Ectopic expression of wheat expansin gene TaEXPA2 improved the salt tolerance of transgenic tobacco by regulating Na+/K+ and antioxidant competence

High salinity is one of the most serious environmental stresses that limit crop growth. Expansins are cell wall proteins that regulate plant development and abiotic stress tolerance by mediating cell wall expansion. We studied the function of a wheat expansin gene, TaEXPA2, in salt stress tolerance by overexpressing it in tobacco. Overexpression of TaEXPA2 enhanced the salt stress tolerance of transgenic tobacco plants as indicated by the presence of higher germination rates, longer root length, more lateral roots, higher survival rates and more green leaves under salt stress than in the wild type (WT). Further, when leaf disks of WT plants were incubated in cell wall protein extracts from the transgenic tobacco plants, their chlorophyll content was higher under salt stress, and this improvement from TaEXPA2 overexpression in transgenic tobacco was inhibited by TaEXPA2 protein antibody. The water status of transgenic tobacco plants was improved, perhaps by the accumulation of osmolytes such as proline and soluble sugar. TaEXPA2‐overexpressing tobacco lines exhibited lower Na⁺ but higher K⁺ accumulation than WT plants. Antioxidant competence increased in the transgenic plants because of the increased activity of antioxidant enzymes. TaEXPA2 protein abundance in wheat was induced by NaCl, and ABA signaling was involved. Gene expression regulation was involved in the enhanced salt stress tolerance of the TaEXPA2 transgenic plants. Our results suggest that TaEXPA2 overexpression confers salt stress tolerance on the transgenic plants, and this is associated with improved water status, Na⁺/K⁺ homeostasis, and antioxidant competence. ABA signaling participates in TaEXPA2‐regulated salt stress tolerance.     

Overexpression of a <Emphasis Type="Italic">Malus</Emphasis> vacuolar Na<Superscript>+</Superscript>/H<Superscript>+</Superscript> antiporter gene (<Emphasis Type="Italic">MdNHX1</Emphasis>) in apple rootstock M.26 and its influence on salt tolerance

Soil salinity is a major factor limiting apple production in some areas. Tonoplast Na⁺/H⁺ antiporters play a critical role in salt tolerance. Here, we isolated MdNHX1, a vacuolar Na⁺/H⁺ antiporter from Luo-2, a salt-tolerant rootstock of apple (Malus × domestica Borkh.), and introduced it into apple rootstock M.26 by Agrobacterium-mediated transformation. PCR and DNA gel blot analyses confirmed successful integration of MdNHX1. RT-PCR analysis indicated that the gene was highly expressed in transgenic plants, but the degree of this expression varied among lines. Its overexpression conferred high tolerance to salt stress. Analysis of ion contents showed that, when exposed to salinity stress, the transgenics compartmentalized more Na⁺ in the roots and also maintained a relatively high K⁺/Na⁺ ratio in the leaves compared with non-transformed plants. Under normal conditions, however, amounts of potassium and sodium did not differ significantly between transgenic and control plants.     

Overexpression of a novel soybean gene modulating Na+ and K+ transport enhances salt tolerance in transgenic tobacco plants

Salt is an important factor affecting the growth and development of soybean in saline soil. In this study, a novel soybean gene encoding a transporter (GmHKT1) was identified and its function analyzed using transgenic plants. GmHKT1 encoded a protein of 419 amino acids, with a potential molecular mass of 47.06 kDa and a predicted pI value of 8.59. Comparison of the genomic and cDNA sequences of GmHKT1 identified no intron. The deduced amino acid sequence of GmHKT1 showed 38–49% identity with other plant HKT‐like sequences. RT‐PCR analysis showed that the expression of GmHKT1 was upregulated by salt stress (150 mM NaCl) in roots and leaves but not in stems. Overexpression of GmHKT1 significantly enhanced the tolerance of transgenic tobacco plants to salt stress, compared with non‐transgenic plants. To investigate the role of GmHKT1 in K⁺ and Na⁺ transport, we compared K⁺ and Na⁺ accumulation in roots and shoots of wild‐type and transgenic tobacco plants. The results suggested that GmHKT1 is a transporter that affected K⁺ and Na⁺ transport in roots and shoots, and regulated Na⁺/K⁺ homeostasis in these organs. Our findings suggest that GmHKT1 plays an important role in response to salt stress and would be useful in engineering crop plants for enhanced tolerance to salt stress.     

Expressing the yeast HAL1 gene in tomato increases fruit yield and enhances K+/Na+ selectivity under salt stress

The yeast HAL1 gene facilitates K⁺/Na⁺ selectivity and salt tolerance of cells. Ectopic expression of HAL1 in transgenic tomato (Lycopersicon esculentum Mill.) plants minimized the reduction in fruit production caused by salt stress. Maintenance of fruit production by transgenic plants was correlated with enhanced growth under salt stress of calli derived from the plants. The HAL1 transgene enhanced water and K⁺ contents in both leaf calli and leaves in the presence of salt, which indicates that HAL1 functions in plants using a similar mechanism to that in yeast, namely by facilitating K⁺/Na⁺ selectivity under salt stress.     

New perspective on the mechanism of alleviating salt stress by spermidine in barley seedlings

Salt stress is considered to be a major limiting factor for plant growth and crop productivity. Salt injuries in plants are mostly due to excess Na⁺ entry. A possible survival strategy of plants under saline environments is the effective compartmentation of excess Na⁺ by sequestering Na⁺ in roots and inhibiting transport of Na⁺ from roots to shoots. Our previous study showed that exogenous application of polyamines (PAs) could attenuate salt injuries in barley plants. In order to further understand such protective roles of PAs against salt stress, the effects of spermidine (Spd) on sodium and potassium distribution in barley (Hordeum vulgare L.) seedlings under saline conditions were investigated. The results showed that exogenous application of Spd induced reductions in Na⁺ levels in roots and shoots with comparison of NaCl-treated plants, while no significant changes in K⁺ levels were observed. Correspondingly, the plants treated with Spd exogenously maintained high values of [K⁺]/[Na⁺] as compared with salt-stressed plants. Moreover, it was shown by X-ray microanalysis that K⁺ and Na⁺ accumulated mainly in the exodermal intercellular space and cortical cells of roots under salinity stress, and low accumulation was observed in endodermal cells and stelar parenchyma, indicating Casparian bands possibly act as ion transport barriers. Most importantly, Spd treatment further strengthened this barrier effects, leading to inhibition of Na⁺ transport into shoots. These results suggest that, by reinforcing barrier effects of Casparian bands, exogenous Spd inhibits Na⁺ transport from roots to shoots under conditions of high salinity which are beneficial for attenuating salt injuries in barley seedlings.     

短期NaCl胁迫对不同小麦品种幼苗K+吸收和Na+、K+积累的影响

为研究盐胁迫下小麦幼苗生长及Na⁺、K⁺的吸收和积累规律,以中国春、洲元9369和长武134等3种耐盐性不同小麦品种为材料,采用非损伤微测技术检测盐胁迫2 d后的根系K⁺离子流变化,并对植株体内的Na⁺、K⁺含量进行测定。结果表明:短期(2d)盐胁迫对小麦生长有抑制作用,且对根系的抑制大于地上部,耐盐品种下降幅度小于盐敏感品种。盐胁迫下,小麦根际的 K⁺大量外流,盐敏感品种中国春K⁺流速显著高于耐盐品种长武134,最高可达15倍。小麦幼苗地上部分和根系均表现为Na⁺积累增加,K⁺积累减少,Na⁺/K⁺比随盐浓度增加而上升。中国春限Na⁺能力显著低于长武134,Na⁺/K⁺则显著高于长武134。综上所述,盐胁迫下造成小麦组织器官中Na⁺/K⁺比上升的主要原因是根系K⁺大量外流和Na⁺的过量积累,耐盐性不同的小麦品种间差异显著,并认为根系对K⁺的保有能力可能是作物耐盐性评价的一个重要指标。     

Isolation of a cDNA clone (PcSrp) encoding serine-rich-protein from Porteresia coarctata T. and its expression in yeast and finger millet (Eleusine coracana L.) affording salt tolerance

A 1.4 Kb cDNA clone encoding a serine-rich protein has been isolated from the cDNA library of salt stressed roots of Porteresia coarctata, and designated as P. coarctata serine-rich-protein (PcSrp) encoding gene. Northern analysis and in situ mRNA hybridization revealed the expression of PcSrp in the salt stressed roots and rhizome of P. coarctata. However, no such expression was seen in the salt stressed leaves and in the unstressed tissues of root, rhizome and leaf, indicating that PcSrp is under the control of a salt-inducible tissue-specific promoter. In yeast, the PcSrp conferred increased NaCl tolerance, implicating its role in salinity tolerance at cellular level. Further, PcSrp was cloned downstream to rice Actin-1 promoter and introduced into finger millet through particle-inflow-gun method. Transgenic plants expressing PcSrp were able to grow to maturity and set seed under 250 mM NaCl stress. The untransformed control plants by contrast failed to survive under similar salt stress. The stressed roots of transgenic plants invariably accumulated higher Na⁺ and K⁺ ion contents compared to roots of untransformed plants; whereas, shoots of transgenics accumulated lower levels of both the ions than that of untransformed plants under identical stress, clearly suggesting the involvement of PcSrp in ion homeostasis contributing to salt tolerance.     

Heterologous expression of Arabidopsis H+‐pyrophosphatase enhances salt tolerance in transgenic creeping bentgrass (Agrostis stolonifera L.)

The Arabidopsis vacuolar H⁺‐pyrophosphatase (AVP1), when over‐expressed in transgenic (TG) plants, regulates root and shoot development via facilitation of auxin flux, and enhances plant resistance to salt and drought stresses. Here, we report that TG perennial creeping bentgrass plants over‐expressing AVP1 exhibited improved resistance to salinity than wild‐type (WT) controls. Compared to WT plants, TGs grew well in the presence of 100 mm NaCl, and exhibited higher tolerance and faster recovery from damages from exposure to 200 and 300 mm NaCl. The improved performance of the TG plants was associated with higher relative water content (RWC), higher Na⁺ uptake and lower solute leakage in leaf tissues, and with higher concentrations of Na⁺, K⁺, Cl^‐ and total phosphorus in root tissues. Under salt stress, proline content was increased in both WT and TG plants, but more significantly in TGs. Moreover, TG plants exhibited greater biomass production than WT controls under both normal and elevated salinity conditions. When subjected to salt stress, fresh (FW) and dry weights (DW) of both leaves and roots decreased more significantly in WT than in TG plants. Our results demonstrated the great potential of genetic manipulation of vacuolar H⁺‐pyrophosphatase expression in TG perennial species for improvement of plant abiotic stress resistance.     

The K<Superscript>+</Superscript>/H<Superscript>+</Superscript> antiporter <Emphasis Type="Italic">AhNHX1</Emphasis> improved tobacco tolerance to NaCl stress by enhancing K<Superscript>+</Superscript> retention

High salinity is the one of important factors limiting plant growth and crop production. Many NHX-type antiporters have been reported to catalyze K⁺/H⁺ exchange to mediate salt stress. This study shows that an NHX gene from Arachis hypogaea L. has an important role in K⁺ uptake and transport, which affects K⁺ accumulation and plant salt tolerance. When overexpressing AhNHX1, the growth of tobacco seedlings is improved with longer roots and a higher fresh weight than the wild type (WT) under NaCl treatment. Meanwhile, when exposed to NaCl stress, the transgenic seedlings had higher K⁺/H⁺ antiporter activity and their roots got more K⁺ uptake. NaCl stress could induce higher K⁺ accumulation in the roots, stems, and leaves of transgenic tobacco seedlings but not Na⁺ accumulation, thus, leading to a higher K⁺/Na⁺ ratio in the transgenic seedlings. Additionally, the AKT1, HAK1, SKOR, and KEA genes, which are involved in K⁺ uptake or transport, were induced by NaCl stress and kept higher expression levels in transgenic seedlings than in WT seedlings. The H⁺-ATPase and H⁺-PPase activities were also higher in transgenic seedlings than in the WT seedlings under NaCl stress. Simultaneously, overexpression of AhNHX1 increased the relative distribution of K⁺ in the aerial parts of the seedlings under NaCl stress. These results showed that AhNHX1 catalyzed the K⁺/H⁺ antiporter and enhanced tobacco tolerance to salt stress by increasing K⁺ uptake and transport.     

Herbivore exposure alters ion fluxes and improves salt tolerance in a desert shrub

Plants have evolved complex mechanisms that allow them to withstand multiple environmental stresses, including biotic and abiotic stresses. Here, we investigated the interaction between herbivore exposure and salt stress of Ammopiptanthus nanus, a desert shrub. We found that jasmonic acid (JA) was involved in plant responses to both herbivore attack and salt stress, leading to an increased NaCl stress tolerance for herbivore-pretreated plants and increase in K⁺/Na⁺ ratio in roots. Further evidence revealed the mechanism by which herbivore improved plant NaCl tolerance. Herbivore pretreatment reduced K⁺ efflux and increased Na⁺ efflux in plants subjected to long-term, short-term, or transient NaCl stress. Moreover, herbivore pretreatment promoted H⁺ efflux by increasing plasma membrane H⁺-adenosine triphosphate (ATP)ase activity. This H⁺ efflux creates a transmembrane proton motive force that drives the Na⁺/H⁺ antiporter to expel excess Na⁺ into the external medium. In addition, high cytosolic Ca²⁺ was observed in the roots of herbivore-treated plants exposed to NaCl, and this effect may be regulated by H⁺-ATPase. Taken together, herbivore exposure enhance s A. nanus tolerance to salt stress by activating the JA-signalling pathway, increasing plasma membrane H ⁺ - ATPase activity, promoting cytosolic Ca²⁺ accumulation, and then restricting K⁺ leakage and reducing Na⁺ accumulation in the cytosol.     

Exogenous proline effects on water relations and ions contents in leaves and roots of young olive

The ability of exogenous compatible solutes, such as proline, to counteract salt inhibitory effects was investigated in 2-year-old olive trees (Olea europaea L. cv. Chemlali) subjected to different saline water irrigation levels supplied or not with exogenous proline. Leaf water relations [relative water content (RWC), water potential], photosynthetic activity, leaf chlorophyll content, and starch contents were measured in young and old leaves. Salt ions (Na⁺, K⁺, and Ca²⁺), proline and soluble sugars contents were determined in leaf and root tissues. Supplementary proline significantly mitigated the adverse effects of salinity via the improvement of photosynthetic activity (Pn), RWC, chlorophyll and carotenoid, and starch contents. Pn of young leaves in the presence of 25 mM proline was at 1.18 and 1.38 times higher than the values recorded under moderate (SS1) and high salinity (SS2) treatments, respectively. Further, the proline supply seems to have a more important relaxing effect on the photosynthetic chain in young than in old leaves of salt-stressed olive plants. The differential pattern of proline content between young and old leaves suggests that there would be a difference between these tissues in distinguishing between the proline taken from the growing media and that produced as a result of salinity stress. Besides, the large reduction in Na⁺ accumulation in leaves and roots in the presence of proline could be due to its interference in osmotic adjustment process and/or its dilution by proline supply. Moreover, the lower accumulation of Na⁺ in proline-treated plants, compared to their corresponding salinity treatment, displayed the improved effect of proline on the ability of roots to exclude the salt ions from the xylem sap flowing to the shoot, and thus better growth rates.     

Salt-tolerant reed plants contain lower Na<Superscript>+</Superscript> and higher K<Superscript>+</Superscript> than salt-sensitive 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.     

Responses to nonaeration and/or salinity stress in hydroponically cultured <Emphasis Type="Italic">Populus nigra</Emphasis> and <Emphasis Type="Italic">Populus alba</Emphasis> cuttings

Growth, photosynthesis, and Na⁺, K⁺, Ca²⁺, and Mg²⁺ distributions were examined in two-year-old hydroponically cultured Populus nigra and Populus alba cuttings exposed to salt stress (0, 50, or 100 mM NaCl) for four or six weeks and to nonaeration stress for one or three weeks, followed by a three-week aeration period in 2/5 Hoagland solution. Salt stress with 100 mM NaCl totally inhibited height increase in P. nigra cuttings. Combined salinity and nonaeration inhibited height increase to a greater degree than either stress alone in both species. Simple salt stress did not affect diameter increase in P. alba, whereas combined high salinity (100 mM NaCl) and nonaeration inhibited diameter increase. Growth and biomass accumulation were more sensitive to salt stress in P. nigra cuttings than in P. alba, although P. alba showed a more rapid decrease in photosynthesis in response to nonaeration stress. Ion distributions in the leaves and roots differed between species. P. alba was superior to P. nigra in terms of Na⁺ exclusion capacity, such that most of the absorbed Na⁺ was confined to the root system, with little reaching the leaves. The distributions of K⁺, Ca²⁺, and Mg²⁺ in the leaves and roots of each species under the two stressors were also analyzed. The lower Na⁺/K⁺ ratio in leaves indicated that P. alba was more tolerant to salt stress than P. nigra.     

盐胁迫对3种平欧杂种榛幼苗叶片解剖结构及离子吸收、运输与分配的影响

研究盐胁迫下3个品种平欧杂种榛幼苗叶片解剖结构和离子代谢特征,以揭示盐胁迫响应与适应机制及不同品种的耐盐性差异。以‘达维’、‘辽榛7号’、‘玉坠’2年生压条苗为材料,在盆栽条件下经轻度、中度、重度(分别为50、100、200 mmol/L NaCl)盐胁迫处理,设对照为0,研究幼苗叶片显微解剖结构参数和Na~+、K~+、Cl~-、Ca²⁺含量的变化及其在根、茎、叶中的吸收、运输和分配特征。不同品种平欧杂种榛叶片厚度、上表皮厚度、下表皮厚度、栅栏组织和海绵组织厚度随着盐胁迫程度的增强呈现出先增加后降低的特点,轻度和中度胁迫下各参数显著高于对照。中度盐胁迫显著提高了各品种叶片结构紧密度。盐胁迫导致平欧杂种榛根、茎、叶Na~+和Cl~-含量明显高于对照。盐胁迫下,Na~+和Cl~-在叶中的绝对含量明显高于茎和根,但二者的增幅以根中最大,叶中最小,表明平欧杂种榛根系首先会吸收并截留一定数量的Na~+和Cl~-,然后将其运输至茎和叶中。与对照相比,轻度和中度盐胁迫下根、茎对K~+和Ca²⁺的吸收保持稳定或减少,叶对K~+和Ca²⁺...     

Salt-Induced Hypodermal Transfer Cells in Roots of Prosopis farcta and Ion Distribution within Young Plants

Prosopis farcta was grown on hydroculture with additions of 0.5, 10, 50, and 100 mM NaCl and without salt treatment. In plants from a 0.5 mM NaCl treatment, Cl^? was taken up into stems and leaves, but Na⁺ was withheld from the shoot. At 10 mM NaCl, shoot K⁺ concentration was below that of the control; Na⁺ and Cl^? were taken up to stems and cotyledons in nearly equimolar amounts. However, in the leaves, Na⁺ concentrations were only half of those of Cl^?. With increasing salt stress, Na⁺ and Cl^? were transported to the shoot, but kept at relatively low levels in the roots. Na⁺/ K⁺ ratios in roots did not increase proportionally to those in the solution. At an external Na⁺/K⁺ of > 5 and a root Na⁺/K⁺ of >1 (10 mM NaCl treatment), K⁺ selectivity was induced which rose exponentially with increasing salt stress; and cell wall protuberances were discovered in the hypodermis at the zone of side root formation. These transfer cells were found neither in roots from the 0.5 mM NaCl treatment nor in the controls. Their possible role in the Na⁺/K⁺ selectivity of the roots of Prosopis farcta is discussed.     

Contribution of <Emphasis Type="Italic">Glomus intraradices</Emphasis> inoculation to nutrient acquisition and mitigation of ionic imbalance in NaCl-stressed <Emphasis Type="Italic">Trigonella foenum-graecum</Emphasis>

The study aimed to investigate the effects of an AM fungus (Glomus intraradices Schenck and Smith) on mineral acquisition in fenugreek (Trigonella foenum-graecum) plants under different levels of salinity. Mycorrhizal (M) and non-mycorrhizal (NM) fenugreek plants were subjected to four levels of NaCl salinity (0, 50, 100, and 200 mM NaCl). Plant tissues were analyzed for different mineral nutrients. Leaf senescence (chlorophyll concentration and membrane permeability) and lipid peroxidation were also assessed. Under salt stress, M plants showed better growth, lower leaf senescence, and decreased lipid peroxidation as compared to NM plants. Salt stress adversely affected root nodulation and uptake of NPK. This effect was attenuated in mycorrhizal plants. Presence of the AM fungus prevented excess uptake of Na⁺ with increase in NaCl in the soil. It also imparted a regulatory effect on the translocation of Na⁺ ions to shoots thereby maintaining lower Na⁺ shoot:root ratios as compared to NM plants. Mycorrhizal colonization helped the host plant to overcome Na⁺-induced Ca²⁺ and K⁺ deficiencies. M plants maintained favorable K⁺:Na⁺, Ca²⁺:Na⁺, and Ca²⁺:Mg²⁺ ratios in their tissues. Concentrations of Cu, Fe, and Zn²⁺ decreased with increase in intensity of salinity stress. However, at each NaCl level, M plants had higher concentration of Cu, Fe, Mn²⁺, and Zn²⁺ as compared to NM plants. M plants showed reduced electrolyte leakage in leaves as compared to NM plants. The study suggests that AM fungi contribute to alleviation of salt stress by mitigation of NaCl-induced ionic imbalance thus maintaining a favorable nutrient profile and integrity of the plasma membrane.     

盐胁迫对二穗短柄草幼苗生长及不同器官中盐离子稳态的影响

采用4种浓度的NaCl溶液(50、100、150、200 mmol/L)处理二穗短柄草和拟南芥(对照)幼苗,测定不同浓度盐胁迫下2种植物幼苗的生长指标和离子分布,以探讨二穗短柄草在盐胁迫下主要阳离子平衡机制.结果表明:(1)盐胁迫显著抑制二穗短柄草根叶的生物量积累.(2)根冠比数据显示,在盐胁迫条件下二穗短柄草能够更好地维系地下部分的生物量积累.(3)在4种浓度盐胁迫下,二穗短柄草叶中Na+含量低于根系,而且K+、Cl-含量和K+/Na+比值始终高于根系,说明在二穗短柄草中Na+从地下到地上的转运受到抑制,但对Cl-的转运缺乏有效的调控.(4)回归分析发现,盐胁迫下二穗短柄草和拟南芥根部Na+与K+含量变化呈正相关关系,而在叶部则不相关,说明二穗短柄草和拟南芥Na+与K+在根部具有相同的离子通道,而在叶部却具有各自独立的转运途径.     

Arbuscular mycorrhizal fungi native from a Mediterranean saline area enhance maize tolerance to salinity through improved ion homeostasis

Soil salinity restricts plant growth and productivity. Na⁺ represents the major ion causing toxicity because it competes with K⁺ for binding sites at the plasma membrane. Inoculation with arbuscular mycorrhizal fungi (AMF) can alleviate salt stress in the host plant through several mechanisms. These may include ion selection during the fungal uptake of nutrients from the soil or during transfer to the host plant. AM benefits could be enhanced when native AMF isolates are used. Thus, we investigated whether native AMF isolated from an area with problems of salinity and desertification can help maize plants to overcome the negative effects of salinity stress better than non‐AM plants or plants inoculated with non‐native AMF. Results showed that plants inoculated with two out the three native AMF had the highest shoot dry biomass at all salinity levels. Plants inoculated with the three native AMF showed significant increase of K⁺ and reduced Na⁺ accumulation as compared to non‐mycorrhizal plants, concomitantly with higher K⁺/Na⁺ ratios in their tissues. For the first time, these effects have been correlated with regulation of ZmAKT2, ZmSOS1 and ZmSKOR genes expression in the roots of maize, contributing to K⁺ and Na⁺ homeostasis in plants colonized by native AMF.