Short-term salt tolerance mechanisms in differentially salt tolerant tomato species

The physiological changes induced by a daily increase of NaCl level, over a period of 4 d, were studied in leaves of the salt-sensitive cultivated tomato species Lycopersicon esculentum and its wild salt-tolerant relative Lycopersicon pennellii. A higher solute contribution to the osmotic adjustment was observed in NaCl-treated leaves of L. pennellii than in those of L. esculentum. This response together with the higher accumulation of inorganic solutes in the wild species and of organic solutes in the cultivated species verified the different salt tolerance mechanisms operating in the two species in the short-term. With regard to the changes induced by salt stress on the free polyamine levels, the putrescine and spermine levels increased with salinity, whereas the spermine levels decreased in both tomato species; nevertheless, the main difference between the two species lays in an earlier and greater accumulation of putrescine induced by salinity in L. pennellii than in L. esculentum. The changes in putrescine levels were associated to changes in amino acids related to its synthesis, and the changes were different in both species. In L. esculentum, the high concentrations of some intermediate compounds (glutamate and arginine) were related to the low accumulation rate of both proline and putrescine. In contrast, in L. pennellii, important reductions in glutamate and arginine levels were found at the end of the salinization period. Moreover, in this last situation, a decline in the putrescine level ran parallel to a high proline accumulation, which suggests that the higher the stress level, the higher the deviation of glutamate to proline occurring in the salt tolerant species. It could be concluded that an early accumulation of the diamine putrescine seems to be associated with salt tolerance in the short-term.     

Silencing of the SlZF-31 gene decreases the salt stress tolerance and drought tolerance of tomato

Plant Cell, Tissue and Organ Culture (PCTOC) - Sceletium tortuosum is a South African protected species with tremendous value in traditional and modern medicine. The plants’ mesembrine-type...     

RFLP mapping of QTLs conferring salt tolerance during the vegetative stage in tomato

 Quantitative trait loci (QTLs) contributing to salt tolerance during the vegetative stage in tomato were investigated using an interspecific backcross between a salt-sensitive Lycopersicon esculentum breeding line (NC84173, maternal and recurrent parent) and a salt-tolerant Lycopersicon pimpinellifolium accession (LA722). One hundred and nineteen BC₁ individuals were genotyped for 151 RFLP markers and a linkage map was constructed. The parental lines and 119 BC₁S₁ families (self-pollinated progeny of the BC₁ individuals) were evaluated for salt tolerance in aerated saline-solution cultures with the salt concentration gradually raised to 700 mM NaCl+70 mM CaCl₂ (equivalent to an electrical conductivity of approximately 64 dS/m and a water potential of approximately −35.2 bars). The two parental lines were distinctly different in salt tolerance: 80% of the LA722 plants versus 25% of the NC84173 plants survived for at least 2 weeks after the final salt concentration was reached. The BC₁S₁ population exhibited a continuous variation, typical of quantitative traits, with the survival rate of the BC₁S₁ families ranging from 9% to 94% with a mean of 51%. Two QTL mapping techniques, interval mapping (using MAPMAKER/QTL) and single-marker analysis (using QGENE), were used to identify QTLs. The results of both methods were similar and five QTLs were identified on chromosomes 1 (two QTLs), 3, 5 and 9. Each QTL accounted for between 5.7% and 17.7%, with the combined effects (of all five QTLs) exceeding 46%, of the total phenotypic variation. All QTLs had the positive QTL alleles from the salt-tolerant parent. Across QTLs, the effects were mainly additive in nature. Digenic epistatic interactions were evident among several QTL-linked and QTL-unlinked markers. The overall results indicate that tomato salt tolerance during the vegetative stage could be improved by marker-assisted selection using interspecific variation. Received: 4 January 1999 / Accepted: 4 January 1999     

Identification of two loci in tomato reveals distinct mechanisms for salt tolerance

Salt stress is one of the most serious environmental factors limiting the productivity of crop plants. To understand the molecular basis for salt responses, we used mutagenesis to identify plant genes required for salt tolerance in tomato. As a result, three tomato salt-hypersensitive (tss) mutants were isolated. These mutants defined two loci and were caused by single recessive nuclear mutations. The tss1 mutant is specifically hypersensitive to growth inhibition by Na(+) or Li(+) and is not hypersensitive to general osmotic stress. The tss2 mutant is hypersensitive to growth inhibition by Na(+) or Li(+) but, in contrast to tss1, is also hypersensitive to general osmotic stress. The TSS1 locus is necessary for K(+) nutrition because tss1 mutants are unable to grow on a culture medium containing low concentrations of K(+). Increased Ca(2)+ in the culture medium suppresses the growth defect of tss1 on low K(+). Measurements of membrane potential in apical root cells were made with an intracellular microelectrode to assess the permeability of the membrane to K(+) and Na(+). K(+)-dependent membrane potential measurements indicate impaired K(+) uptake in tss1 but not tss2, whereas no differences in Na(+) uptake were found. The TSS2 locus may be a negative regulator of abscisic acid signaling, because tss2 is hypersensitive to growth inhibition by abscisic acid. Our results demonstrate that the TSS1 locus is essential for K(+) nutrition and NaCl tolerance in tomato. Significantly, the isolation of the tss2 mutant demonstrates that abscisic acid signaling is also important for salt and osmotic tolerance in glycophytic plants.     

Mapping QTLs conferring salt tolerance during germination in tomato by selective genotyping

This study was conducted to identify genomic regions (quantitative trait loci, QTLs) affecting salt tolerance during germination in tomato. Germination response of an F2 population of a cross between UCT5 (Lycopersicon esculentum, salt-sensitive) and LA716 (L. pennellii, salt-tolerant) was evaluated at a salt-stress level of 175 mM NaCl + 17.5 mM CaCl2 (water potential ca. –950 kPa). Germination was scored visually as radicle protrusion at 6 h intervals for 30 consecutive days. Individuals at both extremes of the response distribution (i.e., salt-tolerant and salt-sensitive individuals) were selected. The selected individuals were genotyped at 84 genetic markers including 16 isozymes and 68 restriction fragment length polymorphisms (RFLPs). Trait-based marker analysis (TBA) which measures changes (differences) in marker allele frequencies in selected lines was used to identify marker-linked QTLs. Eight genomic regions were identified on seven tomato chromosomes bearing genes (QTLs) with significant effects on this trait. The results confirmed our previous suggestion that salt tolerance during germination in tomato is polygenically controlled. The salt-tolerant parent contributed favorable QTL alleles on chromosomes 1, 3, 9 and 12 whereas the salt sensitive parent contributed favorable QTL alleles on chromosomes 2, 7 and 8. The identification of favorable alleles in both parents suggests the likelihood of recovering transgressive segregants in progeny derived from these parental genotypes. The results can be used for marker-assisted selection and breeding of salt-tolerant tomatoes.     

The yeast HAL1 gene improves salt tolerance of transgenic tomato

Overexpression of the HAL1 gene in yeast has a positive effect on salt tolerance by maintaining a high internal K(+) concentration and decreasing intracellular Na(+) during salt stress. In the present work, the yeast gene HAL1 was introduced into tomato (Lycopersicon esculentum Mill.) by Agrobacterium tumefaciens-mediated transformation. A sample of primary transformants was self-pollinated, and progeny from both transformed and non-transformed plants (controls) were evaluated for salt tolerance in vitro and in vivo. Results from different tests indicated a higher level of salt tolerance in the progeny of two different transgenic plants bearing four copies or one copy of the HAL1 gene. In addition, measurement of the intracellular K(+) to Na(+) ratios showed that transgenic lines were able to retain more K(+) than the control under salt stress. Although plants and yeast cannot be compared in an absolute sense, these results indicate that the mechanism controlling the positive effect of the HAL1 gene on salt tolerance may be similar in transgenic plants and yeast.     

Response of two tomato cultivars differing in salt tolerance to inoculation with mycorrhizal fungi under salt stress

Effects of arbuscular mycorrhizal fungi (AMF) and salt stress on nutrient acquisition and growth of two tomato cultivars exhibiting differences in salt tolerance were investigated. Plants were grown in a sterilized, low-P (silty clay) soil-sand mix. Salt was applied at saturation extract (EC_e) values of 1.4 (control), 4.9 (medium) and 7.1 dS m^–1 (high salt stress). Mycorrhizal colonization occurred irrespective of salt stress in both cultivars, but AMF colonization was higher under control than under saline soil conditions. The salt-tolerant cultivar Pello showed higher mycorrhizal colonization than the salt-sensitive cultivar Marriha. Shoot dry matter (DM) yield and leaf area were higher in mycorrhizal than nonmycorrhizal plants of both cultivars. Shoot DM and leaf area but not root DM were higher in Pello than Marriha. The enhancement in shoot DM due to AMF inoculation was 22% and 21% under control, 31% and 58% under medium, and 18% and 59% under high salinity for Pello and Marriha, respectively. For both cultivars, the contents of P, K, Zn, Cu, and Fe were higher in mycorrhizal than nonmycorrhizal plants under control and medium saline soil conditions. The enhancement in P, K, Zn, Cu, and Fe acquisition due to AMF inoculation was more pronounced in Marriha than in the Pello cultivar under saline conditions. The results suggest that Marriha benefited more from AMF colonization than Pello under saline soil conditions, despite the fact that Pello roots were highly infected with the AMF. Thus, it appears that Marriha is more dependent on AMF symbiosis than Pello. Accepted: 22 January 2001     

番茄品种资源芽苗期和幼苗期的耐盐性及耐盐指标评价

对芽苗期和幼苗期番茄耐盐鉴定指标进行检验,明确番茄芽苗期和幼苗期耐盐性的相关性,筛选耐盐的番茄品种资源,以便为耐盐育种和栽培提供材料和方法.试验采用不同浓度的NaCl水溶液人工模拟盐胁迫,以多项指标盐害系数隶属函数值和总隶属函数值为依据,比较了20个番茄品种资源芽苗期和幼苗期的耐盐性及两个时期耐盐性的相关性,并利用单一指标盐害系数隶属函数值和总隶属函数值进行了聚类分类.结果表明:芽苗期和幼苗期番茄耐盐性有所不同,耐盐和中等耐盐材料相同率为53.85%.进行番茄耐盐性筛选时可以把发芽势、发芽率、发芽指数、萌发活力指数、下胚轴长、地上鲜重作为芽苗期耐盐鉴定指标,把地上部鲜重、根鲜重、地上部干重、根干重、壮苗指数、根/冠比作为幼苗期耐盐鉴定指标.     

Garden-waste-vermicompost leachate alleviates salinity stress in tomato seedlings by mobilizing salt tolerance mechanisms

The incidence of salinity-induced plant stress as a result of natural and anthropogenic factors in arid and semi-arid agricultural lands is great. In South Africa alone, 9 % of irrigated agricultural land is salt-affected. Commercial fertilizers used for improving soil nutrient levels are costly and affect the quality, lifespan and sustainability of soil and water resources. Organic farming practices are based on cost-effective and environmentally-aware management systems. Vermicompost leachate (VCL) is a vermicompost-derived liquid product that has become recognised as a suitable soil amendment product. Commercial tomato (Lycopersicon esculentum Mill var. Heinz-1370) seedlings were subjected to sodium chloride (NaCl) concentrations of 0, 25, 50 and 100 mM and were treated with 1:10 (v/v) WizzardWorms VCL prepared in Hoagland’s nutrient solution under greenhouse conditions. Morphological characters of VCL-treated tomato seedlings showed improved root growth and stimulated overall aboveground growth with significantly higher numbers of leaves, greater stem thickness and increased leaf area, even at a high NaCl-tested concentration (100 mM). The accumulation of compatible solutes such as proline and total soluble sugars indicate an induced salt tolerance or adaptive mechanism in VCL-treated tomato seedlings. The current investigation demonstrates the potential of an organic liquid to maximise tomato productivity by improving seedling growth performance under salt stress conditions.     

宏基因组学应用于耐盐酶类及耐盐基因研究的进展

耐盐酶在高盐浓度下仍具备催化活性和稳定性,在高盐食品和海产品加工、洗涤及其它高盐环境生物技术领域被广泛应用;耐盐基因在高盐条件下可以使微生物维持正常功能,获取并研究不同环境中的耐盐基因对揭示微生物的耐盐机制,以及实现其在高盐环境中的定向应用具有的重要意义。宏基因组学避开纯培养技术探知微生物的多样性及其功能,为我们提供了一种发现新基因、开发新的微生物活性物质和研究微生物群落结构及其功能的新技术。文中结合本课题组的研究工作,综述了利用宏基因组学获取耐盐酶类及耐盐基因的策略,同时着重介绍利用宏基因组学从海洋、土壤、胃肠道等环境中获取耐盐酶类及耐盐基因的研究。     

RFLP mapping of QTLs conferring salt tolerance during germination in an interspecific cross of tomato

 Most cultivars of tomato (Lycopersicon esculentum) are sensitive to salinity during seed germination and at later stages. Genetic resources for salt tolerance have been identified within the related wild species of tomato. The purpose of the present study was to identify quantitative trait loci (QTLs) for salt tolerance during germination in an inbred backcross (BC₁S₁) population of an interspecific cross between a salt-sensitive tomato breeding line (NC84173, maternal and recurrent parent) and a salt-tolerant Lycopersicon pimpinellifolium accession (LA722). Onehundred and nineteen BC₁ individuals were genotyped for 151 restriction fragment length polymorphism (RFLP) markers and a genetic linkage map was constructed. The parental lines and 119 BC₁S₁ families (self-pollinated progeny of 119 BC₁ individuals) were evaluated for germination at an intermediate salt-stress level (150 mM NaCl+15 mM CaCl₂, water potential approximately −850 kPa). Germination was scored visually as radicle protrusion at 8-h intervals for 28 consecutive days. Germination response was analyzed by survival analysis and the time to 25, 50, and 75% germination was determined. In addition, a germination index (GI) was calculated as the weighted mean of the time from imbibition to germination for each family/line. Interval mapping, single-marker analysis and distributional extreme analysis, were used to identify QTLs and the results of all three mapping methods were generally similar. Seven chromosomal locations with significant effects on salt tolerance were identified. The L. pimpinellifolium accession had favorable QTL alleles at six locations. The percentage of phenotypic variation explained (PVE) by individual QTLs ranged from 6.5 to 15.6%. Multilocus analysis indicated that the cumulative action of all significant QTLs accounted for 44.5% of the total phenotypic variance. A total of 12 pairwise epistatic interactions were identified, including four between QTL-linked and QTL-unlinked regions and eight between QTL-unlinked regions. Transgressive phenotypes were observed in the direction of salt sensitivity. The graphical genotyping indicated a high correspondence between the phenotypes of the extreme families and their QTL genotypes. The results indicate that tomato salt tolerance during germination can be improved by marker-assisted selection using interspecific variation. Received: 29 January 1998 / Accepted: 4 June 1998     

Coordinate induction of glutathione biosynthesis and glutathione-metabolizing enzymes is correlated with salt tolerance in tomato

The acclimation of reduced glutathione (GSH) biosynthesis and GSH-utilizing enzymes to salt stress was studied in two tomato species that differ in stress tolerance. Salt increased GSH content and GSH:GSSG (oxidized glutathione) ratio in oxidative stress-tolerant Lycopersicon pennellii (Lpa) but not in Lycopersicon esculentum (Lem). These changes were associated with salt-induced upregulation of gamma-glutamylcysteine synthetase protein, an effect which was prevented by preincubation with buthionine sulfoximine. Salt treatment induced glutathione peroxidase and glutathione-S-transferase but not glutathione reductase activities in Lpa. These results suggest a mechanism of coordinate upregulation of synthesis and metabolism of GSH in Lpa, that is absent from Lem.     

Overexpression of SlSOS2 (SlCIPK24) confers salt tolerance to transgenic tomato

The Ca(2+)-dependent SOS pathway has emerged as a key mechanism in the homeostasis of Na(+) and K(+) under saline conditions. We have identified and functionally characterized the gene encoding the calcineurin-interacting protein kinase of the SOS pathway in tomato, SlSOS2. On the basis of protein sequence similarity and complementation studies in yeast and Arabidopsis, it can be concluded that SlSOS2 is the functional tomato homolog of Arabidopsis AtSOS2 and that SlSOS2 operates in a tomato SOS signal transduction pathway. The biotechnological potential of SlSOS2 to provide salt tolerance was evaluated by gene overexpression in tomato (Solanum lycopersicum L. cv. MicroTom). The better salt tolerance of transgenic plants relative to non-transformed tomato was shown by their faster relative growth rate, earlier flowering and higher fruit production when grown with NaCl. The increased salinity tolerance of SlSOS2-overexpressing plants was associated with higher sodium content in stems and leaves and with the induction and up-regulation of the plasma membrane Na(+)/H(+) (SlSOS1) and endosomal-vacuolar K(+), Na(+)/H(+) (LeNHX2 and LeNHX4) antiporters, responsible for Na(+) extrusion out of the root, active loading of Na(+) into the xylem, and Na(+) and K(+) compartmentalization.     

Evaluation of salt tolerance in cultivated and wild tomato species through in vitro shoot apex culture

The possibility of using in vitro shoot apex culture to evaluate salt tolerance of cultivated (Lycopersicon esculentum Mill.) and wild (Lycopersicon pennellii (Correll) D'Arcy) tomato species was determined and related to the response obtained by callus culture. Both apices and calluses were grown on media supplemented with 0, 35, 70, 105, 140, 175 and 210 mM NaCl, and growth and physiological traits were determined. Most apices of L. esculentum did not develop roots from low NaCl levels, whereas the apices of L. pennellii were able to develop roots at the different salt levels. This different degree of salt tolerance between L. esculentum and L. pennellii was not, however, clearly shown on the basis of the shoot growth of the plantlets. The callus response was similar to that shown by the rooting parameters, as callus growth in response to increased salinity was much greater in L. pennellii than in the tomato cultivar. K⁺decreased more and proline accumulated less with salinity in shoots of L. esculentum compared to L. pennellii, whereas the opposite response was obtained in calluses. The results obtained in this study suggest that rooting parameters are the most useful traits for rapid evaluation and screening of tomato species and segregating populations through in vitro shoot apex culture. This revised version was published online in June 2006 with corrections to the Cover Date.     

Regulation of K+ transport in tomato roots by the TSS1 locus. Implications in salt tolerance

The tss1 tomato (Lycopersicon esculentum) mutant exhibited reduced growth in low K+ and hypersensitivity to Na+ and Li+. Increased Ca2+ in the culture medium suppressed the Na+ hypersensitivity and the growth defect on low K+ medium of tss1 seedlings. Interestingly, removing NH4+ from the growth medium suppressed all growth defects of tss1, suggesting a defective NH4(+)-insensitive component of K+ transport. We performed electrophysiological studies to understand the contribution of the NH4(+)-sensitive and -insensitive components of K+ transport in wild-type and tss1 roots. Although at 1 mm Ca2+ we found no differences in affinity for K+ uptake between wild type and tss1 in the absence of NH4+, the maximum depolarization value was about one-half in tss1, suggesting that a set of K+ transporters is inactive in the mutant. However, these transporters became active by raising the external Ca2+ concentration. In the presence of NH4+, a reduced affinity for K+ was observed in both types of seedlings, but tss1 at 1 mm Ca2+ exhibited a 2-fold higher Km than wild type did. This defect was again corrected by raising the external concentration of Ca2+. Therefore, membrane potential measurements in root cells indicated that tss1 is affected in both NH4(+)-sensitive and -insensitive components of K+ transport at low Ca2+ concentrations and that this defective transport is rescued by increasing the concentration of Ca2+. Our results suggest that the TSS1 gene product is part of a crucial pathway mediating the beneficial effects of Ca2+ involved in K+ nutrition and salt tolerance.     

Wounding increases salt tolerance in tomato plants: evidence on the participation of calmodulin-like activities in cross-tolerance signalling

Cross-tolerance is the phenomenon by which a plant resistance to a stress results in resistance to another form of stress. It has previously been shown that salt stress causes the accumulation of proteinase inhibitors and the activation of other wound-related genes in tomato plants (Solanum lycopersicum). However, very little is known about how different stresses interact with one another, and which are the signalling components that interrelate the responses triggered by different stress types. In the present work, it is shown that mechanical wounding increases salt-stress tolerance in tomato plants through a mechanism that involves the signalling peptide systemin and the synthesis of JA. Data are also provided indicating that calmodulin-like activities are necessary for the downstream signalling events that lead to cross-tolerance between wounding and salt stress. Finally, evidence was gathered supporting the hypothesis that LeCDPK1, a Ca2+ -dependent protein kinase from tomato previously described in our laboratory, could participate in this cross-tolerance mechanism interrelating the signalling responses to wounding and salt stress.     

Advances in salt tolerance

Summary Advances in and prospects for the development of salt tolerant crops are discussed. The genetic approach to the salinity problem is fairly new, but research has become quite active in a short span of time. Difficulties and opportunities are outlined. Salinity varies spatially, temporally, qualitatively, and quantitatively. In addition, the responses of plants to salt stress vary during their life cycle. Selection and breeding, including the use of wide crosses, are considered the best short-term approaches to the development of salt tolerant crops, but the new biotechnological and molecular biological techniques will make increasingly important contributions. Cooperation is called for among soil and water scientists, agronomists, plant physiologists and biochemists, cytologists, and plant geneticists, breeders, and biotechnologists. Given such cooperation and adequate support for these endeavors, the potential for increasing productivity in salt-affected areas can be realized.     

The increase in unsaturation of fatty acids of phosphatidylglycerol in thylakoid membrane enhanced salt tolerance in tomato

Overexpression of chloroplastic glycerol-3-phosphate acyltransferase gene (LeGPAT) in tomato increased cis-unsaturated fatty acid content in phosphatidylglycerol (PG) of thylakoid membrane. By contrast, suppressing the expression of LeGPAT decreased the content of cis-unsaturated fatty acid in PG. Under salt stress, sense transgenic plants exhibited higher activities of chloroplastic antioxidant enzymes, lower content of reactive oxygen species (ROS) and less ion leakage compared with the wild type (WT) plants. The net photosynthetic rate (P _N) and the maximal photochemical efficiency (F_v/F_m) of photosystem II (PSII) decreased more slightly in sense lines but more markedly in the antisense ones, compared to WT. D1 protein, located in the reactive center of the PSII, is the primary target of photodamage and has the highest turnover rate in the chloroplast. Under salt stress, compared with WT, the content of D1 protein decreased slightly in sense lines and significantly in the antisense ones. In the presence of streptomycin (SM), the net degradation of the damaged D1 protein was faster in sense lines than in other plants. These results suggested that, under salt-stress conditions, increasing cis-unsaturated fatty acids in PG by overexpression of LeGPAT can alleviate PSII photoinhibition by accelerating the repair of D1 protein and improving the activity of antioxidant enzymes in chloroplasts.     

Comparative physiological and biochemical evaluation of salt and nickel tolerance mechanisms in two contrasting tomato genotypes

Plant tolerance against a combination of abiotic stresses is a complex phenomenon, which involves various mechanisms. Physiological and biochemical analyses of salinity (NaCl) and nickel (Ni) tolerance in two contrasting tomato genotypes were performed in a hydroponics experiment. The tomato genotypes selected were proved to be tolerant (Naqeeb) and sensitive (Nadir) to both salinity and Ni stress in our previous experiment. The tomato genotypes were exposed to combinations of NaCl (0, 75 and 150 mM) and Ni (0, 15, and 20 mg l⁻¹) for 28 days. The results revealed that the tolerant and sensitive tomato genotypes showed similar response to NaCl and Ni stress; however, the level of response was significantly different in both genotypes. The tolerant tomato genotype showed less reduction in growth than the sensitive genotype against both NaCl and Ni stress. Root and shoot ionic analysis showed a decrease in Na and increase in K concentration by increasing Ni levels in the growth medium. Moreover, accumulation of Na and Ni in tissues showed a decrease in membrane stability index and an increase in malondialdehyde contents. The activity of superoxide dismutase, catalase, peroxidase and glutathione reductase under NaCl and Ni stress was significantly higher in the tolerant compared to the sensitive genotype. Enhanced activity of many antioxidant enzymes in Naqeeb under stress conditions is among the other mechanisms that enabled the genotype to better detoxify reactive oxygen species and therefore Naqeeb tolerated the stresses better than Nadir.