Neveen Talaat

Excessive salt accumulation in soils is a major ecological and agronomical problem, in particular in arid and semiarid areas. While important physiological insights about the mechanisms of salt tolerance in plants have been gained, the transfer of such knowledge into crop improvement has been limited. The identification and exploitation of soil microorganisms (especially rhizosphere bacteria and mycorrhizal fungi) that interact with plants by alleviating stress opens new alternatives for a pyramiding strategy against salinity as well as
new approaches to discover new mechanisms involved in stress tolerance. Considering the kingdom of fungi, arbuscular mycorrhizal fungi (AMF) stand out as the most significant and widespread group of plant growth–promoting microorganisms. Ectomycorrhizal fungi (EMF) are also important symbionts of particular relevance for many woody plants. Considering the kingdom of bacteria, a wide range of microorganisms including different species and strains of Bacillus, Burkholderia, Pseudomonas, and the well-known nitrogen-fixing organisms
Rhizobium, Bradyrhizobium, Azotobacter, Azospirillum,
and Herbaspirillum are classically regarded as
important plant growth–promoting rhizobacteria
(PGPR). Today, it is widely accepted that
AMF, EMF, and PGPR promote plant growth and
increase tolerance against stress conditions, at
least in part, because they facilitate water and
nutrient uptake and distribution as well as alter
plant hormonal status, and this ability has been
attributed to various mechanisms. This chapter
addresses the significance of soil biota in alleviation
of salinity stress and their beneficial effects
on plant growth and productivity. Moreover,
it emphasizes new perspectives and challenges
in physiological and molecular studies on salt
stress alleviation by soil biota.

Abstract Dual application [24-epibrassinolide (EBL) and spermine (Spm)] influence on the antioxidant machinery in water-stressed plants has received no attention. The present study, as a first investigation, was conducted with an aim to investigate the effects of EBL, Spm and their dual application on the \{ROS\} scavenging antioxidant defense machinery in plants subjected to drought conditions. This approach was assessed as possible mechanisms of drought tolerance and how these applications protect plants against oxidative stress. To achieve this goal, two maize hybrids (Giza 10 and Giza 129) were subjected to well-watered conditions and water-stressed conditions (75% and 50% of field capacity) with and without \{EBL\} and/or Spm foliar application. The grains were sown in plastic pots containing clay-loam (sand 37%, silt 28%, clay 35%) soil (Inceptisols; FAO), under greenhouse condition. Water deficiency significantly reduced growth, productivity, and membrane stability index, particularly in hybrid Giza 10. However, the follow-up treatment with the dual application (25 mg l?1 Spm + 0.1 mg l?1 EBL) detoxified the stress generated by drought and significantly improved the above parameters, particularly in hybrid Giza 129. Drought stress significantly increased \{H2O2\} and \{O2\} ? contents and caused oxidative stress to lipids assessed by the increase in \{MDA\} content. However, they were significantly decreased in stressed plants treated with the dual application. Moreover, dual application alleviated the detrimental effects of drought on the electrolyte leakage. Activities of superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase and levels of ascorbate, glutathione, proline, and glycinebetaine were increased in response to drought treatments as well as foliar applications. Dual application significantly alleviated drought-induced inhibition in the activities of monodehydroascorbate reductase and dehydroascorbate reductase as well as in the ratios of AsA/DHA and GSH/GSSG. Overall, dual application improved the plant drought tolerance and decreased the accumulation of \{ROS\} by enhancing their scavenging through elevation of antioxidant enzymes activity and improving the redox state of ascorbate and glutathione.

Abstract No information is available regarding the influence of effective microorganisms (EM) on protein synthesis and polyamine balance in plants grown under saline conditions. Thus, as a first approach, this study sheds light on some different mechanisms that may protect EM-treated plants against salt excess. The response of common bean (Phaseolus vulgaris L.) cv. Nebraska to soil salinization [0.1 dS m?1 (non-saline), 2.5 and 5.0 dS m?1] and/or \{EM\} application was investigated. Plants grown in saline soils exhibited a significant decline in productivity, membrane stability index, nitrate reductase activity, nitrate and protein content, K+ concentration, and K+/Na+ ratio. However, \{EM\} application ameliorated the deleterious effects of salinity and significantly improved the above parameters. Soil salinity induced oxidative damage through increased lipid peroxidation and hydrogen peroxide content. \{EM\} application significantly reduced the oxidative damage. Polyamines responded to salinity stress by increasing its content, particularly putrescine level. The \{EM\} treatment changed the polyamine balance under saline conditions, a high increase in spermidine and spermine levels was observed. Moreover, \{EM\} application significantly reduced the activities of diamine oxidase and polyamine oxidase in salt-stressed plants. Both the modulation of polyamine pool and the regulation of protein synthesis can be one of the most important mechanisms used by EM-treated plants to improve plant adaptation to saline soils.

No information is available regarding effective microorganisms (EM) influence on the enzymatic and non-enzymatic antioxidant defence system involved in the ascorbate-glutathione cycle under saline conditions. Therefore, as a first approach, this article focuses on the contribution of EM to the scavenging capacity of the ascorbate-glutathione cycle in salt-stressed plants. It investigates some mechanisms underlying alleviation of salt toxicity by EM application. Phaseolus vulgaris cv. Nebraska plants were grown under non-saline or saline conditions (2.5 and 5.0 dSm(-1)) with and without EM application. Lipid peroxidation and H2O2 content were significantly increased in response to salinity, while they decreased with EM application in both stressed and non-stressed plants. Activities of ascorbate peroxidase (APX; EC 1.11.1.11) and glutathione reductase (GR; EC 1.6.4.2) increased under saline conditions; these increases were more significant in salt-stressed plants treated by EM. Activities of monodehydroascorbate reductase (MDHAR; EC 1.6.5.4) and dehydroascorbate reductase (DHAR; EC 1.8.5.1) decreased in response to salinity; however, they were significantly increased in stressed plants treated with EM. Ascorbate and glutathione contents were increased with the increasing salt concentration; moreover they further increased in stressed plants treated with EM. Ratios of AsA/DHA and GSH/GSSG decreased under saline conditions, whereas they were significantly increased with EM treatment in the presence or in the absence of soil salinization. The EM treatment detoxified the stress generated by salinity and significantly improved plant growth and productivity. Enhancing the H2O2-scavenging capacity of the ascorbate-glutathione cycle in EM-treated plants may be an efficient mechanism to attenuate the activation of plant defences.

Although there is evidence for a positive involvement of the antioxidant defense system in plant response to salt stress, there is poor information regarding the influence of mycorrhizal symbiosis on enzymatic and nonenzymatic antioxidant defense in wheat under saline conditions. The present article focuses on the contribution of mycorrhizae to antioxidant defense in salt-stressed wheat plants. Two wheat (Triticum aestivum L.) cultivars, Sids 1 and Giza 168, were grown under nonsaline or two saline conditions (4.7 and 9.4 dS m–1) with and without arbuscular mycorrhizal fungi (AMF) inoculation. Salt stress considerably decreased root colonization and plant productivity, particularly in Giza 168. Interestingly, mycorrhizal colonization alleviated the adverse effect of salt stress and significantly enhanced plant productivity, especially in Sids 1. The concentration of glycinebetaine, the activities of antioxidative enzymes (superoxide dismutase, peroxidase, catalase, and glutathione reductase) and the concentrations of antioxidant molecules (glutathione and ascorbate) were increased under saline conditions; these increases were more significant in salt-stressed mycorrhizal plants, especially in Sids 1. Salt stress induced oxidative damage through increased lipid peroxidation, electrolyte leakage, and hydrogen peroxide concentration, particularly in Giza 168. Mycorrhizal colonization altered plant physiology and significantly reduced oxidative damage. Elimination of reactive oxygen species (ROS) can be one of the mechanisms how AMF improve wheat adaptation to saline soils and increase its productivity.

Abstract Little information is available concerning arbuscular mycorrhizal fungi (AMF) influence on carbon and nitrogen metabolisms in wheat under saline conditions. Thus, this study will shed light on some different mechanisms that play a role in the protection of wheat plants colonized by \{AMF\} against hyperosmotic salinity. Two wheat (Triticum aestivum L.) cultivars, Sids 1 and Giza 168, were grown under non-saline or saline conditions (4.7 and 9.4 dS m−1) with and without \{AMF\} inoculation. Root colonization was adversely affected by increasing salinity level, particularly in Giza 168. Soil salinity decreased plant productivity, membrane stability index, photochemical reactions of photosynthesis, the concentrations of N, K+, nitrate, chlorophyll, carbohydrates, and protein, the relative water content, and the activities of nitrate reductase and carbonic anhydrase. The reduction was more pronounced in Giza 168. Mycorrhizal symbiosis protected wheat against the detrimental effect of salinity and significantly improved the above parameters, especially in Sids 1. Under saline conditions, wheat plants colonized by \{AMF\} had higher gas exchange capacity (increased net \{CO2\} assimilation rate and stomatal conductance, and decreased intercellular \{CO2\} concentration), compared with non-mycorrhizal ones. Concentrations of soluble sugars, free amino acids, proline and glycinebetaine increased under saline conditions; these increases were more marked in salt-stressed plants colonized by AMF, especially in Sids 1. Soil salinization induced oxidative damage through increased lipid peroxidation and hydrogen peroxide levels, particularly in Giza 168. Mycorrhizal colonization altered plant physiology and significantly reduced the oxidative damage in plants exposed to salinity. Enhanced metabolism of carbon and nitrogen can be one of the most important mechanisms of plant adaptation to saline soils that are activated by AMF. This is the first report dealing with mycorrhization effect on the activity of carbonic anhydrase under saline conditions.

Two wheat (Triticum aestivum L.) cultivars, Sids 1 and Giza 168, were grown under non-saline or saline conditions (4.7 and 9.4 dS m-1) and were sprayed with 0.00, 0.05 and 0.10 mg l-1 24-epibrassinolide (EBL). Salt stress markedly decreased plant productivity and N, P, and K uptake, particularly in Giza 168. A follow-up treatment with 0.1 mg l-1 EBL detoxified the stress generated by salinity and considerably improved the above parameters, especially in Sids 1. Organic solutes (soluble sugars, free amino acids, proline and glycinebetaine), antioxidative enzymes (superoxide dismutase, peroxidase, catalase and glutathione reductase), antioxidant molecules (glutathione and ascorbate) and Ca and Mg levels were increased under saline condition, and these increases proved to be more significant in salt-stressed plants sprayed with EBL, par- ticularly at 0.1 mg l-1 EBL with Sids 1. Sodium concen- tration, lipid peroxidation, hydrogen peroxide content and electrolyte leakage were increased under salt stress and significantly decreased when 0.1 mg l-1 EBL was sprayed. Clearly, EBL alleviates salt-induced inhibition of pro- ductivity by altering the physiological and biochemical properties of the plant.

Two wheat (Triticum aestivum L.) cultivars, Sids 1 and Giza 168, were grown under non-saline or saline conditions (4.7 and 9.4 dS m-1) with and without arbus- cular mycorrhizal fungi (AMF) inoculation. Salt stress considerably decreased root colonization, plant productiv- ity and N, P, K?, Fe, Zn and Cu concentrations, while it increased Na? level, particularly in Giza 168. Mycorrhizal colonization significantly enhanced plant productivity and N, P, K?, Fe, Zn and Cu acquisition, while it diminished Na? uptake, especially in Sids 1. Salinity increased putrescine level in Giza 168, however, values of spermi- dine and spermine increased in Sids 1 and decreased in Giza 168. Mycorrhization changed the polyamine balance under saline conditions, an increase in putrescine level associated with low contents of spermidine and spermine in Giza 168 was observed, while Sids 1 showed a decrease in putrescine and high increase in spermidine and spermine. Moreover, mycorrhizal inoculation significantly reduced the activities of diamine oxidase and polyamine oxidase in salt-stressed wheat plants. Modulation of nutrient acquisi- tion and polyamine pool can be one of the mechanisms used by AMF to improve wheat adaptation to saline soils. This is the first report dealing with mycorrhization effect
on diamine oxidase and polyamine oxidase activities under salt stress.