Abstract

Salinity is an ever‐present threat to crop yields, especially in countries where irrigation is an essential aid to agriculture. Although the tolerance of saline conditions by plants is variable, crop species are generally intolerant of one‐third of the concentration of salts found in seawater. Attempts to improve the salt tolerance of crops through conventional breeding programmes have met with very limited success, due to the complexity of the trait: salt tolerance is complex genetically and physiologically. Tolerance often shows the characteristics of a multigenic trait, with quantitative trait loci (QTLs) associated with tolerance identified in barley, citrus, rice, and tomato and with ion transport under saline conditions in barley, citrus and rice. Physiologically salt tolerance is also complex, with halophytes and less tolerant plants showing a wide range of adaptations. Attempts to enhance tolerance have involved conventional breeding programmes, the use of in vitro selection, pooling physiological traits, interspecific hybridization, using halophytes as alternative crops, the use of marker‐aided selection, and the use of transgenic plants. It is surprising that, in spite of the complexity of salt tolerance, there are commonly claims in the literature that the transfer of a single or a few genes can increase the tolerance of plants to saline conditions. Evaluation of such claims reveals that, of the 68 papers produced between 1993 and early 2003, only 19 report quantitative estimates of plant growth. Of these, four papers contain quantitative data on the response of transformants and wild‐type of six species without and with salinity applied in an appropriate manner. About half of all the papers report data on experiments conducted under conditions where there is little or no transpiration: such experiments may provide insights into components of tolerance, but are not grounds for claims of enhanced tolerance at the whole plant level. Whether enhanced tolerance, where properly established, is due to the chance alteration of a factor that is limiting in a complex chain or an effect on signalling remains to be elucidated. After ten years of research using transgenic plants to alter salt tolerance, the value of this approach has yet to be established in the field.

Received 20 March 2003; Accepted 3 July 2003

Why should we want to improve crop salt tolerance?

Earth is a salty planet, with most of its water containing about 30 g of sodium chloride per litre. This salt solution has affected, and continues to affect, the land on which crops are, or might be, grown. Although the amount of salt‐affected land (about 900×106 ha) is imprecisely known, its extent is sufficient to pose a threat to agriculture (Flowers and Yeo, 1995; Munns, 2002) since most plants, and certainly most crop plants, will not grow in high concentrations of salt: only halophytes (by definition) grow in concentrations of sodium chloride higher than about 400 mM. Consequently, salinity is a threat to food supply. Although there is currently food enough for the world population, more than 800 million people are chronically undernourished (Conway, 1997). Growth of the human population by 50%, from 6.1 billion in mid‐2001 to 9.3 billion by 2050 (http://www.unfpa.org/swp/2001/), means that crop production must increase if food security is to be ensured, especially for those who live on about $1 per day.

Approximately half of the world’s land surface is ‘perennial desert or drylands’ (United Nations Development Programme, see http://www.undp.org/seed/unso/pub‐htm/dryland‐population.pdf). These areas can only be made more productive by irrigation (and irrigation increased by almost one‐third between 1979 and 1999, from 207 to 274 million hectares; http://apps.fao.org/page/collections?subset=agriculture). Unfortunately, a strong link with salinization (Ghassemi et al., 1995), throws an immediate question over the sustainability of using irrigation to increase food production and it has been argued elsewhere (Shannon and Noble, 1990; Flowers and Yeo, 1995) that the primary value of increasing the salt tolerance of crops will be to the sustainability of irrigation. Given the amount by which food production will have to be increased, it seems reasonable to predict that changing the salt tolerance of crops will be an important aspect of plant breeding in the future, if global food production is to be maintained.

Historical perspective

The need to produce salt‐tolerant crops was evident in ancient times (Jacobsen and Adams, 1958) and the possible ways to increase tolerance have been extensively rehearsed. Epstein et al. (1980) described technical and biological ‘fixes’ to the problem of salinity. The ‘biological fix’ was founded on salt tolerance having a genetic basis, for which the evidence (Epstein et al., 1980) was the existence of a salt‐tolerant flora (halophytes) and differences in salt tolerance between genotypes within species. Varietal differences in salt tolerance have been known since the 1930s (Epstein, 1977) and intraspecific selection for salt tolerance was, by the 1980s, shown to be possible with rice (Akbar and Yabuno, 1977) and barley (Epstein et al., 1980).

In spite of early promise, the ‘biological fix’ has been slow in arriving. In 1993, Flowers and Yeo (1995) reviewed the evidence for the paucity of new salt‐tolerant cultivars and concluded that the number was likely to be fewer than 30. Since 1993, there have been just three registrations of salt‐resistant cultivars in Crop Science (Owen et al., 1994; Al‐Doss and Smith, 1998; Dierig et al., 2001) and one patent registered in the US (Dobrenz, 1999); one other patent claims a method to increase tolerance in cereals by the incorporation of a late embryogenesis abundant (LEA) protein (Wu and Ho, 1996). Flowers and Yeo (1995) concluded that, although salinity might be of profound local importance, it had not yet had sufficient impact on regional agricultural production to warrant the effort necessary to produce new salt‐tolerant cultivars.

The complexity of salt tolerance

Assessment of tolerance

Ultimately, salt tolerance of crops is tested as yield from farmers’ fields. However, evaluating field performance under saline conditions is notoriously difficult because of the variability of salinity within fields (Richards, 1983; Shannon and Noble, 1990; Daniells et al., 2001) and the enormous potential for interactions with other environmental factors, ranging from gaseous pollutants, soil fertility and drainage to temperature, light flux density and transpirational water loss. Consequently, prediction of ‘field’ performance is commonly carried out in trial plots, or using a solution‐based method where the salinity of the medium can be readily adjusted to required values (Maas and Hoffmann, 1977; Francois and Maas, 1994). The latter often precludes measuring yield through lack of space and estimates of tolerance obtained from such experiments may not always be borne out by the response of plants in the field (Rowland et al., 1989; Daniells et al., 2001). Evaluating tolerance is made more complex by variation in sensitivity to salt during the life cycle. For example, it has long been known that grain yield in rice is much more depressed by salt than is vegetative growth (Khatun and Flowers, 1995): germination is relatively salt resistant. In tomato, tolerance at germination is not correlated with the ability to grow under salt stress: both are controlled by different mechanisms (Foolad and Lin, 1997), although it is possible to find genotypes with similar tolerance at germination and during vegetative growth (Foolad and Chen, 1999).

Genetics of salt tolerance

Perhaps the first attempt to evaluate the inheritance of salt tolerance was made by Lyon (1941). An interspecific cross of Lycopersicon esculentum and L. pimpinellifolium showed fruit yield of the hybrid was more sensitive to increasing salt (sodium sulphate) than that of either parent. Other crosses of wild and cultivated tomato also suggested a complex genetics. Heterosis was apparent under saline (NaCl) conditions in the elongation of stems in hybrids of L. esculentum produced with three wild species (L. cheesmanii, L. peruvianum, and L. pennellii =Solanum pennellii) by Tal and Shannon (1983). Stem elongation was a dominant trait in hybrids with S. pennellii, but not with L. cheesmanii as the parent. Total dry matter production of another F1 hybrid, between L. esculentum and L. pennellii, showed hybrid vigour (Saranga et al., 1991) under saline conditions. Analysis of other species has also suggested that the genetics of salt tolerance is complex.

In rice, sterility, an important factor in yield under saline conditions, is determined by at least three genes (Akbar et al., 1972; Akbar and Yabuno, 1977). In diallel analysis the effects of salinity on the seedling stage and on sterility suggested both additive and dominance effects, some with high heritability (Moeljopawiro and Ikehashi, 1981; Akbar et al., 1986). Evidence of dominance of tolerance is also seen with pigenopea (Cajanus cajan), where a cross with Atylosia albicans (one of the most salt‐tolerant relatives of pigeonpea) produced intergeneric hybrids that behaved as the wild parent, indicating dry weight production was determined by a dominant genetic factor (Subbarao et al., 1990). There is also evidence of dominance in the salt tolerance of sorghum. Diallel analysis, based on assessing tolerance to NaCl as relative root length in salt‐treated as compared with control plants, showed that there were both additive and dominance effects of NaCl (Azhar and McNeilly, 1988). These examples suggest that while the assessment of tolerance is complicated by changes occurring during the ontogeny of a plant and may be technically difficult under field conditions, there is evidence of a genetically complex trait (Shannon, 1985), showing heterosis, dominance and additive effects.

Physiological complexity

As well as the genetic evidence, there is physiological evidence to support the view that salt tolerance is a complex trait. Halophytes show a wide range of adaptations from the morphological to the biochemical (Flowers et al., 1986; Leach et al., 1990; Flowers and Dalmond, 1992; Glenn et al., 1999; Tester and Davenport, 2003), adaptations that include the ability to remove salt through glandular activity. Although control of ion uptake is exercized at the root, the ability to secrete ions has evolved into a successful strategy for salt tolerance. Some (but by no means all) halophytes utilize salt‐secreting glands to remove excess ions from their leaves (Thomson et al., 1988), reducing the need for very tight balancing of ion accumulation and growth (Flowers and Yeo, 1988). Within less tolerant species, intraspecific variation in tolerance is also associated with variation in a wide variety of physiological traits (Yeo et al., 1990; Cuartero et al., 1992; Foolad, 1997; Wahid et al., 1997; Tozlu et al., 1999a, b).

Perhaps the best investigated of the traits relating to salt tolerance are those associated with the ion contents of plants grown in the presence of salts. Although there was an early hint that an ability to exclude chloride in Glycine showed simple Mendelian inheritance (Abel, 1969), there proved to be complications introduced by interactions with phosphate (Grattan and Maas, 1984, 1988). In vine, chloride exclusion appeared to be inherited either as a qualitative or a quantitative trait, depending on the parents (Sykes, 1992), while in inter‐specific crosses of Citrus, chloride accumulation showed continuous variation amongst progeny suggesting that this is a polygenic trait, although with a strong heritable basis (see Sykes, 1992). In Trifolium repens, heritability of chloride accumulation, estimated from parent–progeny regressions, is only moderate (0.24–0.37, Rogers et al., 1997), but sufficient for net accumulation of chloride to be a useful tool in the selection of salt‐tolerant genotypes.

For the other dominant ion in saline soils, sodium, there has been considerably more research, often associated with an estimation of the other major monovalent cation in plants, potassium. The tolerance of plants to sodium chloride is commonly, but not uniquely, related to the concentration of sodium in the shoot. For tomato, Foolad (1997) reported, from a parent–progeny comparison, that sodium accumulation under saline conditions was under genetic control, with more than 90% of the genetic variation attributable to additive effects: dominance had little influence. Analysis of sodium and potassium accumulation between Fn and Fn+1 families of rice growing under saline conditions showed that net accumulation of both sodium and potassium to be heritable (with narrow sense heritabilities of between 0.4 and 0.5), although shoot sodium and potassium concentrations were unrelated, suggesting that the pathways for net accumulation of sodium and potassium in rice are separate (Garcia et al., 1997a). A high degree of heterosis and large environmental effects on Na/K ratios (Gregorio and Senadhira, 1993) are characteristic of this aspect of salt tolerance in rice behaving as a quantitative trait.

The means by which sodium enters plants is still poorly understood. At low external concentrations, potassium may enter roots through K carriers, while at higher concentrations, non‐selective cation channels (Demidchik et al., 2002) are possible means of transport. Channels that are activated when the transmembrane potential is hyperpolarized are highly selective for K; other channels, activated when the membrane potential is depolarized, are less selective and could be one means by which sodium enters cells (Maser et al., 2002b). Sodium can also enter via KUP/HAK/KT potassium transporters, cyclic‐nucleotide‐gated channels, glutamate‐activated channels, LCT transporters, and HKT transporters, although the relative roles of each seem likely to vary across species (Maser et al., 2002b; Tester and Davenport, 2003). HKT1 plays a role in net Na accumulation into wheat (Laurie et al., 2002) and into the distribution of ions between roots and shoots of arabidopsis (Maser et al., 2002a). The rice OsHKT1 is down‐regulated after osmotic shock (with 150 mM NaCl) of plants growing in a low (micromolar) potassium concentration and more so in a vigorous tolerant landrace than in a sensitive dwarfed variety (Golldack et al., 2002).

Much has been written about the importance of the ability of plants to discriminate between sodium and potassium, for which a simple index, the K/Na ratio, can be determined for plants and plant parts. In bread wheat, the discrimination between potassium and sodium in their transport to the shoot, manifested as K/Na ratio in shoot tissue, is apparently determined by a locus described as Kna1 and confirmed by RFLP analysis to be completely linked to five markers on the long arm of Chromosome 4D (Gorham et al., 1997). That the ratio of K to Na in a plant is determined at a single locus, if proved true, is surprising, given the number of proteins that might contribute to Na and K transport from root to shoot, unless they, or their control, are clustered in a particular chromosomal location. The K/Na discrimination trait can be transferred from durum to bread wheat (Dvorak et al., 1994). However, control of the K/Na discrimination itself cannot be confined to the D genome, as in durum wheat (Triticum turgidum L. ssp durum) discrimination equivalent to that found in the hexaploid bread wheat has been found in lines which contain no D genome (Munns et al., 1999).

Quantitative trait loci

There is, then, considerable evidence to support the view that salt tolerance and its sub‐traits might be determined by multiple gene loci. In an intergeneric cross of tomato, quantitative trait loci (QTL) were found associated with fruit yield in plants growing under saline conditions (Breto et al., 1994), although some of the QTL identified were later shown to be dependent on the parentage of the cross (Monforte et al., 1997a). An important conclusion stemming from this work was that QTL are treatment‐sensitive. Some QTL associated with aspects of fruit yield were found regardless of whether the plants were grown with or without salt; others were detected only under saline or under non‐saline conditions (Monforte et al., 1997b). Other crosses have also identified both stress‐ (salt and cold) specific and stress‐non‐specific QTL: the stress‐non‐specific QTL generally exhibited larger individual effects and accounted for a greater portion of the total phenotypic variation under each condition than the stress‐specific QTL (Foolad et al., 1999). As for the QTL identified for fruit yield, QTL associated with germination depend upon the conditions under which germination is assessed (Foolad et al., 1999). A similar situation exists for citrus, where about half of the potential QTL identified depended on the presence or absence of salinity (Tozlu et al., 1999a), and in rice (Gong et al., 1999, 2001) where less than 10% of the QTL were detected both in the presence and absence of salt. Clearly, the major determinants of yield vary with the environmental conditions and quantitative traits typically exhibit a large environment×genotype interaction.

The use of tomato has also been important in establishing that QTL associated with tolerance vary with the stage of plant development. The QTL associated with tolerance at germination (Foolad et al., 1997, 1998) and vegetative growth (Foolad and Chen, 1999; Foolad et al., 2001) differ (Foolad, 1999). Such differences are not restricted to tomato and have been demonstrated in arabidopsis (Quesada et al., 2002) and cereals; both barley (Mano and Takeda, 1997) and rice (Prasad et al., 2000). QTL associated with aspects of ion transport have also been reported in citrus (Tozlu et al., 1999b) and in rice (Koyama et al., 2001).

Conclusions

There is sufficient evidence to be confident that salt tolerance is a multigenic trait. Research on the physiology of salt tolerance suggests that the overall trait is determined by a number of sub‐traits any of which might, in turn, be determined by any number of genes. These sub‐traits generally include an ability to minimize the net accumulation of sodium and/or chloride ions and to select potassium from a background of high sodium concentration.

Approaches to enhancing tolerance

Flowers and Yeo (1995) suggested five possible ways, which were appropriate at that time, to develop salt‐tolerant crops: (1) develop halophytes as alternative crops; (2) use interspecific hybridization to raise the tolerance of current crops; (3) use the variation already present in existing crops; (4) generate variation within existing crops by using recurrent selection, mutagenesis or tissue culture, and (5) breed for yield rather than tolerance. These all remain possible solutions to the problem. Although conventional forms of mutagenesis have not, in general, delivered salt‐tolerant genotypes (Flowers and Yeo, 1995; but see Tester and Davenport, 2003), mutagenesis has unearthed a number of salt‐sensitive types (Borsani et al., 2002; Zhu, 2002). Bohnert and Jensen (1996) claimed that an important approach had been missed by Flowers and Yeo: they wrote ‘tolerance breeding must be accompanied by transformation’; and that ‘successful releases of tolerant crops will require large‐scale “metabolic engineering” which must include the transfer of many genes’. While such an approach was not feasible in the early 1990s (Flowers and Yeo, 1996); this approach is now being widely advocated. Some 13 species (Table 1A) have been transformed with nearly 40 genes in experiments reported between 1993 and 2003 (Table 1B). The majority of experiments have used rice, tobacco and arabidopsis; transformations involving the synthesis of compatible solutes have been more popular than any other, with those involving glycine betaine the most commonly performed (Table 1A). There is an increasing number of claims in this literature that overall tolerance can be manipulated through alteration in the activity of one or two genes (see below), which was not something claimed by Bohnert and Jensen (1996). For a trait as complex as salt tolerance this seems intuitively unlikely. The fundamental issue to be resolved is the importance of individual components or sub‐traits of salt tolerance and whether the manipulation of individual or of many genes is required to alter complex traits. If altering a single gene can alter tolerance, this suggests either that changing the concentration of a few key components has a substantial effect on a wide range of other processes or that salt tolerance is not as complex as it appears or that a key limit to tolerance might be altered in any given species (or genotype). Substantiating, or otherwise, claims that tolerance is altered by transformation is clearly of major importance both for our understanding of complex traits and for the practicalities of their manipulation.

Genetic engineering of salt tolerance: evaluation of success

The evaluation of transgenic material requires some comment. The material to be tested should be genetically stable (it has been suggested that it should be in its fourth or fifth generation by Bajaj et al., 1999) and a comparison of as many transformed lines as possible made with the performance of the parental (wild‐type) line under saline and non‐saline conditions (Table 2). It is important to know whether or not the overall growth of the transgenic plant has been affected, as vigour itself is an important determinant of salt tolerance. For crops, claims of enhanced tolerance should be made on the basis of yield. Unfortunately, there were no such reports by 1999 (Bajaj et al., 1999) and the situation had changed little by early 2003. Given the paucity of data on crop yield (just five reports of estimates of crop yields, Guo et al., 1997; Wang et al., 2000; Zhang and Blumwald, 2001; Zhang et al., 2001; Li et al., 2002), the success, or otherwise, of a transformation in altering salt tolerance has generally to be evaluated against the nature of the data that is presented. Those claims based on quantitative estimates of the growth of fourth or fifth generation transgenic lines should be seen as stronger than claims based upon photographic evidence of the performance of plants of the primary transformants grown in salt alone. In the following analysis, papers are allocated to one of five categories (Table 3). Only those data relating to the growth of plants under conditions in which transpiration occurs have been evaluated: it is transpiration that transports ions to the shoots, where their presence brings about injury and death. Photographs of plants in culture medium are unconvincing as evidence for a successful alteration of crop yield. Quantitative measures of growth are required for plants grown in the presence and absence of salt: the ability to germinate in salt is, in general, a poor indicator of performance in the field. It is also important that salt be added in such a way that it is not the effect of water or osmotic stress that is being evaluated and this generally requires an increase of salt concentration of 50 mM or less per day and determination of the consequences days or weeks later, depending on the salt tolerance of the species (Munns, 1993, 2002).

Analysis of publications to date shows that of the 68 reports produced between 1993 and early 2003 (Table 3; see also supplementary data online) only 19 describe quantitative estimates of plant growth. Of these, four papers (Table 4) contain quantitative data on the response of transformants and wild type of six species without and with salinity applied in an appropriate manner. About half of all the papers (35, Table 3 and supplementary data online) report data on experiments conducted under conditions where there is little or no transpiration: such experiments may provide insights into components of tolerance, but are not grounds for claims of enhanced tolerance at the whole plant level—in such a system, the fern Ceratopteris, where single gene mutants alter the salt tolerance in the gametophytic generation (Warne et al., 1995) might be a useful genetic model.

Those experiments where the effects of transformation were determined in saline and non‐saline soil or hydroponic culture suggest that real changes in salt tolerance can be effected, but generally not without consequences for the growth that occurs in the absence of salt. Over‐expression of the gene Alfin1 in alfalfa increased its salt tolerance and promoted root growth and shoot growth (Table 4A), under normal and saline conditions, producing larger plants than the wild type (Winicov, 2000). Alfin1 is a putative transcription factor, but its mode of action in altering overall salt tolerance is still unclear. Adding to the uncertainty of how some genes affect overall tolerance is the consequence of transforming tomato with the yeast gene HAL1. HAL1 alters the salt tolerance of tomato (Gisbert et al., 2000) and increases the K/Na ratio in transgenic plants. However, these transgenic plants, when grown in the absence of salt, had half the shoot dry weight of the wild type (Table 4A). A similar effect of an introduced gene on growth is also seen following the transformation of tobacco with mannitol‐1‐phosphate dehydrogenase. Here mannitol that accumulated as a consequence of the transformation made only a small contribution to the osmotic potential of the transformed plants, which were smaller than the wild‐type, although they were less affected in relative terms by salinity (Table 4A). A similar situation was reported by Huang et al. (2000) for arabidopsis, canola and tobacco transformed to oxidize choline to glycinebetaine (Table 4A). In all of these cases, the effects of the genes are not simply on tolerance, making the evaluation of the effects complex. Tolerance, judged in relative terms (i.e. yield in the saline conditions expressed as a proportion of yield in non‐saline conditions), although an important indicator, is unlikely to impress a farmer unless the absolute yield is adequate. A genotype whose yield is hardly affected by salinity may well still be out‐performed by a vigorous, high‐yielding genotype which loses 50% of its yield under saline conditions, if the ‘salt‐tolerant’ genotype is intrinsically low‐yielding (Dewey, 1962). A similar situation has been previously reported for some hybrids between established crop varieties and wild relatives (Table 4B).

There is other, albeit weaker, evidence that transformation of plants with genes whose products affect transcription does appear to alter salt tolerance. Tobacco transformed with a gene isolated from a cDNA library prepared from salt‐treated plants of Atriplex hortensis, by screening with a fragment of arabidopsis AtDREB2A encoding a DNA‐binding domain, apparently increased the tolerance of tobacco to salt (Shen et al., 2003) without causing a dwarf phenotype, although quantitative data are lacking. A yeast kinase (a functional homologue of the yeast Dbf2 kinase) enhanced tolerance of tobacco cells to salt in tissue culture (Lee et al., 1999). However, the assessment of salt tolerance in tissue culture is a poor predictor of tolerance in the whole plant and tolerance in cultured cells is not translated to tolerance in plants in the field (Flowers and Yeo, 1995). Transformation of arabidopsis with the protein kinase coded by AtGSK1 (a GSK/Shaggy‐like protein kinase) induced anthocyanin synthesis, a symptom of NaCl stress, in the absence of NaCl (Piao et al., 2001) and promoted survival in soil irrigated with NaCl (but with 300 mM, which is likely to have produced an initial osmotic shock rather than a salt stress).

Recent research has shown that rice, transformed to overexpress genes that brought about the synthesis of trehalose, contained a reduced concentration of Na in the shoot and grew better than non‐transformed (control) plants when in the presence of 100 mM NaCl (Garg et al., 2002). Trehalose concentrations in the plants grown in 100 mM NaCl were relatively low, at less than 0.1 mg g–1 fresh weight (approximately 5 mM in the cytoplasm if this were 10% of the water volume; Flowers et al., 1991). It has been suggested that solutes such as trehalose are likely to function through their ability to scavenge reactive oxygen species (Zhu, 2001) and the protection afforded to the machinery of protein synthesis may be particularly important for normal repair processes (Chen and Murata, 2002). Another possible explanation for at least some of the compounds such as trehalose is that they act in a signalling cascade. Although trehalose is commonly present in bacteria, fungi and insects, its concentration in plants is very low and it may even be toxic: recent evidence suggests that this toxicity may stem from its role in the regulation of carbon metabolism (Muller et al., 1999; Wingler, 2002). Earlier research on tobacco had shown that transformants producing trehalose were stunted in growth (Romero et al., 1997) and experiments on rice had shown that treatment of plants with exogenous trehalose reduced sodium accumulation, but had a significant effect on root morphology (Garcia et al., 1997b). In the more recent experiment of Garg et al. (2002), the synthesis of trehalose was under the influence of a stress‐inducible promoter, so that growth under control conditions was presumably no different from the wild type (the authors note that non‐stressed plants appeared normal, but did not, unfortunately, support this with quantitative data). The use of stress‐inducible promoters may be an important way in which to avoid inhibition under non‐stressed conditions (Kasuga et al., 1999), if there are yield penalties from expressing genes under a constitutive promoter.

Although the targets of genetic engineering have largely been compatible solutes, there have been some attempts to manipulate one of the underlying causes of salt damage, the net accumulation of sodium ions. Down‐regulation of HKT1 in wheat increased resistance to salinity under conditions of low K supply (Laurie et al., 2002) and transformation of the cyanobacterium Synechococcus with a Na/H antiporter increased its tolerance to salt (Waditee et al., 2002). For higher plants, however, any enhancement of antiporter activity would have to be targeted to root cells, for in the aerial parts of the plant enhanced Na/H antiporter activity would only exacerbate the consequences of ion accumulation in cell walls (Oertli, 1968; Flowers et al., 1991; Munns, 2002). Even in the roots, it is likely that ions removed from cortical cell walls would have osmotic consequences (Yeo, 1998): only in situations where there was a large volume of external solution (e.g. marine algae) would there be a chance of ions effluxing from the cytoplasm being washed from cell walls. Manipulating the vacuolar proton gradient to enhance ion accumulation has also led to claims of enhanced salt tolerance in transgenic plants (Gaxiola et al., 2002). However, there is only qualitative evidence for Arabidopsis (Apse et al., 1999) and the evidence obtained with Brassica napus (Zhang et al., 2001) and tomato (Zhang and Blumwald, 2001) does not include (other than a photograph) the effects of salt (200 mM) on the wild type. In other experiments, B. napus continued to yield in 200 mM NaCl (Ashraf et al., 2001) as does the tomato cultivar Moneymaker (TJ Flowers and SA Flowers, unpublished data). While the ability to accumulate sodium in leaf vacuoles is clearly a trait that is important for dicotyledonous halophytes, in such species this ability is coupled with other traits such as the regulation of transpiration, the synthesis of compatible solutes and an ability to function with low cytoplasmic potassium concentrations (Flowers and Dalmond, 1992).

Conventional breeding programmes

Strategies for breeding for salt tolerance in cross‐pollinating species by cycles of recurrent selection were described long ago (Dewey, 1962): for a self‐pollinating species the same process would require the use of male‐sterile lines to facilitate out‐crossing (Ramage, 1980). These approaches depend on adequate heritability of the overall trait, for which there is evidence for wild grasses (Ashraf et al., 1986), sorghum (0.74, Maiti et al., 1994), maize (0.4, Maiti et al., 1996), and tomato (Saranga et al., 1992, 1993). In both rice (Yeo et al., 1988) and Trifolium (Rogers and Noble, 1992; Rogers et al., 1997), it has proved possible to select lines whose ion contents, when grown under saline conditions, are either higher or lower than those of the parental types. By way of contrast, Saranga et al. (1992) concluded that for tomato (a cross between L. esculentum and L. pennellii), selection for ion contents would not improve the breeding process.

Use of in vitro selection

The use of in vitro selection was widely advocated during the 1980s, but did not result in cultivars in farmers’ fields (Rowland et al., 1989). More recently, selections for alfalfa (Winicov, 1991; Winicov and Bastola, 1997) look promising and there may be a use for somaclonal variants within breeding programmes (Zhu et al., 2000).

Pooling physiological traits

The possibility of pooling physiological traits has been advocated for rice (Yeo et al., 1990), screening methods evaluated (Garcia et al., 1995) and the approach proved successful in generating salt‐resistant lines (Gregorio et al., 2002). The methodology does not require a deep knowledge of the genetics of traits, merely that they display sufficient heritability and that suitable screening procedures can be developed. The methods may be applicable to crops other than rice (Cuartero et al., 1992; Ellis et al., 1997; Foolad, 1997; Isla et al., 1998; Munns et al., 2002).

Interspecific hybridization

The introduction of genes from wild salt‐tolerant species has been explored for tomato (Rush and Epstein, 1981; Tal and Shannon, 1983; Saranga et al., 1991; Perez Alfocea et al., 1994), tomato/potato (Sherraf et al., 1994), wheat (Dvorak and Ross, 1986; Gorham et al., 1986; Mahmood and Quarrie, 1993; Martin et al., 1993; William and Mujeebkazi, 1993; King et al., 1997a, b), and pigeonpea (Subbarao et al., 1990). However, the approach has not led to the release of salt‐tolerant crops, although there is a recent proposal for a new salt‐tolerant cereal, tritipyrum (King et al., 1997b),

Halophytes as alternative crops

Historical evidence suggests that farmers shift from more sensitive to more tolerant crops as salinity in their fields rises (Jacobsen and Adams, 1958). The natural end of such a succession would be the use of halophytes, whose potential as crops has been explored (Malcolm, 1969; O’Leary, 1984; O’Leary et al., 1985; Lovett, 1993; Troyodieguez et al., 1994; Zahran, 1994; Brown and Glenn, 1999; Brown et al., 1999; Glenn et al., 1999), but is yet to be fully realized. Since the domestication of wild species was, in the past, a successful strategy, this must remain a useful approach for generating salt‐tolerant crops in the future, especially given the wide range of halophytes available.

Use of marker‐aided selection

The multigenic nature of salt tolerance has clearly been established and quantitative trait loci associated with aspects of germination, ion transport and yield. One obvious use of QTL in plant breeding for salt tolerance is in marker‐aided selection (or marker‐assisted selection, MAS). The drawbacks in using marker‐assisted breeding are ‘linkage drag’ of undesirable traits due to the large size of regions of chromosomes identified by QTL (Asins, 2002) and the fact that environment and genetic background have a significant influence on the QTL that are identified (see above). In a wider context, QTL might be used to identify genes that are important in salt tolerance and it is noteworthy, given the complexity of salt tolerance, that so few QTL are identified (Yeo et al., 2000) within any given genome. This may be an indication that traits are determined by a limited number of sites and/or that genes associated with physiological traits are clustered on chromosomes. However, the fact that a QTL represents many, perhaps hundreds, of genes remains a problem to finding key loci within a QTL. The easiest way forward may be through the identification of candidate genes. Of the five QTL associated with the effects of salinity on vegetative growth in arabidopsis (Quesada et al., 2002) one was located close to the location of SOS2 (which codes for a serine/threonine protein kinase) and another close to the positions of RD29A and RD29B (genes coding for hydrophilic proteins involved in ABA signal transduction).

Conclusions

It is conceivable that approaches that identify specific genes that are up‐ or down‐regulated either through the analysis of RNA (Kawasaki et al., 2001) or proteins (Salekdeh et al., 2002) might provide a specific focus for transformation, although choosing key genes for tolerance is currently far from happening. Transgenic technology will undoubtedly continue to aid the search for the cellular mechanisms that underlie tolerance, but the complexity of the trait is likely to mean that the road to engineering such tolerance into sensitive species will be long. In the meantime, it would be expedient to continue to invest in other avenues such as the manipulation of ion excretion from leaves through salt glands and the domestication of halophytes. Experience suggests authors should avoid hyperbole in their titles and summaries, as this does little service to the long‐term aim of improving the salt tolerance of crops in the field.

Supplementary data

Table S1 is a list, in date order, of papers where plants have been transformed to investigate or alter their salt tolerance, together with the category (see Table 3) assigned to the paper. It can be found at Journal of Experimental Botany online.

Acknowledgements

I would like to thank the Royal Society for a grant that allowed me to visit Western Australia, a region undergoing a salinity crisis; Tim Colmer, Rana Munns and Rob Walker for their hospitality and Wieland Fricke and Sam Flowers for their comments on the text.

Table 1.

Species and genes used in the transformation of plants where authors claimed enhancement of salt tolerance

(A) Species No. of experiments reported 
Arabidopsis thaliana 14 
Brassica napus and B. juncea 
Citrus (Carrizo citrange)  
Cucumis melo (melon) 
Diospyros kaki (Japanese persimmon) 
Lycopersicon esculentum (tomato) 
Medicago sativa (alfalfa) 
Nicotiana tabaccum (tobacco) 19 
Oryza sativa (rice) 17 
Solanum melongena (eggplant) 
Solanum tuberosum (potato) 
Triticum aestivum (wheat)  
(B) Transformed for No. of experiments  
Apoplastic invertase, Apo‐Inv 
Arginine decarboxylase, ADC 
Betaine aldehyde dehydrogenase, BADH; betB, choline dehydrogenase (CDH);  15 
 choline oxidase, codA (glycinebetaine)  
Ca2+‐dependent protein kinase, CDPK 
Ca/H antiporter, CAX1 
Calcium‐binding protein, EhCaBP 
Calicneurin; protein kinase, CaN  
Ca protein kinase, OsCDPK7 
Glutathione S‐transferase, GST and glutathione peroxidase, GPX 
Glyceraldehyde‐3‐phosphate dehydrogenase, GPD 
Glycogen‐synthase kinase‐3, AtGSK1 
Glutamine synthetase, GS2 
Heat shock protein, DnaK/HSP70 
High‐affinity potassium transporter, *HKT1a 
Isopentenyl transferase, ipt (increased cytokinin) 
Late embryo abundant protein, HVA1 (a LEA) 
Mannitol 1‐phosphate dehydrogenase, mt1D (mannitol) 
Myo‐inositol O‐methyltransferase, IMT1 (ononitol) 
Omega‐3 fatty acid desaturase, fad7 (fatty acid processing) 
Osmotin‐like protein 
Proline dehydrogenase; Delta (1)‐pyrroline‐5‐carboxylate synthetase (proline) 
Proline transporter, AhProT1 
Proton sodium exchanger, *HNX1a 
Putative transcription factor, Alfin1 
Rare Cold Inducible gene 3, RCI3 
Rice Hal2 like, RHL 
S‐adenosylmethionine decarboxylase, SAMDC (spermine, spermidine) 
Serine/threonine kinase, AT‐DBF2 
Sorbitol‐6‐phosphate dehydrogenase, SPD (sorbitol) 
SR‐like, putative splicing protein 
Transcription factors, DREB1A; AhDREB1 
Trehalose‐6‐phosphate synthase/phosphatase, TPSP (trehalose) 
Yeast halotolerance gene, Hal2 
Yeast halotolerance gene, Hal1 
Yeast mitochondrial superoxide dismutase, Mn‐SOD 
Vacuolar H+‐pyrophosphatase, AVP1 
(A) Species No. of experiments reported 
Arabidopsis thaliana 14 
Brassica napus and B. juncea 
Citrus (Carrizo citrange)  
Cucumis melo (melon) 
Diospyros kaki (Japanese persimmon) 
Lycopersicon esculentum (tomato) 
Medicago sativa (alfalfa) 
Nicotiana tabaccum (tobacco) 19 
Oryza sativa (rice) 17 
Solanum melongena (eggplant) 
Solanum tuberosum (potato) 
Triticum aestivum (wheat)  
(B) Transformed for No. of experiments  
Apoplastic invertase, Apo‐Inv 
Arginine decarboxylase, ADC 
Betaine aldehyde dehydrogenase, BADH; betB, choline dehydrogenase (CDH);  15 
 choline oxidase, codA (glycinebetaine)  
Ca2+‐dependent protein kinase, CDPK 
Ca/H antiporter, CAX1 
Calcium‐binding protein, EhCaBP 
Calicneurin; protein kinase, CaN  
Ca protein kinase, OsCDPK7 
Glutathione S‐transferase, GST and glutathione peroxidase, GPX 
Glyceraldehyde‐3‐phosphate dehydrogenase, GPD 
Glycogen‐synthase kinase‐3, AtGSK1 
Glutamine synthetase, GS2 
Heat shock protein, DnaK/HSP70 
High‐affinity potassium transporter, *HKT1a 
Isopentenyl transferase, ipt (increased cytokinin) 
Late embryo abundant protein, HVA1 (a LEA) 
Mannitol 1‐phosphate dehydrogenase, mt1D (mannitol) 
Myo‐inositol O‐methyltransferase, IMT1 (ononitol) 
Omega‐3 fatty acid desaturase, fad7 (fatty acid processing) 
Osmotin‐like protein 
Proline dehydrogenase; Delta (1)‐pyrroline‐5‐carboxylate synthetase (proline) 
Proline transporter, AhProT1 
Proton sodium exchanger, *HNX1a 
Putative transcription factor, Alfin1 
Rare Cold Inducible gene 3, RCI3 
Rice Hal2 like, RHL 
S‐adenosylmethionine decarboxylase, SAMDC (spermine, spermidine) 
Serine/threonine kinase, AT‐DBF2 
Sorbitol‐6‐phosphate dehydrogenase, SPD (sorbitol) 
SR‐like, putative splicing protein 
Transcription factors, DREB1A; AhDREB1 
Trehalose‐6‐phosphate synthase/phosphatase, TPSP (trehalose) 
Yeast halotolerance gene, Hal2 
Yeast halotolerance gene, Hal1 
Yeast mitochondrial superoxide dismutase, Mn‐SOD 
Vacuolar H+‐pyrophosphatase, AVP1 

a Asterisk indicates a specific prefix, e.g. At.

Table 2.

Possible combinations for experiments reporting the evaluation of transgenic plants Column 1 lists experimental sites and column 2 the parameters that might be evaluated—yielding six possible combinations. In each combination, either quantitative or qualitative date might be reported on transformant and/or wild type, ideally under saline and non‐saline conditions, but often only the transformant under saline conditions. Ideally, an experiment evaluating the effects of a transformation would report quantitative data on yield of field grown plants of both wild‐type and transformant lines in the presence or absence of salinity.

Experimental location Parameter evaluated Data Material tested Treatments 
Field Yield Quantitative Transformant lines Plus salinity 
Greenhouse or other controlled environment Growth Qualitative Wild type Minus salinity 
In vitro’ (including germination)    Plus and minus salinity 
Experimental location Parameter evaluated Data Material tested Treatments 
Field Yield Quantitative Transformant lines Plus salinity 
Greenhouse or other controlled environment Growth Qualitative Wild type Minus salinity 
In vitro’ (including germination)    Plus and minus salinity 
Table 3.

Criteria used to define categories of experiments where an enhancement of salt tolerance was claimed as a consequence of a transformation for plants and the numbers of experiments reported (between 1993 and early 2003) in each of those categories

Criteria Category Number in category 
Data Material tested Treatments   
Quantitative Transformants and wild type Plus and minus salinity 
Quantitative Material missing Treatments missing 1a 13 
Qualitative Transformants and wild type Plus and minus salinity 
Qualitative Material missing Treatments missing 2a 12 
All in vitro   35 
Criteria Category Number in category 
Data Material tested Treatments   
Quantitative Transformants and wild type Plus and minus salinity 
Quantitative Material missing Treatments missing 1a 13 
Qualitative Transformants and wild type Plus and minus salinity 
Qualitative Material missing Treatments missing 2a 12 
All in vitro   35 
Table 4.

The effects of transformation (A) and hybridization (B) on the growth (g dry weight or fresh weight) or yield of plants in the presence and absence of salt

Species Minus NaCl Plus NaCl Reference 
 Wild type Transformant Wild ype  Transformant NaCl (mM)  
(A)       
Alfalfa 0.50 2.18 – – – Winicov, 2000a 
Tomato 1.4  0.67  0.70  0.72  150  Gisbert et al., 2000b 
Tobacco 73.6  55.0  40.4  41.5  150 Karakas et al., 1997c 
Arabidopsis 0.311 0.189 0.05 0.052 100 Huang et al., 2000d 
Canola 1.51 0.60 1.09 0.60 300 Huang et al., 2000d 
Tobacco 1.174 0.231 0.661 0.209 150 Huang et al., 2000e 
 Cultivated parent Hybrid Cultivated parent Hybrid   
(B)       
Wheat 121.2 75.4 23.0 21.2 150 King et al., 1997bf 
Tomato 7438 5132 2343 1244 150 Rush and Epstein, 1981g 
Species Minus NaCl Plus NaCl Reference 
 Wild type Transformant Wild ype  Transformant NaCl (mM)  
(A)       
Alfalfa 0.50 2.18 – – – Winicov, 2000a 
Tomato 1.4  0.67  0.70  0.72  150  Gisbert et al., 2000b 
Tobacco 73.6  55.0  40.4  41.5  150 Karakas et al., 1997c 
Arabidopsis 0.311 0.189 0.05 0.052 100 Huang et al., 2000d 
Canola 1.51 0.60 1.09 0.60 300 Huang et al., 2000d 
Tobacco 1.174 0.231 0.661 0.209 150 Huang et al., 2000e 
 Cultivated parent Hybrid Cultivated parent Hybrid   
(B)       
Wheat 121.2 75.4 23.0 21.2 150 King et al., 1997bf 
Tomato 7438 5132 2343 1244 150 Rush and Epstein, 1981g 

a Weighted mean of three transformed lines.

b Shoot dry weight.

c Total PLANT dry weight.

d Dry weight per plant: choline‐supplemented wild‐type (WT) and betaine‐producing transgenic lines.

e Fresh weight per plant: choline‐supplemented wild‐type (WT) and betaine‐producing transgenic lines.

f Grains per plant.

g Average fruit yield, g fresh weight per plant.

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