Root apoplastic barriers block Na+ transport to shoots in rice (Oryza sativa L.)

Rice is an important crop that is very sensitive to salinity. However, some varieties differ greatly in this feature, making investigations of salinity tolerance mechanisms possible. The cultivar Pokkali is salinity tolerant and is known to have more extensive hydrophobic barriers in its roots than does IR20, a more sensitive cultivar. These barriers located in the root endodermis and exodermis prevent the direct entry of external fluid into the stele. However, it is known that in the case of rice, these barriers are bypassed by most of the Na(+) that enters the shoot. Exposing plants to a moderate stress of 100 mM NaCl resulted in deposition of additional hydrophobic aliphatic suberin in both cultivars. The present study demonstrated that Pokkali roots have a lower permeability to water (measured using a pressure chamber) than those of IR20. Conditioning plants with 100 mM NaCl effectively reduced Na(+) accumulation in the shoot and improved survival of the plants when they were subsequently subjected to a lethal stress of 200 mM NaCl. The Na(+) accumulated during the conditioning period was rapidly released when the plants were returned to the control medium. It has been suggested that the location of the bypass flow is around young lateral roots, the early development of which disrupts the continuity of the endodermal and exodermal Casparian bands. However, in the present study, the observed increase in lateral root densities during stress in both cultivars did not correlate with bypass flow. Overall the data suggest that in rice roots Na(+) bypass flow is reduced by the deposition of apoplastic barriers, leading to improved plant survival under salt stress.     

The involvement of the transpirational bypass flow in sodium uptake by high- and low-sodium-transporting lines of rice developed through intravarietal selection

We report the characterization of high- and low-sodium-transporting lines developed by intravarietal selection within a cultivar, IR36, of rice (Oryza sativa L.). The purpose was to investigate the mechanistic basis of sodium uptake in material in which differences in salt uptake could be isolated from the many other morphological and physiological characteristics that affect the phenotypic expression of salt tolerance. The lines differed in mean sodium transport by a factor of 2. They differed in vigour and water use efficiency, which are characters that modify the effects of salt transport, by only 12% or 13%. The lines did not differ significantly in other physiological traits that are components of salt resistance: compartmentalization at the leaf and cellular levels. There was a strong correlation between the transport of sodium and a tracer for apoplastic pathways (trisodium, 3-hydroxy-5,8,10-pyrene trisulphonic acid, PTS) in both lines. The regression coefficient for sodium transport on PTS transport was the same in both lines. The individual variation in PTS transport was similar to that in sodium transport, and the variation in the transport of both was very much greater than the variation in any other character studied. The high-sodium-transporting line took up proportionately more PTS than the low-sodium-transporting line. It is concluded that the transpirational bypass flow is of major importance in sodium uptake by rice and that selection for differences in sodium transport has been brought about by selection for heritable differences in the bypass flow.     

Screening of rice (Oryza sativa L.) genotypes for physiological characters contributing to salinity resistance,and their relationship to overall performance

Summary Phenotypic resistance of salinity is expressed as the ability to survive and grow in a salinised medium. Some subjective measure of overall performance has normally been used in plant breeding programmes aimed at increasing salinity resistance, not only to evaluate progeny, but to select parents. Salinity resistance has, at least implicitly, been treated as a single trait. Physiological studies of rice suggest that a range of characteristics (such as low shoot sodium concentration, compartmentation of salt in older rather than younger leaves, tolerance to salt within leaves and plant vigour) would increase the ability of the plant to cope with salinity. We describe the screening of a large number of rice genotypes for overall performance (using an objective measure based on survival) and for the aforementioned physiological traits. There was wide variation in all the characters studied, but only vigour was strongly correlated with survival. Shoot sodium concentration, which a priori is expected to be important, accounted for only a small proportion of the variability in the survival of salinity. Tissue tolerance (the cellular component of resistance reflecting the ability to compartmentalise salt within leaves) revealed a fivefold range between genotypes in the tolerance of their leaves to salt, but this was not correlated positively with survival. On the basis of such (lack of) correlation, these traits would be rejected in normal plant breeding practice, but we discuss the fallacies involved in attempting correlation between individual traits and the overall performance of a salt-sensitive species in saline conditions. We conclude that whilst overall performance (survival) can be used to evaluate the salt resistance of a genotype, it is not the basis on which parents should be selected to construct a complex character through breeding. It was the norm for varieties which had one good characteristic affecting salt resistance to be unexceptional or poor in the others. This constitutes experimental evidence that the potential for salt resistance present in the rice genome has not been realised in genotypes currently extant. The results are discussed in relation to the use of physiological traits in plant breeding, with particular reference to environmental stresses that do not affect a significant part of a species' ecological range.     

Genetic behaviour of physiological traits conferring cytosolic K+/Na+ homeostasis in wheat

A plant's ability to maintain an optimal cytosolic K(+)/Na(+) ratio has long been cited as a key feature of salinity tolerance. As traditional whole-leaf nutrient analysis does not account for tissue and organelle-specific ion sequestration, the predictive value of this index at the whole-plant level is not always satisfactory. Consequently, suitable in situ methods for functionally assessing the activity of the key membrane transporters contributing to this trait at the cellular level need to be developed. The aim of this work was to investigate the extent to which plasma membrane transporter-mediated Na(+) exclusion and KOR-mediated K(+) retention traits, measured with the microelectrode ion flux measuring (MIFE) technique, are inheritable in wheat, and whether the MIFE technique has the potential to be used in combination with molecular markers to determine QTLs for these transporter proteins. Experiments involved two bread (Triticum aestivum) and two durum (Triticum turgidum) wheat lines contrasting in their salinity tolerance. Net Na(+), K(+) and H(+) fluxes were measured from 6-day-old roots of parental lines and their F(1) hybrids upon addition and removal of NaCl. These results were complemented by assessment of whole-plant physiological and agronomic characteristics. We show evidence for a strong heritability of plasma membrane transporter-mediated Na(+) exclusion and K(+) retention traits in wheat at the cellular level. This opens the prospect of using the MIFE technique to map the position of these transporters on particular loci of wheat chromosomes. The next obvious step would be to pyramid these traits in one ideotype with superior salinity tolerance.     

Silicon deposition in the root reduces sodium uptake in rice (Oryza sativa L.) seedlings by reducing bypass flow

Sodium chloride reduces the growth of rice seedlings, which accumulate excessive concentrations of sodium and chloride ions in their leaves. In this paper, we describe how silicon decreases transpirational bypass flow and ion concentrations in the xylem sap in rice (Oryza sativa L.) seedlings growing under NaCl stress. Salt (50 mM NaCl) reduced the growth of shoots and roots: adding silicate (3 mM) to the saline culture solution improved the growth of the shoots, but not roots. The improvement of shoot growth in the presence of silicate was correlated with reduced sodium concentration in the shoot. The net transport rate of Na from the root to shoot (expressed per unit of root mass) was also decreased by added silicate. There was, however, no effect of silicate on the net transport of potassium. Furthermore, in salt-stressed plants, silicate did not decrease the transpiration, and even increased it in seedlings pre-treated with silicate for 7 d prior to salt treatment, indicating that the reduction of sodium uptake by silicate was not simply through a reduction in volume flow from root to shoot. Experiments using trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS), an apoplastic tracer, showed that silicate dramatically decreased transpirational bypass flow in rice (from about 4.2 to 0.8%), while the apparent sodium concentration in the xylem, which was estimated indirectly from the flux data, decreased from 6.2 to 2.8 mM. Direct measurements of the concentration of sodium in xylem sap sampled using Philaenus spumarius confirmed that the apparent reduction was not a consequence of sodium recycling. X-ray microanalysis showed that silicon was deposited in the outer part of the root and in the endodermis, being more obvious in the latter than in the former. The results suggest that silicon deposition in the exodermis and endodermis reduced sodium uptake in rice (Oryza sativa L.) seedlings under NaCl stress through a reduction in apoplastic transport across the root.     

Silicon reduces sodium uptake in rice (Oryza sativa L.) in saline conditions and this is accounted for by a reduction in the transpirational bypass flow

Rice is relatively sensitive to salinity and is classified as a silicon accumulator. There have been reports that silicon can reduce sodium uptake in crop grasses in saline conditions, but the mechanism by which silicon might alleviate salinity damage is unclear. We report on the effects of silicon on growth, gas exchange and sodium uptake in rice genotypes differing in salt tolerance. In non-saline media there were no effects of supplementary silicate upon shoot fresh or dry weight or upon root dry weight, indicating that the standard culture solution was not formally deficient with respect to silicon. Plants grown with supplementary silicate had slightly, but significantly, shorter leaves than plants grown in a standard culture solution. Salinity reduced growth and photosynthetic gas exchange. Silicate supplementation partly overcame the reduction in growth and net photosynthesis caused by salt. This amelioration was correlated with a reduction in sodium uptake. Silicate supplementation increased the stomatal conductance of salt-treated plants, showing that silicate was not acting to reduce sodium uptake via a reduction in the transpiration rate. Silicate reduced both sodium transport and the transport of the apoplastic tracer trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS). This implies that the mode of action of silicate was by partial blockage of the transpirational bypass flow, the pathway by which a large proportion of the uptake of sodium in rice occurs. Mechanisms by which silicate might reduce the transpirational bypass flow directly are discussed.     

Cell-specific localization of Na+ in roots of durum wheat and possible control points for salt exclusion

Sodium exclusion from leaves is an important mechanism for salt tolerance in durum wheat. To characterize possible control points for Na(+) exclusion, quantitative cryo-analytical scanning electron microscopy was used to determine cell-specific ion profiles across roots of two durum wheat genotypes with contrasting rates of Na(+) transport from root to shoot grown in 50 mm NaCl. The Na(+) concentration in Line 149 (low transport genotype) declined across the cortex, being highest in the epidermal and sub-epidermal cells (48 mm) and lowest in the inner cortical cells (22 mm). Na(+) was high in the pericycle (85 mm) and low in the xylem parenchyma (34 mm). The Na(+) profile in Tamaroi (high transport genotype) had a similar trend but with a high concentration (130 mm) in the xylem parenchyma. The K(+) profiles were generally inverse to those of Na(+). Chloride was only detected in the epidermis. These data suggest that the epidermal and cortical cells removed most of the Na(+) and Cl(-) from the transpiration stream before it reached the endodermis, and that the endodermis is not the control point for salt uptake by the plant. The pericycle as well as the xylem parenchyma may be important in the control of net Na(+) loading of the xylem.     

Inhibition of H(+)-transporting ATPase, Ca(2+)-transporting ATPase and H+/K(+)-transporting ATPase by strophanthidin.

Studies of the effect of strophanthidin on H(+)-transporting ATPase, Ca(2+)-transporting ATPase and H+/K(+)-transporting ATPase activities are reported. Inhibition observations and kinetic results suggest the existence of a common digitalis aglycone binding site located on the extracellular surface of the enzyme, which is affected competitively by the binding of potassium to H(+)-transporting ATPase, Ca(2+)-transporting ATPase, as well as H+/K(+)-transporting ATPase and Na+/K(+)-transporting ATPase. This may lead to a better understanding of the mechanism of the pharmacological action of cardiac glycosides and imply the possibility that the positive inotropic effect may result from the inhibition of both Ca(2+)-transporting ATPase and Na+/K(+)-transporting ATPase.     

Salt in the wound: a possible role of Na+ gradient in chlamydial infection

Genome analysis has revealed the presence of key components of the Na(+) chemiosmotic cycle, including the primary Na(+) pump (Na(+)-translocating NADH:ubiquinone oxidoreductase), in the cytoplasmic membrane of two ubiquitous human pathogens, Chlamydia trachomatis and Chlamydiophyla pneumoniae. This observation seemed paradoxical in the case of obligatory intracellular parasites because the Na(+) cycle is thought to be primarily a mechanism that enhances the adaptive potential in free-living bacteria that are often facing drastic changes in the salinity and pH of the environment. We present a model suggesting that operation of the Na(+) cycle may play an important role in the course of chlamydial infection, when the Na(+) and H(+) homeostasis of the host cell become severely impaired. This introduces the intriguing possibility of the application of drugs targeting Na(+)-transporting enzymes to chlamydial infections, which are notoriously difficult to treat.     

Accumulation and localisation of sodium ions within the shoots of rice (Oryza sativa) varieties differing in salinity resistance

Oryza sativa L. (rice) is a salt-sensitive crop species which is relatively ineffective in controlling the influx of sodium and chloride ions to the shoot. Nonetheless, there is considerable varietal and individual variability in salinity resistance, much of which must derive, therefore, from differences in the fates and subsequent effects of saline ions after they have entered the plant. The destination of sodium ions within the plant has been investigated, in saline conditions, by examining the time-course of sodium ion concentrations in different leaves of four varieties and breeding lines of rice of differing salinity resistance. Radionuclide tracers were employed to study short term effects and the degree of retranslocation of these sodium ions. Sodium was not distributed uniformly but accumulated in the older leaves before the younger ones. At least some leaves were maintained at sub-lethal salt concentrations in at least the more salt resistant varieties. Radionuclide tracer studies showed that the discontinuous distribution of sodium (from leaf to leaf) is constitutive, and cannot be explained by time of exposure or differential leaf growth rates, and that significant quantities of sodium were not subsequently retranslocated, either within the plant or to the root medium.     

The Na+ transporter AtHKT1;1 controls retrieval of Na+ from the xylem in Arabidopsis

HKT-type transporters appear to play key roles in Na(+) accumulation and salt sensitivity in plants. In Arabidopsis HKT1;1 has been proposed to influx Na(+) into roots, recirculate Na(+) in the phloem and control root : shoot allocation of Na(+). We tested these hypotheses using (22)Na(+) flux measurements and ion accumulation assays in an hkt1;1 mutant and demonstrated that AtHKT1;1 contributes to the control of both root accumulation of Na(+) and retrieval of Na(+) from the xylem, but is not involved in root influx or recirculation in the phloem. Mathematical modelling indicated that the effects of the hkt1;1 mutation on root accumulation and xylem retrieval were independent. Although AtHKT1;1 has been implicated in regulation of K(+) transport and the hkt1;1 mutant showed altered net K(+) accumulation, (86)Rb(+) uptake was unaffected by the hkt1;1 mutation. The hkt1;1 mutation has been shown previously to rescue growth of the sos1 mutant on low K(+); however, HKT1;1 knockout did not alter K(+) or (86)Rb(+) accumulation in sos1.     

盐胁迫下不同水稻种质形态指标与耐盐性的相关分析

选用中国、韩国和国际水稻研究所(IRRI)不同耐盐能力的籼、粳、爪哇稻16份为材料进行芽期和苗期试验。随着NaCl浓度增加,种子开始发芽的时间推迟、发芽过程延长、发芽率降低;品种的发芽率能有效地指示其芽期耐盐能力。NaCl浓度对许多苗期形态指标有显影响,其中叶片盐害指数能反映品种的苗期耐盐能力;随着NaCl处理时间的增长,叶片盐害指数与NaCl浓度间的相关性越来越大。同一品种在芽期的耐盐能力和苗期表现不一致,两的相关系数很低。     

Paxillus involutus Strains MAJ and NAU mediate K(+)/Na(+) homeostasis in ectomycorrhizal Populus x canescens under sodium chloride stress

Salt-induced fluxes of H(+), Na(+), K(+), and Ca(2+) were investigated in ectomycorrhizal (EM) associations formed by Paxillus involutus (strains MAJ and NAU) with the salt-sensitive poplar hybrid Populus × canescens. A scanning ion-selective electrode technique was used to measure flux profiles in non-EM roots and axenically grown EM cultures of the two P. involutus isolates to identify whether the major alterations detected in EM roots were promoted by the fungal partner. EM plants exhibited a more pronounced ability to maintain K(+)/Na(+) homeostasis under salt stress. The influx of Na(+) was reduced after short-term (50 mm NaCl, 24 h) and long-term (50 mm NaCl, 7 d) exposure to salt stress in mycorrhizal roots, especially in NAU associations. Flux data for P. involutus and susceptibility to Na(+)-transport inhibitors indicated that fungal colonization contributed to active Na(+) extrusion and H(+) uptake in the salinized roots of P. × canescens. Moreover, EM plants retained the ability to reduce the salt-induced K(+) efflux, especially under long-term salinity. Our study suggests that P. involutus assists in maintaining K(+) homeostasis by delivering this nutrient to host plants and slowing the loss of K(+) under salt stress. EM P. × canescens plants exhibited an enhanced Ca(2+) uptake ability, whereas short-term and long-term treatments caused a marked Ca(2+) efflux from mycorrhizal roots, especially from NAU-colonized roots. We suggest that the release of additional Ca(2+) mediated K(+)/Na(+) homeostasis in EM plants under salt stress.     

Difference in sodium spatial distribution in the shoot of two canola cultivars under saline stress

Among different mechanisms of salt resistance, regulation of ion distribution among various tissues and intracellular compartmentation are of great importance. In this study, we investigated the effects of salt stress on growth, photosynthesis, and Na(+) accumulation and distribution in leaf apoplast and symplast of two canola (Brassica napus L.) cultivars (NYY 1 and BZY 1). The results showed that the declines in shoot dry mass, leaf water potential and net photosynthetic rate of BZY 1 (salt sensitive) were higher than those of NYY 1 (salt resistant) in response to salt stress. Stomatal limitation to photosynthesis was mainly affected under moderate salinity, whereas the reduction in assimilation rate under severe salt stress was due to both stomatal and non-stomatal limitations. We also found that more Na(+) was distributed to leaf veins in NYY 1 than in BZY 1; simultaneously, less Na(+) accumulated in the leaf blade in NYY 1 than in BZY 1. The percentage of Na(+) in the leaf symplast in NYY 1 was markedly lower than that in BZY 1. Also, Na(+) diffusion in leaves through apoplastic and symplastic pathways of BZY 1 was stronger than that in NYY 1, and the transpiration rate in BZY 1, especially at the leaf edges, decreased more than in NYY 1. Our results showed that NYY 1 accumulated less Na(+) in the shoot, especially in leaf blades, and confined Na(+) to the apoplast to avoid leaf salt toxicity, which could be one reason for the higher resistance of NYY 1 than BZY 1 plants to salt stress.     

Nitric oxide functions as a signal in salt resistance in the calluses from two ecotypes of reed

Calluses from two ecotypes of reed (Phragmites communis Trin.) plant (dune reed [DR] and swamp reed [SR]), which show different sensitivity to salinity, were used to study plant adaptations to salt stress. Under 200 mm NaCl treatment, the sodium (Na) percentage decreased, but the calcium percentage and the potassium (K) to Na ratio increased in the DR callus, whereas an opposite changing pattern was observed in the SR callus. Application of sodium nitroprusside (SNP), as a nitric oxide (NO) donor, revealed that NO affected element ratios in both DR and SR calluses in a concentration-dependent manner. N(omega)-nitro-l-arginine (an NO synthase inhibitor) and 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxyde (a specific NO scavenger) counteracted NO effect by increasing the Na percentage, decreasing the calcium percentage and the K to Na ratio. The increased activity of plasma membrane (PM) H(+)-ATPase caused by NaCl treatment in the DR callus was reversed by treatment with N(omega)-nitro-l-arginine and 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxyde. Western-blot analysis demonstrated that NO stimulated the expression of PM H(+)-ATPase in both DR and SR calluses. These results indicate that NO serves as a signal in inducing salt resistance by increasing the K to Na ratio, which is dependent on the increased PM H(+)-ATPase activity.