT, while there was no significant difference in DMA distribution. Inhibition of PC synthesis by l-buthionine-sulphoximine decreased As(III) deposition in the top node but increased As accumulation in the grain and flag leaf. The results suggest that rice nodes serve as a filter restricting As(III) distribution to the grain. Furthermore, Lsi2 plays a role in As(III) distribution in rice nodes and phytochelatins are important compounds for As(III) storage in the nodes.
Highlights
Rice is the staple food for about half of the world’s population, but it is the most important dietary source of inorganic arsenic (As), a class-one carcinogen (Meharg et al, 2009; Li et al, 2011)
When excised panicles were exposed to As(III), the lsi2 mutant distributed more As to the node and flag leaf but less As to the grain compared with the WT, while there was no significant difference in dimethylarsinic acid (DMA) distribution
enlarged VB (EVB) and diffuse VB (DVB) are connected to the flag leaf and the panicle, respectively, and the transfer of minerals between these two types of vascular bundle determines the relative distribution of minerals between the flag leaf and the grain
Summary
Rice is the staple food for about half of the world’s population, but it is the most important dietary source of inorganic arsenic (As), a class-one carcinogen (Meharg et al, 2009; Li et al, 2011). Lsi is an aquaporin channel belonging to the nodulin26-like intrinsic protein (NIP) family It is localized on the plasma membranes of the distal side of both exodermal and endodermal cells, and allows silicic acid and As(III) to permeate into the cells (Ma et al, 2006). A new arsenate reductase, named HAC1 (Chao et al, 2014) or ATQ1 (Sanchez-Bermejo et al, 2014), has recently been identified in A. thaliana This enzyme is critical for limiting As accumulation in A. thaliana shoots; the loss of function of HAC1 results in decreased As(V) reduction in the roots, diminished As(III) efflux to the external medium and hyperaccumulation of As in the shoots (Chao et al, 2014)
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