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 SOS1, 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 QTL in monocot crop plants are also based on this HKT-mediated mechanism. Sodium toxicity and salt tolerance in plants Physiological studies have shown that salinity stress in plants is multifactorial, including osmotic stress [1] and cellular sodium (Na + ) toxicity, such as inhibition of vital enzymes and metabolic processes [2][3][4][5][6][7][8][9][10][11][12][13][14]. Photosynthetic processes are among the most sensitive to salinity and, therefore, salinity stress directly reduces carbon fixation and biomass production in plants [5,[15][16][17][18]. Sodium transport processes have major roles in salinity tolerance, including organellar Na + sequestration [4,8,9,15,19,20]; Na + extrusion by plasma membrane Na + -H + exchange transporters, such as AtSOS1 [21,22] and exclusion of Na + from leaves and shoots [11,[23][24][25][26][27][28][29]. In addition, reducing Na + uptake or increasing cytoplasmic potassium (K + ) levels relative to Na + increases Na + tolerance in plants [30][31][32][33]. However, given that multiple independent cationic nutrient uptake transporters mediate Na + uptake from the soil into roots (reviewed in Refs [6,34]), engineering of reduced Na + influx into plant roots is likely a more challenging endeavor. The identification and characterization of Na + -permeable transporters is therefore pivotal to understanding plant Na + toxicity and tolerance [13,[35][36][37][38].Recent research has demonstrated that members of the high-affinity K + transporter (HKT) transporter/channel family mediate important Na + tolerance mechanisms in plants. The TaHKT2;1 gene from wheat (Triticum aestivum) (previously named HKT1), was the first HKT transporter gene found in plants [39]. It was shown to mediate high-affinity Na + -K + cotransport and also preferred Na + -selective low-affinity Na + transport in the presence of a Corresponding author: Horie, T. (horie@rib.okayama-u.ac.jp). Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please ...

The plant hormone abscisic acid (ABA) mediates seed dormancy, controls seedling development and triggers tolerance to abiotic stresses, including drought. Core ABA signaling components consist of a recently identified group of ABA receptor proteins of the PYRABACTIN RESISTANCE (PYR)/REGULATORY COMPONENT OF ABA RECEPTOR (RCAR) family that act as negative regulators of members of the PROTEIN PHOSPHATASE 2C (PP2C) family. Inhibition of PP2C activity enables activation of SNF1-RELATED KINASE 2 (SnRK2) protein kinases, which target downstream components, including transcription factors, ion channels and NADPH oxidases. These and other components form a complex ABA signaling network. Here, an in depth analysis of the evolution of components in this ABA signaling network shows that (i) PYR/RCAR ABA receptor and ABF-type transcription factor families arose during land colonization of plants and are not found in algae and other species, (ii) ABA biosynthesis enzymes have evolved to plant- and fungal-specific forms, leading to different ABA synthesis pathways, (iii) existing stress signaling components, including PP2C phosphatases and SnRK kinases, were adapted for novel roles in this plant-specific network to respond to water limitation. In addition, evolutionarily conserved secondary structures in the PYR/RCAR ABA receptor family are visualized.

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.

Drought stress triggers an increase in the level of the plant hormone abscisic acid (ABA), which initiates a signaling cascade to close stomata and reduce water loss. Recent studies have revealed that guard cells control cytosolic ABA concentration through the concerted actions of biosynthesis, catabolism as well as transport across membranes. Substantial progress has been made at understanding the molecular mechanisms of how the ABA signaling core module PYR/PYL/RCAR-PP2C-SnRK2 controls the activity of anion channels and thereby stomatal aperture. In this review, we focus on our current mechanistic understanding of ABA signaling in guard cells including the role of the second messenger Ca2+ as well as crosstalk with biotic stress responses.

The transcriptome of endosymbiotic Bradyrhizobium japonicum bacteroids was assessed, using RNA extracted from determinate soybean root nodules. Results were compared with the transcript profiles of B. japonicum cells grown in either aerobic or microaerobic culture. Microoxia is a known trigger for the induction of symbiotically relevant genes. In fact, one third of the genes induced in bacteroids at day 21 after inoculation are congruent with those up-regulated in culture by a decreased oxygen concentration. The other induced genes, however, may be regulated by cues other than oxygen limitation. Both groups of genes provide a rich source for the possible discovery of novel functions related to symbiosis. Samples taken at different timepoints in nodule development have led to the distinction of genes expressed early and late in bacteroids. The experimental approach applied here is also useful for B. japonicum mutant analyses. As an example, we compared the transcriptome of wild-type bacteroids with that of bacteroids formed by a mutant defective in the RNA polymerase transcription factor sigma54. This led to a collection of hitherto unrecognized B. japonicum genes potentially transcribed in planta in a sigma54-dependent manner.