Tomoaki Horie

Plant salt-tolerance mechanisms

Crop performance is severely affected by high salt concentrations in soils. To engineer more salt-tolerant plants it is crucial to unravel the key components of the plant salt-tolerance network. Here we review our understanding of the core salt-tolerance mechanisms in plants. Recent studies have shown that stress sensing and signaling components can play important roles in regulating the plant salinity stress response. We also review key Na+ transport and detoxification pathways and the impact of epigenetic chromatin modifications on salinity tolerance. In addition, we discuss the progress that has been made towards engineering salt tolerance in crops, including marker-assisted selection and gene stacking techniques. We also identify key open questions that remain to be addressed in the future.

Altered shoot/root Na+ distribution and bifurcating salt sensitivity in Arabidopsis by genetic disruption of the Na+ transporter AtHKT1

Sodium (Na+) is toxic to most plants, but the molecular mechanisms of plant Na+ uptake and distribution remain largely unknown. Here we analyze Arabidopsis lines disrupted in the Na+ transporter AtHKT1. AtHKT1 is expressed in the root stele and leaf vasculature. athkt1 null plants exhibit lower root Na+ levels and are more salt resistant than wild‐type in short‐term root growth assays. In shoot tissues, however, athkt1 disruption produces higher Na+ levels, and athkt1 and athkt1/sos3 shoots are Na+‐hypersensitive in long‐term growth assays. Thus wild‐type AtHKT1 controls root/shoot Na+ distribution and counteracts salt stress in leaves by reducing leaf Na+ accumulation.

HKT transporter-mediated salinity resistance mechanisms in Arabidopsis and monocot crop plants

The salinization of irrigated lands is increasingly detrimental to plant biomass production and agricultural productivity, as most plant species are sensitive to high concentrations of sodium (Na(+)), which causes combined Na(+) toxicity and osmotic stress. Plants have multiple Na(+)-transport systems to circumvent Na(+) toxicity. Essential physiological functions of major Na(+) transporters and their mechanisms mediating salinity resistance have been identified in Arabidopsis , including the AtSOS1, AtNHX and AtHKT1;1 transporters. As we discuss here, recent studies have demonstrated that a class of xylem-parenchyma-expressed Na(+)-permeable plant HKT transporters represent a primary mechanism mediating salt tolerance and Na(+) exclusion from leaves in Arabidopsis, and that major salt-tolerance quantitative trait loci in monocot crop plants are also based on this HKT-mediated mechanism.

A conserved primary salt tolerance mechanism mediated by HKT transporters: a mechanism for sodium exclusion and maintenance of high K+/Na+ ratio in leaves during salinity stress

Increasing soil salinity is a serious threat to agricultural productions worldwide in the 21st century. Several essential Na(+) transporters such as AtNHX1 and AtSOS1 function in Na(+) tolerance under salinity stress in plants. Recently, evidence for a new primary salt tolerance mechanism has been reported, which is mediated by a class of HKT transporters both in dicots such as Arabidopsis and monocot crops such as rice and wheat. Here we present a review on vital physiological functions of HKT transporters including AtHKT1;1 and OsHKT1;5 in preventing shoot Na(+) over-accumulation by mediating Na(+) exclusion from xylem vessels in the presence of a large amount of Na(+) thereby protecting leaves from salinity stress. Findings of the HKT2 transporter sub-family are also updated in this review. Subjects regarding function and regulation of HKT transporters, which need to be elucidated in future research, are discussed.

Rice OsHKT2;1 transporter mediates large Na+ influx component into K+-starved roots for growth

Excessive accumulation of sodium in plants causes toxicity. No mutation that greatly diminishes sodium (Na+) influx into plant roots has been isolated. The OsHKT2;1 (previously named OsHKT1) transporter from rice functions as a relatively Na+‐selective transporter in heterologous expression systems, but the in vivo function of OsHKT2;1 remains unknown. Here, we analyzed transposon‐insertion rice lines disrupted in OsHKT2;1. Interestingly, three independent oshkt2;1‐null alleles exhibited significantly reduced growth compared with wild‐type plants under low Na+ and K+ starvation conditions. The mutant alleles accumulated less Na+, but not less K+, in roots and shoots. OsHKT2;1 was mainly expressed in the cortex and endodermis of roots. 22Na+ tracer influx experiments revealed that Na+ influx into oshkt2;1‐null roots was dramatically reduced compared with wild‐type plants. A rapid repression of OsHKT2;1‐mediated Na+ influx and mRNA reduction were found when wild‐type plants were exposed to 30 mM NaCl. These analyses demonstrate that Na+ can enhance growth of rice under K+ starvation conditions, and that OsHKT2;1 is the central transporter for nutritional Na+ uptake into K+‐starved rice roots.

Glycine residues in potassium channel-like selectivity filters determine potassium selectivity in four-loop-per-subunit HKT transporters from plants

Plant HKT proteins comprise a family of cation transporters together with prokaryotic KtrB, TrkH, and KdpA transporter subunits and fungal Trk proteins. These transporters contain four loop domains in one polypeptide with a proposed distant homology to K+ channel selectivity filters. Functional expression in yeast and Xenopus oocytes revealed that wheat HKT1 mediates Na+-coupled K+ transport. Arabidopsis AtHKT1, however, transports only Na+ in eukaryotic expression systems. To understand the molecular basis of this difference we constructed a series of AtHKT1/HKT1 chimeras and introduced point mutations to AtHKT1 and wheat HKT1 at positions predicted to be critical for K+ selectivity. A single-point mutation, Ser-68 to glycine, was sufficient to restore K+ permeability to AtHKT1. The reverse mutation in HKT1, Gly-91 to serine, abrogated K+ permeability. This glycine in P-loop A of AtHKT1 and HKT1 can be modeled as the first glycine of the K+ channel selectivity filter GYG motif. The importance of such filter glycines for K+ selectivity was confirmed by interconversion of Ser-88 and Gly-88 in the rice paralogues OsHKT1 and OsHKT2. Surprisingly, all HKT homologues known from dicots have a serine at the filter position in P-loop A, suggesting that these proteins function mainly as Na+ transporters in plants and that Na+/K+ symport in HKT proteins is associated with a glycine in the filter residue. These data provide experimental evidence that the glycine residues in selectivity filters of HKT proteins are structurally related to those of K+ channels.

Sodium Transporters in Plants. Diverse Genes and Physiological Functions

Soil salinity represents an increasing threat to agricultural production. High sodium (Na+) concentrations in soils are toxic to most higher plants. More than 40% of irrigated lands worldwide show increased salt levels. Several studies have shown that under saline conditions, Na+ influx into root

Drought stress alters water relations and expression of PIP-type aquaporin genes in Nicotiana tabacum plants.

Plasma membrane intrinsic proteins (PIPs), a type of aquaporins, mediate water transport in many plant species. In this study, we investigated the relationship between the functions of PIP-type water channels and water relations of tobacco plants (Nicotiana tabacum cv. Samsun) under drought stress. Drought stress treatments have led to reductions in the stomatal conductance, transpiration, water potential and turgor pressure in leaves, and also the sap flow rate and osmotic hydraulic conductance in roots. In contrast, leaf osmotic pressure was increased in response to drought stress. Interestingly, the accumulation of NtPIP1;1 and NtPIP2;1 transcripts was significantly decreased, but only that of the NtAQP1 transcript was increased under drought stress. Functional analysis using Xenopus laevis oocytes revealed that NtPIP2;1 shows marked water transport activity, but the activities of NtAQP1 and NtPIP1;1 are weak or almost negligible, respectively, when expressed alone. However, co-expression of NtPIP1;1 with NtPIP2;1 significantly enhanced water transport activity compared with that of NtPIP1;1- or NtPIP2;1-expressing oocytes, suggesting that these two aquaporins may function as a water channel, forming a heterotetramer. Heteromerization of NtPIP1;1 and NtPIP2;1 was also suggested by co-expression analyses of NtPIP1;1-GFP (green fluorescent protein) and NtPIP2;1 in Xenopus oocytes. Re-watering treatments recovered water relation parameters and the accumulation of the three NtPIP transcripts to levels similar to control conditions. These results suggest that NtPIP1;1 and NtPIP2;1 play an important role in water transport in roots, and that expression of NtPIP1;1 and NtPIP2;1 is down-regulated in order to reduce osmotic hydraulic conductance in the roots of tobacco plants under drought stress.

Salinity tolerance mechanisms in glycophytes: An overview with the central focus on rice plants

Elevated Na+ levels in agricultural lands are increasingly becoming a serious threat to the world agriculture. Plants suffer osmotic and ionic stress under high salinity due to the salts accumulated at the outside of roots and those accumulated at the inside of the plant cells, respectively. Mechanisms of salinity tolerance in plants have been extensively studied and in the recent years these studies focus on the function of key enzymes and plant morphological traits. Here, we provide an updated overview of salt tolerant mechanisms in glycophytes with a particular interest in rice (Oryza sativa) plants. Protective mechanisms that prevent water loss due to the increased osmotic pressure, the development of Na+ toxicity on essential cellular metabolisms, and the movement of ions via the apoplastic pathway (i.e. apoplastic barriers) are described here in detail.

Differential Sodium and Potassium Transport Selectivities of the Rice OsHKT2;1 and OsHKT2;2 Transporters in Plant Cells

Na+ and K+ homeostasis are crucial for plant growth and development. Two HKT transporter/channel classes have been characterized that mediate either Na+ transport or Na+ and K+ transport when expressed in Xenopus laevis oocytes and yeast. However, the Na+/K+ selectivities of the K+-permeable HKT transporters have not yet been studied in plant cells. One study expressing 5′ untranslated region-modified HKT constructs in yeast has questioned the relevance of cation selectivities found in heterologous systems for selectivity predictions in plant cells. Therefore, here we analyze two highly homologous rice (Oryza sativa) HKT transporters in plant cells, OsHKT2;1 and OsHKT2;2, that show differential K+ permeabilities in heterologous systems. Upon stable expression in cultured tobacco (Nicotiana tabacum) Bright-Yellow 2 cells, OsHKT2;1 mediated Na+ uptake, but little Rb+ uptake, consistent with earlier studies and new findings presented here in oocytes. In contrast, OsHKT2;2 mediated Na+-K+ cotransport in plant cells such that extracellular K+ stimulated OsHKT2;2-mediated Na+ influx and vice versa. Furthermore, at millimolar Na+ concentrations, OsHKT2;2 mediated Na+ influx into plant cells without adding extracellular K+. This study shows that the Na+/K+ selectivities of these HKT transporters in plant cells coincide closely with the selectivities in oocytes and yeast. In addition, the presence of external K+ and Ca2+ down-regulated OsHKT2;1-mediated Na+ influx in two plant systems, Bright-Yellow 2 cells and intact rice roots, and also in Xenopus oocytes. Moreover, OsHKT transporter selectivities in plant cells are shown to depend on the imposed cationic conditions, supporting the model that HKT transporters are multi-ion pores.