3 Plant adaptive responses to salinity stress
Miguel A. Botella, Abel Rosado, Ray A. Bressanand Paul M. Hasegawa
3.1 Salt stress effects on plant survival, growth and development
Salt is so abundant on our planet that it is a major constraint to global foodcrop production, jeopardizing the capacity of agriculture to sustain the bur-geoning human population increase (Flowers, 2004). It is estimated that 20%of all cultivated land and nearly half of irrigated land is salt-affected, greatlyreducing yield well below the genetic potential (van Schilfgaarde, 1994;Munns, 2002; Flowers, 2004). Soil salinity is becoming a more acute problem,primarily because of declining irrigation water quality (Rhoades & Loveday,1990; Ghassemi et al., 1995; Flowers, 2004). Consequently, new strategies toenhance crop yield stability on saline soils represent a major research priority.Remediation of salinized soils is a possibility. However, this approach ishypothetically based and unsubstantiated by compelling evidence of feasibility(Tester & Davenport, 2003). A complementary strategy is to increase salttolerance of crop plants either by genetic introgression, using both traditionalas well as molecular marker-assisted breeding techniques, and biotechnology(Hasegawa et al., 2000; Koyama et al., 2001; Borsani et al., 2003; Flowers,2004).Limited success in increasing the yield stability of crops grown on saline
soils is due, in some measure, to a minimal understanding about how salinityand other abiotic stresses affect the most fundamental processes of cellularfunction including cell division, differentiation and expansion which havea substantial impact on plant growth and development (Hasegawa et al., 2000;Zhu, 2001). In fact, many years ago we suggested that plant adaptation tosalinity stress most probably involves phenological responses that are import-ant for fitness in a saline environment but may adversely affect crop yield(Binzel et al., 1985; Bressan et al., 1990). Regardless, some degree of yieldstability is achievable in salt stress environments (Epstein et al., 1980; Yeo &Flowers, 1986; Flowers & Yeo, 1995; Flowers, 2004). For example, wildspecies and primitive cultivars of tomato are a repository of genetic variationfor salt adaptive capacity (Jones, 1986; Cuartero et al., 1992).Whether sufficient genetic diversity for salt tolerance exists or is accessible
in cultivars commonly used in breeding programs is problematic (Yeo &Flowers, 1986; Flowers, 2004). The narrow gene pool for salt tolerance in
Plant Abiotic StressEdited by Matthew A. Jenks, Paul M. Hasegawa
Copyright 2005 by Blackwell Publishing Ltd
modern crop varieties infers that domestication has selected against the cap-acity for salt adaptation. That is, negative linkages exist between loci respon-sible for high yield and those necessary for salt tolerance. Current researchefforts directed at identification of salt tolerance determinants and dissectionof the integration between salt tolerance and plant growth and developmentwill provide substantial resources and insights for plant breeding and biotech-nology strategies to improve crop yield stability. Molecular marker-basedbreeding strategies will identify loci responsible for salt tolerance and, to theextent possible, facilitate the separation of physically linked loci that nega-tively influence the yield (Foolad & Lin, 1997; Flowers et al., 2000; Fooladet al., 2001). Monogenic introgression of salt tolerance determinants directlyinto high-yielding modern crop varieties should simplify breeding efforts toimprove yield stability and facilitate pyramiding of genes, which, in allprobability will be required.
3.1.1 NaCl causes both ionic and osmotic stresses
Salt at higher concentrations in apoplast of cells generates primary andsecondary effects that negatively affect survival, growth and development.Primary effects are ionic toxicity and disequilibrium, and hyperosmolality.Both Na and Cl are inhibitory to cytosolic and organellar processes (Niuet al., 1995; Serrano et al., 1999; Zhu et al., 1998). Concentrations > 0.4 Minhibit most enzymes because of disturbances to hydrophobicelectrostaticbalance that is necessary to maintain the protein structure (Wyn Jones &Pollard, 1983). However, Na concentrations of 0.1 M are cytotoxic indicat-ing that the ion directly affects specific biochemical and physiological pro-cesses (Serrano, 1996). High concentrations of salt also impose hyperosmoticshock by lowering the water potential causing turgor reduction or loss thatrestricts cell expansion (Munns & Termaat, 1986; Hasegawa et al., 2000; Zhu,2002). Sufficiently negative apoplastic water potential can lead to cellularwater loss simulating dehydration that occurs during the episodes of severedrought.
3.1.2 Secondary effects of salt stress
Principal secondary effects of NaCl stress include disturbance of K acquisi-tion, membrane dysfunction, impairment of photosynthesis and other bio-chemical processes, generation of reactive oxygen species (ROS) andprogrammed cell death (Serrano et al., 1999; Hasegawa et al., 2000; Rodri-guez-Navarro, 2000; Zhu, 2003). K is an essential element for plants andNa competes with K for uptake into cells, particularly when the externalconcentration is substantially greater than that of the nutrient (Niu et al., 1995;Rodriguez-Navarro, 2000). Ca2 alleviates Na toxicity through different
38 PLANT ABIOTIC STRESS
mechanisms, including the control of K=Na selective accumulation andothers that are now being elucidated (Lauchli, 1990; Kinrade, 1998, 1999;Liu & Zhu, 1998; Rubio et al., 2004).Ionic and hyperosmotic stresses lead to secondary metabolic effects that
plants must alleviate in order to survive and to reestablish growth and devel-opment (Hasegawa et al., 2000; Zhu, 2002). For example, salt stress inducesoxidative stress by increasing superoxide radicals (O.2 ), hydrogen peroxide(H2O2) and hydroxyl radicals (OH) (Moran et al., 1994; Borsani et al., 2001a,2001b). These molecules, known as ROS, are produced in aerobic cellularprocesses such as electron transport in the mitochondria or the chloroplast, orduring photorespiration (Chinnusamy et al., 2004).
3.2 Plant genetic models for dissection of salt tolerance mechanismsand determinant function
Plants that are native to saline environments (halophytes) or not (glycophytes)use many conserved cellular and organismal processes to tolerate salt (Hase-gawa et al., 2000; Zhu, 2003). Notwithstanding, some plants are salt avoiders,including those that use adaptive morphological structures such as glands orbladders on leaves as salt sinks (Adams et al., 1998; Tester & Davenport,2003). However, even plants that possess such avoidance capabilities utilizeconserved physiological processes once metabolically active cells are exposedto salt stress.For some period, researchers have understood that crops have genetic
variation for salt tolerance (Flowers et al., 1977; Epstein et al., 1980; Yeo &Flowers, 1986; Dvorak & Zhang, 1992; Flowers & Yeo, 1995; Munns, 2002;Flowers, 2004). Considerable emphasis over decades has been to dissect thecritical mechanisms by which halophytes are able to tolerate extreme salinity(Flowers et al., 1977; Flowers & Yeo, 1995; Bohnert et al., 1995; Adams et al.,1998). This research characterized physiological, biochemical and molecularresponses of several halophytes, including Salicornia, Sueada, Mesembryan-themum, etc., and determined many unique salt-adaptive features innate tothese genotypes (Flowers et al., 1977, 1986; Bohnert et al., 1995; Adams et al.,1998; Bressan et al., 2001). Further insight came from the studies of wheat andtomato and their halophytic relatives (Dvorak & Zhang, 1992; Dubcovskyet al., 1994; Dvorak et al., 1994; Foolad et al., 2001, 2003). Research withtobacco cells and plants, and sorghum plants established that glycophytescould adapt to high levels of salinity, provided that stress imposition occursin moderate increments (Amzallag et al., 1990; Hasegawa et al., 1994).Together, this research determined that all plants have some capacity totolerate salt stress and many focal adaptation determinants are conserved inhalophytes and glycophytes.
PLANT ADAPTIVE RESPONSES TO SALINITY STRESS 39
3.2.1 Arabidopsis thaliana as a model for glycophyte responses to salt stress
In the last decade there has been unprecedented progress in the identificationof salt tolerance determinants, including components of signaling pathwaysthat control plant responses to high salinity (Hasegawa et al., 2000; Zhu, 2000,2002, 2003; Bressan et al., 2001; Xiong et al., 2002; Xiong & Zhu, 2003;Shinozaki et al., 2003). This progress is due primarily to the use of Arabi-dopsis as a molecular genetic model system in abiotic stress research, whichhas facilitated the identification of numerous salt adaptation determinants byloss- or gain-of-function experimental approaches (Kasuga et al., 1999;Bressan et al., 2001; Apse & Blumwald, 2002; Zhu, 2002, 2003). Arabidopsiswill continue to be a model for dissection of plant salt stress responses beingparticularly pivotal for defining signal pathways networking and control nodeintegration that coordinate determinant function.Tomato is another glycophyte model that has been valuable for the dissec-
tion plant salt stress tolerance mechanisms because of its molecular genetictractability and substantial genetic variation for adaptive capacity that ispresent in the primary gene pool (Borsani et al., 2001a; Borsani et al., 2002;Rubio et al., 2004). Tomato is a widely distributed annual vegetable crop thatis adapted to diverse climates. In spite of this, production is concentrated in afew warm and relatively dry areas where salinity is a serious problem(Szabolcs, 1994; Cuartero & Fernandez-Munoz, 1999). For this reason, exten-sive physiological studies, particularly under field conditions, have beenperformed using tomato as a model crop plant to evaluate yield stability(Cuartero & Fernandez-Munoz, 1999).
3.2.2 Thellungiella halophila (salt cress) a halophyte moleculargenetic model
Although all plants are able to tolerate some degree of salt stress, thoseindigenous to saline environments have substantially greater stress adaptationcapacity. With the exception of specialized adaptations that are unique to somehalophytes (Flowers et al., 1977, 1986; Bohnert et al., 1995; Adams et al.,1998; Bressan et al., 2001), there is only a rudimentary understanding ofdeterminant function that mediates extreme salt tolerance of these plants. Itis now time to dissect the integral processes of halophytism using a geneticallyfacile plant model (Bressan et al., 2001; Inan et al., 2004; Taji et al., 2004).Focal questions are: Do halophytes have unique salt tolerance determinants,better alleles that encode more effective determinants, or regulatory cascadesthat endow these plants with greater capacity to mount a more effectivedefense in response to salt imposition?Salt cress (T. halophila) is a halophyte relative of Arabidopsis that also
exhibits a high degree of freezing tolerance (Bressan et al., 2001; Inan et al.,
40 PLANT ABIOTIC STRESS
2004; Taji et al., 2004). Salt cress is capable of reproduction after exposure to500 mM NaCl or 158C (Inan et al., 2004). It is a tractable molecular geneticmodel because of its small plant size, prolific seed production, short lifecycle, small genome ( 2X Arabidopsis) and ease of transformation (Bressanet al., 2001). Furthermore, sequences in the genome (derived from cDNAs)average about 90% identity with Arabidopsis, which should facilitate orthol-ogy-based cloning of determinant genes (Inan et al., 2004). Preliminary resultsalso indicate that salt cress, like many halophytes, has substantially betterwater-use efficiency than crop plants (Lovelock & Ball, 2002). Therefore, itmay be possible to use salt cress for the genetic dissection of water-useefficiency.
3.3 Plant adaptations to NaCl stress
Ion and osmotic homeostasis is necessary for plants to be salt tolerant.Intracellular ion homeostasis requires determinants that control toxic ionuptake and facilitate their compartmentalization into the vacuole (Niu et al.,1995; Hasegawa et al., 2000; Zhu, 2003). Since the vacuole is a focal com-partment for cell expansion, ion accumulation in this organelle facilitatesosmotic adjustment that drives growth with minimal deleterious impact oncytosolic and organellar machinery. Controlling ion uptake at the plasmamembrane and vacuolar compartmentalization requires intracellular coordin-ation of transport determinants, and the regulatory molecules and system(s) arebeginning to be identified (Zhu, 2003). Osmotic homeostasis is accomplishedby accumulation of compatible osmolytes in the cytosol for intracellularosmotic homeostasis (Hasegawa et al., 2000). Processes that function tomaintain ionic and osmotic homeostatic balance in a tissue and an organismalcontext are less deciphered. For example, root to shoot coordination thatrestricts ion movement to the aerial part of the plant minimizes salt load tothe shoot meristem and metabolically active cells, but mechanisms involvedare not well understood. Establishment of ionic and osmotic homeostasis aftersalt stress is essential not only to prevent cellular death but is also required forthe physiological and biochemical steady states necessary for growth andcompletion of the life cycle (Bohnert et al., 1995).
3.3.1 Intracellular ion homeostatic processes
Both Na and Cl can be cytotoxic, however, primary focus has been thedissection of intracellular Na homeostasis mechanisms (Blumwald et al.,2000; Hasegawa et al., 2000; Tester & Davenport, 2003; Zhu, 2003). Plantplasma membranes have an inside negative potential of between 120and 200 mV (Niu et al., 1995; Hirsch et al., 1998; Borsani et al., 2001a,
PLANT ADAPTIVE RESPONSES TO SALINITY STRESS 41
2001b; Rubio et al., 2004). Thus, passive entry of Na could concentratethe ion in the cytosol to > 103-fold relative to the apoplast (Niu et al., 1995).Cytosolic Na concentrations > 100 mM disturb the normal functioning ofthe cell (Serrano et al., 1999), but plant cells can adapt to Na concentrations< 100 mM in the cytosol (Flowers et al., 1986; Binzel et al., 1988). Asindicated in Figure 3.1, intracellular homeostasis is achieved by tight coord-inate control of Na entry and compartmentalization of Na and Cl into thevacuole, and there is some insight about how this regulation occurs (Niu et al.,1995; Blumwald et al., 2000; Hasegawa et al., 2000; Tester & Davenport,2003; Zhu, 2003).
126.96.36.199 Na influx and efflux across the plasma membrane
Preventing or reducing net Na entry into cells restricts accumulation of theion in the cytosol, thereby increasing plant salt tolerance. Na uptake acrossthe plasma membrane has been attributed to low Na permeability of systemsthat transport K (Amtmann & Sanders, 1999; Maathuis & Amtmann, 1999;Blumwald et al., 2000). Transport systems that have high affinity for K butalso low affinity for Na inc...