Candidates for Breeding Target Genes Related to Cesium Transport in Plants After the Fukushima Daiichi Nuclear Power Plant Accident

Abstract

In this chapter, I summarize the knowledge about Cs⁺ transport genes in plants, which would be potential candidates for breeding targets of low-Cs crops. After the Fukushima Daiichi Nuclear Power Plant accident, vigorous studies about Cs transport in crops have revealed various Cs⁺ transport systems. Most reported Cs⁺ transport systems were potassium ion transporters (KUP/HAK/KT family), but some were free from potassium ion transport systems such as SNARE Sec22p/SEC22 and ATP-binding cassette (ABC) proteins. Diversity of Cs⁺ transport systems would encourage projects of making low-Cs crops.

6.1 Introduction

A large area in Eastern Japan was contaminated by radiocesium (¹³⁷Cs and ¹³⁴Cs) derived from the Tokyo Electric Power Company’s Fukushima Daiichi Nuclear Power Plant (TEPCO-FDNPP) accident in 2011. At that time, there had been a lot of knowledge about Cs behavior in the environment (air, soil, water, etc.) and living things (plants, animals, microorganisms, etc.). Most of the knowledge had been obtained through research on the Chornobyl accident. Therefore, we, local agricultural researchers around Fukushima, searched for the Cs knowledge among the old journals to comprehend what would happen in food contamination because of the accident. As a result, we could obtain numerous meaningful and practical information about counter measurement in agriculture, for example, potassium amelioration on Cs accumulation in crops and contribution of soil minerals on Cs fixation in soil. In addition, there was a lot of data about Cs accumulation tendency among crops; for example, berries and mushrooms likely have higher Cs concentrations, and grains have much lower Cs.

Cs⁺ transport mechanism in plants has been vigorously researched before the TEPCO-FDNPP accident in 2011. Previous researches revealed that the uptake mechanism of Cs⁺ from soil to root was similar to that of K⁺ (Collander 1941; Bange and Overstreet 1960; White and Broadley 2000). Consequently, reports on K⁺ transporters mediating Cs⁺ in plants increased (Kim et al. 1998; Qi et al. 2008). Especially HAK5, a high-affinity transport system belonging to the K⁺ uptake permease transporter/high-affinity K⁺ transporter/K⁺ transporter (KUP/HAK/KT) family, was found to be the main contributor mediating Cs⁺ uptake from soil to root (Qi et al. 2008). Furthermore, in addition to the KUP/HAK/KT family, KAT1 and SKOR1 were shown to mediate Cs⁺ by electrophysiology (see the review of Zhu and Smolders 2000; White and Broadley 2000). Other than potassium transporters, cyclic nucleotide (CN)-gated and calmodulin (CaM)-regulated channels (CNGCs) have been candidates to transport Cs⁺. For example, AtCNGC10 might allow Cs⁺ to cells when AtCNGC10 was expressed in E. coli (Li et al. 2005).

TEPCO-FDNPP accident has been promoting the research on every aspect of Cs⁺, and the studies on the molecular mechanism of Cs⁺ transport in plants have also progressed. In this chapter, I summarized the reports for 11 years, from 2011 to 2022, about transporters and transportation systems, which have possibilities to be employed for making low-Cs crops (Fig. 6.1).

Fig. 6.1

Cesium ion transport mechanism in plant roots. (a) Cesium transport from soil solution to root cell. For example, KUP/HAK/KT family and ATP-binding cassette (ABC) proteins, ABCG37 and ABCG33, are transporters mediating the Cs⁺ transport process. (b) Cesium transport from cytosol to vacuole inside cells. For example, the SNARE protein, Sec22p/SEC22, was reported to be involved in the transport system

6.2 Potassium Transporters, KUP/HAK/KT Family

The potassium transporter of Arabidopsis, HAK5, was already known as the primary contributor of Cs⁺ uptake from soil to plant under low potassium conditions before the TEPCO-FDNPP accident, 2011 (Qi et al. 2008). However, the primary transporters mediating Cs⁺ uptake in rice had been unknown. In 2017, OsHAK1 was identified to mediate Cs⁺ uptake from soil to plants by reverse genetics (Nieves-Cordones et al. 2017). In the paper, OsHAK1 was firstly expressed in yeast and shown to mediate Cs⁺ uptake in yeast. Then OsHAK1-edited mutant plants were produced using the CRISPR/Cas9 genome editing technique and examined Cs⁺ uptake in hydroponic culture solution and soil derived from Fukushima. As a result, Cs concentration in OsHAK1-edited mutant plants was drastically reduced compared with the wild-type plant when the plants grew on the low potassium soil. The results implied that OsHAK1 would be a breeding target for making low-cesium rice. However, OsHAK1-edited mutants have a penalty for obtaining potassium from low-K soil (the potassium concentration was about half that of wild-type plants), resulting in low biomass in OsHAK1-edited mutants under low potassium conditions (Nieves-Cordones et al. 2017). The strategy of simple knock-out OsHAK1 seems not to work. On the other hand, upgrading ion selectivity would be a potential strategy for making low-Cs rice. Cs⁺ concentration in soil is so low (~10 μM) that the OsHAK1 selectivity of K⁺ against Cs⁺ has not been subjected to evolutionary pressure. Interestingly, a single mutation of amino acid residue (F130S) in the Arabidopsis HAK5 increases K⁺ selectivity over Cs⁺ (Alemán et al. 2014), suggesting room for improving K⁺ selectivity over Cs⁺ in other plants and crops. OsHAK1 was again identified as a causal gene of low-Cs rice in the same year by a forward genetic strategy using Japonica rice cv. Akitakomachi (Rai et al. 2017). In the paper, potassium concentration and biomass of oshak1 were not changed compared with the wild-type. The results indicate that some cultivars could obtain enough potassium from other K⁺ transport systems even though the cultivar lacks OsHAK1. It would be a valuable strategy to find rice cultivars with strong potassium absorption under low K condition and then mutate the OsHAK1.

OsSOS2 is not directly related to potassium or cesium transport, but mutation in OsSOS2 reduced Cs in rice grain (Ishikawa et al. 2017). Under Na⁺ stress, OsSOS2 activates OsSOS1, which is a Na⁺ efflux transporter. The expression levels of OsHAK1, OsHAK5, OsAKT1, and OsHKT2;1 were about half in the OsSOS2 null mutant compared with WT under low-K (0.02 mM) Na existence (10 mM) condition. It was suggested that some of the four potassium transporters (OsHAK1, OsHAK5, OsAKT1, and OsHKT2;1) mediate Cs⁺ uptake and transport to grains. Because the mutant did not have any penalty in growth, it would be an attractive plant to grow under radiocesium-contaminated land.

6.3 SNARE Sec22p/SEC22

The SNARE protein Sec22p/SEC22 functions for Cs accumulation in yeast and plants (Dräxl et al. 2013). The yeast mutant, sec22⊿, reduced Cs accumulation in the cell but did not change Rb and K concentration. The mathematical model predicted the Cs deposition in the vacuole. They also investigated the homologous gene, SEC22, in the model plant Arabidopsis. They found a sec22 mutant that loses its expression in root, stem, and leaves but remains the expression in reproductive organs and young tissue. As a result, the sec22 mutant reduced Cs concentration by half in leaves and 25% in roots. Even though the detailed mechanism of Sec22p/SEC22 for Cs⁺ transport remains unknown, the SNARE protein would be a breeding target to produce low-Cs crops.

Retaining toxic elements in the vacuole of roots is one of the promising methods to reduce toxic element accumulation in grains. There is an excellent example of cadmium. OsHMA3 is a Cd⁺ transporter that mediates Cd from cytosol to vacuole. OsHMA3-overexpression rice reduced Cd translocation from roots to shoots, resulting in 94–98% diminished Cd concentration in grains (Lu et al. 2019). In the case of Cs, there have been no reports about transporters specifically mediating Cs⁺ from the cytosol to the vacuole in roots. It is unknown that the SNARE Sec22p/SEC22 protein can be used for increasing Cs concentration in the vacuole. Further studies about Cs⁺ transport in the vacuole are needed.

6.4 ATP-Binding Cassette (ABC) Proteins, ABCG37 and ABCG33

The ATP binding cassette (ABC) transporters play essential roles in transporting various substances. Recently, ABC transporters are also involved in transporting minerals and metals; for example, arsenic (OsABCC1 and OsABCC2: Song et al. 2010), cadmium and mercury (OsABCC1 and OsABCC2: Park et al. 2012), cadmium (AtABCB25/AtATM3: Kim et al. 2006; AtABCG36/AtPDR8: Kim et al. 2007), lead (AtABCG4: Lee et al. 2005), and cesium (ABCG37 and ABCC33: Ashraf et al. 2021).

ABCG33 and ABCG37 have been shown to mediate Cs⁺ in the wide range of Cs concentrations in the medium (0.4–10 mM). Importantly, ABCG33 and ABCG37 did not transport Rb⁺ in yeast, suggesting that the ABCG33 and ABCG 37 can recognize the difference between Cs⁺ and Rb⁺. Furthermore, the plant of double knock-out of ABCG33 and ABCG37 showed reduced Cs⁺ uptake compared with every single knock-out plant and wild type. Interestingly, the tendency of reduced Cs⁺ uptake manner of the mutants was not changed under high and low K⁺ conditions, indicating that potassium conditions do not influence the ABCG33 and ABCG37 function as Cs⁺ transport. The Cs⁺ uptake system that does not depend on K condition would help make crops of low-Cs contents under a standard-K field.