ATP Binding Cassette Proteins ABCG37 and ABCG33 are required for potassium-independent cesium uptake in Arabidopsis roots

Abstract

26Radiocesium, accumulated in the soil by nuclear accidents is a major environmental concern.27 The transport process of cesium (Cs + ) is tightly linked to the indispensable plant nutrient 28 potassium (K + ) as they both belong to the group I alkali metal with similar chemical properties. 29 Most of the transporters that had been characterized to date as Cs + transporters are directly or 30 indirectly linked to K + . Using a combinatorial approach of physiology, genetics, cell biology and 31 root uptake assay, here we identified two ATP-Binding Cassette (ABC) proteins, ABCG37 and 32 ABCG33 as facilitators of Cs + influx. The gain-of-function mutant of ABCG37 (abcg37-1) 33 showed hypersensitive response to Cs + -induced root growth inhibition, while the double knock 34 out mutant of ABCG33 and ABCG37 (abcg33-1abcg37-2) showed resistance. Single loss-of-35 function mutant of ABCG33 and ABCG37 did not show any alteration in Cs + response. Short 36 term uptake experiment with radioactive Cs + revealed reduced Cs + uptake in abcg33-1abgc37-2 37 compared with wild type in presence or absence of K + . Potassium response and content were 38 unaffected in the double mutant background confirming that Cs + uptake by ABCG33 and 39 ABCG37 is independent of K + . Collectively, this work identified two ABC proteins as new Cs + 40 influx carriers, which act redundantly and independent of K + uptake pathway. 41 42 43 44 45 46 47 48 49 50 51 52 drawbacks; 1) the K + transporter functioning at low external potassium concentration shows little 83 discrimination against Cs + , while the K + channel is dominant at high external K + concentration 84 with high discrimination against Cs + (Zhu and Smolders, 2000), 2) high concentration of 85 potassium itself is toxic to plants regardless of cesium concentration (Hampton et al., 2004), and 863) it is also economically impractical to provide large amount of potassium to soil. Further, it is a 87 general consensus among the scientists that the mechanism by which Cs + is taken up by plant 88 roots is not completely understood. K + transporters and channel are only partially responsible for 89 Cs + uptake and translocation (Zhu and Smolders, 2000). Taken together, these results suggest 90 that there are alternative routes for Cs + transport in plant which needs to be explored to 91 understand the molecular mechanism of Cs + uptake. 92One of the major pathways for plant to detoxify toxic metals is through transporting to 93 sequestering them in the vacuole. The ATP binding cassette (ABC) transporters, also called 94 multidrug resistance proteins, are ubiquitous in plant and animal kingdom and play an important 95 role in transporting various substances, including metals. For instance, tonoplast-localized 96 AtABCC1 and AtABCC2 transport As (Song et al., 2010), Cd, and Hg (Park et al., 2012) inside 97 vacuole. AtATM3/AtABCB25 is involved in cadmium transport (Kim et al., 2006). Plasma 98 membrane-localized AtABCG36/AtPDR8 functions as efflux carrier of Cd (Kim et al., 2007). 99 AtABCG40...

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