Switchgrass (Panicum virgatum L.)—a perennial, warm-season (C4) species—evolved across North America into multiple, divergent populations. The resulting natural variation within the species presents considerable morphological diversity and a wide range of adaptation. The species was adopted as a crop—initially as a forage—only in the last 50 yr. Its potential uses have recently been expanded to include biofuels. Management of switchgrass for biofuels is informed by an understanding of the plant's biology. Successful establishment requires attention to seed dormancy and weed control as well as proper depth and date of planting. The plant's growth rate is closely tied to temperature, but timing of reproductive development is linked to photoperiod. Accordingly, the period of vegetative growth can be extended by planting lower-latitude cultivars at higher latitudes. This strategy may provide a yield advantage, but cold tolerance can become limiting. Switchgrass is thrifty in its use of applied N; it appears a...

The Biology and Agronomy of Switchgrass for Biofuels

Ionic stress is one of the most important components of salinity and is brought about by excess Na+ accumulation, especially in the aerial parts of plants. Since Na+ interferes with K+ homeostasis, and especially given its involvement in numerous metabolic processes, maintaining a balanced cytosolic Na+/K+ ratio has become a key salinity tolerance mechanism. Achieving this homeostatic balance requires the activity of Na+ and K+ transporters and/or channels. The mechanism of Na+ and K+ uptake and translocation in glycophytes and halophytes is essentially the same, but glycophytes are more susceptible to ionic stress than halophytes. The transport mechanisms involve Na+ and/or K+ transporters and channels as well as non-selective cation channels. Thus, the question arises of whether the difference in salt tolerance between glycophytes and halophytes could be the result of differences in the proteins or in the expression of genes coding the transporters. The aim of this review is to seek answers to this question by examining the role of major Na+ and K+ transporters and channels in Na+ and K+ uptake, translocation and intracellular homeostasis in glycophytes. It turns out that these transporters and channels are equally important for the adaptation of glycophytes as they are for halophytes, but differential gene expression, structural differences in the proteins (single nucleotide substitutions, impacting affinity) and post-translational modifications (phosphorylation) account for the differences in their activity and hence the differences in tolerance between the two groups. Furthermore, lack of the ability to maintain stable plasma membrane (PM) potentials following Na+-induced depolarization is also crucial for salt stress tolerance. This stable membrane potential is sustained by the activity of Na+/H+ antiporters such as SOS1 at the PM. Moreover, novel regulators of Na+ and K+ transport pathways including the Nax1 and Nax2 loci regulation of SOS1 expression and activity in the stele, and haem oxygenase involvement in stabilizing membrane potential by activating H+-ATPase activity, favorable for K+ uptake through HAK/AKT1, have been shown and are discussed.

/pdf/the-role-of-na-and-k-transporters-in-salt-stress-adaptation-54zd00mx6p.pdf

The Role of Na+ and K+ Transporters in Salt Stress Adaptation in Glycophytes.

Sodium (Na) toxicity is one of the most formidable challenges for crop production world-wide. Nevertheless, despite decades of intensive research, the pathways of Na(+) entry into the roots of plants under high salinity are still not definitively known. Here, we review critically the current paradigms in this field. In particular, we explore the evidence supporting the role of nonselective cation channels, potassium transporters, and transporters from the HKT family in primary sodium influx into plant roots, and their possible roles elsewhere. We furthermore discuss the evidence for the roles of transporters from the NHX and SOS families in intracellular Na(+) partitioning and removal from the cytosol of root cells. We also review the literature on the physiology of Na(+) fluxes and cytosolic Na(+) concentrations in roots and invite critical interpretation of seminal published data in these areas. The main focus of the review is Na(+) transport in glycophytes, but reference is made to literature on halophytes where it is essential to the analysis.

/pdf/sodium-transport-in-plants-a-critical-review-tbnfms6iaw.pdf

Sodium transport in plants: a critical review

Because plants grow under many different types of soil and environmental conditions, we investigated the hypothesis that multiple pathways for K+ uptake exist in plants. We have identified a new family of potassium transporters from Arabidopsis by searching for homologous sequences among the expressed sequence tags of the GenBank database. The deduced amino acid sequences of AtKUP (for Arabidopsis thaliana K+ uptake transporter) cDNAs are highly homologous to the non-plant Kup and HAK1 potassium transporters from Escherichia coli and Schwanniomyces occidentalis, respectively. Interestingly, AtKUP1 and AtKUP2 are able to complement the potassium transport deficiency of an E. coli triple mutant. In addition, transgenic Arabidopsis suspension cells overexpressing AtKUP1 showed increased Rb+ uptake at micromolar concentrations with an apparent Km of ~22 μM, indicating that AtKUP1 encodes a high-affinity potassium uptake activity in vivo. A small, low-affinity Rb+ uptake component was also detected in AtKUP1-expressing cells. RNA gel blot analysis showed that the various members of the AtKUP family have distinct patterns of expression, with AtKUP3 transcript levels being strongly induced by K+ starvation. It is proposed that plants contain multiple potassium transporters for high-affinity uptake and that the AtKUP family may provide important components of high- and low-affinity K+ nutrition and uptake into various plant cell types.

AtKUP1: an Arabidopsis gene encoding high-affinity potassium transport activity. [Erratum: Apr 1998, v. 10 (4), p. 639.]

Flooding is a common environmental variable with salinity. Submerged organs can suffer from O(2) deprivation and the resulting energy deficits can compromise ion transport processes essential for salinity tolerance. Tolerance of soil waterlogging in halophytes, as in glycophytes, is often associated with the production of adventitious roots containing aerenchyma, and the resultant internal O(2) supply. For some species, shallow rooting in aerobic upper soil layers appears to be the key to survival on frequently flooded soils, although little is known of the anoxia tolerance in halophytes. Halophytic species that inhabit waterlogged substrates are able to regulate their shoot ion concentrations in spite of the hypoxic (or anoxic) medium in which they are rooted, this being in stark contrast with most other plants which suffer when salinity and waterlogging occur in combination. Very few studies have addressed the consequences of submergence of the shoots by saline water; these have, however, demonstrated tolerance of temporary submergence in some halophytes.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2008.02483.x

Flooding tolerance in halophytes

To understand the mechanism of ion homeostasis in salt tolerant and sensitive plants, we isolated cDNAs for K(+) transporter PhaHAK1-u and PhaHAK5-u from reed plants. PhaHAK1-u belongs to group I and PhaHAK5-u belongs to group IV by phylogenetic analysis, respectively. PhaHAK5-u is predicted to be a plasma membrane transporter, and shows high-affinity K(+) transporter. Expression of PhaHAK5 was found in salt-sensitive reed plants, but not in any parts of salt-tolerant reed plants maintained under both control and K(+) starvation conditions. Under the NaCl stress, the K(+) uptake ability of the yeast strain expressing PhaHAK5-u was remarkably lower than that of the yeast strain expressing PhaHAK1-u, and PhaHAK5-u showed Na(+) permeability. These results suggest that PhaHAK5 is one of the routes by which Na(+) enters cells.

High-affinity K + transporter PhaHAK5 is expressed only in salt-sensitive reed plants and shows Na + permeability under NaCl stress

Three grass species, switchgrass ( Panicum virgatum ), foxtail millet ( Setaria italica ), and Old World bluestem ( Bothriochloa ischaemum ), referred to as introduced, traditional, and indigenous species, respectively, in the Loess hilly gully region on Loess Plateau in China, were grown in a growth chamber. Water was withheld to let the soil gravimetric moisture content (SGMC) decline gradually from 14.92% for 15 days. The gas exchange characteristics, leaf water conditions, root and shoot growth, and water use efficiency (WUE) of the three grasses were compared. The SGMC of foxtail millet declined faster than that of switchgrass and Old World bluestem, and it maintained higher leaf water potential at a lower soil water content, indicating that it might have greater drought tolerance. During the process of soil drying, the stomatal conductance (Gs), photosynthesis rate, and transpiration rate (Tr) of foxtail millet and Old World bluestem declined linearly, whereas those of switchgrass declined parabolically. The Gs of each grass was significantly different between the stressed and well-watered (WW) conditions. When the SGMC declined to ≈ 11.0%, 9.5%, and 8.6%, respectively, the leaf water potential was significantly different between the two growing conditions, showing the appearance of non-hydraulic and hydraulic signals. As a result of greater stomatal adjustment ability and a smaller Gs, foxtail millet had a high WUE. Among the three grasses, switchgrass had a high Tr and root/shoot (R/S) ratio under both WW and stressed conditions, and water stress significantly improved its R/S ratio. The results showed that foxtail millet seedlings have the best drought adaptability in the flexible soil‐water environment, whereas switchgrass seedlings have the worst, but a high R/S ratio might be advantageous under drought conditions once the seedlings are established.

Gas exchange, biomass partition, and water relationships of three grass seedlings under water stress

We isolated PhaHAK2 cDNAs from salt-tolerant and salt-sensitive reed plants. PhaHAK2 belongs to group II by phylogenetic analysis, and was predicted to be a high-affinity plasma membrane K(+) transporter. Yeast transformed with the PhaHAK2-u from salt-sensitive reed plants (Phragmites australis) had a decreased ability to take up K(+) in the presence of NaCl and showed a higher Na(+) permeability than yeast transformed with PhaHAK2-n or PhaHAK2-e from two salt-tolerant reed plants. These results suggest a possibility that the continuous K(+) uptake by PhaHAK2 and maintenance of high K(+)/Na(+) ratio under salt stress condition is one of the causes of the salt-tolerance in reed plants.

Cloning and functional analysis of the K+ transporter, PhaHAK2, from salt-sensitive and salt-tolerant reed plants

Reed plants (Phragmites australis Trinius) grow not only in fresh and brackish water areas but also in arid and high salinity regions. Reed plants obtained from a riverside (Utsunomiya) were damaged by 257 mM NaCl, whereas desert plants (Nanpi) were not. When the plants were grown under salt stress, the shoots of the Utsunomiya plants contained high levels of sodium and low levels of potassium, whereas the upper part of the Nanpi plants contained low levels of sodium and high levels of potassium. One month salt stress did not affect potassium contents in either Utsunomiya or Nanpi plants, but it did dramatically increase sodium contents only in the Utsunomiya plants. The ratio of K+ to Na+ was maintained at a high level in the upper parts of the Nanpi plants, whereas the ratio markedly decreased in the Utsunomiya plants in the presence of NaCl. Accumulation of Na+ in the roots and Na+ efflux from the roots were greater in the Nanpi plants than in the Utsunomiya plants. These results suggest that the salt tolerance mechanisms of Nanpi reed plants include an improved ability to take up K+ to prevent an influx of Na+ and an improved ability to exclude Na+ from the roots.

Nobumasa Ichizen

Papers

High-affinity K + transporter PhaHAK5 is expressed only in salt-sensitive reed plants and shows Na + permeability under NaCl stress

Gas exchange, biomass partition, and water relationships of three grass seedlings under water stress

Cloning and functional analysis of the K+ transporter, PhaHAK2, from salt-sensitive and salt-tolerant reed plants

Salt-tolerant reed plants contain lower Na + and higher K + than salt-sensitive reed plants

Impacts of switchgrass (Panicum virgatum L.) planting on soil erosion in the hills of the Loess Plateau in China