Neveen Talaat

Soil salinization is a constant threat to crop productivity and ecology worldwide. The conventional approach, breeding salt-tolerant plant cultivars, has often failed to efficiently alleviate this devastating environmental stress factor. In contrast, the use of a diverse array of microorganisms harbored by plants has attracted increasing attention because of the remarkable beneficial effects of them on plants. Among these microorganisms, halophilic and halotolerant rhizomicrobes is one of the most important extremophilic microorganisms. They can be found in saline or hypersaline ecosystems and have developed different adaptations to survive in salty environments. Their proteins and encoding genes are magnificently engineered to function in a milieu containing 2–5 M salt and represent a valuable repository and resource for reconstruction and visualizing processes of habitat selection and adaptive evolution. Indeed, the natural occurrence of these microorganisms in saline soils opens up a possible important role of them in increasing the salt tolerance in crops. They are capable of eliciting physical, chemical, and molecular changes in plants which enhanced their tolerance and promoted their growth, and thus they can refine agricultural practices and production under saline conditions. Likewise, their ability to serve as bioinoculants could be a more ready utilizable and sustainable solution to ameliorate the deleterious salt effects on plants. However, the ecology of their interactions with plants is still under investigation and not fully understood. This chapter aims to introduce the halotolerant rhizomicrobes and shed light on their special mechanisms to adapt to salinity conditions. A special section would be dedicated for their potential to be exploited in engineering salt tolerance in crops.

This review highlights the recent advances concerning the role of polyamines and brassinosteroids in stress tolerance of plants with a special accent on drought. Alterations of the endogenous polyamine and brassinosteroid levels and their function in alleviation of drought stress are discussed. Possibilities for application of exogenous polyamines and brassinosteroids to lessen the stress injuries and to increase drought tolerance are also summarized. Genetic and molecular approaches for improving plant tolerance to drought via modification of polyamine levels and involvement of brassinosteroids in signal transduction pathways under stress are presented. Interaction of polyamines and brassinosteroids with phytohormones and osmolytes under drought stress is reviewed. We describe also the recent investigations in Bulgaria and Egypt concerning the modulation of plant reactions to drought stress by application of polyamines and brassinosteroids.

No information is available concerning the influence of dual application of 24-epibrassinolide (EBL) and spermine (Spm) on the nitrogen metabolism in plants subjected to drought conditions. As a first report, this investigation assesses the role of EBL, Spm, and their dual application on polyamine and protein pools in water-stressed plants. It explores the ameliorative effects of these foliar applications under water deficiency. Two maize hybrids (Giza 10 and Giza 129) were treated with or without EBL and/or Spm foliar applications under well-irrigated and drought-stressed conditions (75 and 50 {%} of field capacity). Dual application (25 mg l−1 Spm + 0.1 mg l−1 EBL) significantly relieved the drought-induced inhibition on the activities of ribulose-1,5-bisphosphate carboxylase and nitrate reductase and the contents of relative water, nitrate, and protein, particularly in hybrid Giza 129. Changes in the content of free polyamines and in the activity of polyamine biosynthetic and catabolic enzymes were detected when water-stressed plants were treated with EBL and/or Spm. Putrescine content and arginine decarboxylase activity were significantly increased in stressed hybrid Giza 10 plants treated by the dual application. However, spermidine and Spm levels as well as ornithine decarboxylase and S-adenosylmethionine decarboxylase activities were significantly increased in stressed hybrid Giza 129 plants treated with the dual application. Diamine oxidase, polyamine oxidase, protease activity, carbonyl content, and ethylene formation were increased in response to water stress and significantly decreased when stressed plants were treated by the dual application. Total free amino acids, phenols, and flavonoids concentration were increased with the increasing water stress level; moreover, they further increased in stressed plants treated with the dual application. Overall, the combined utilization of EBL and Spm serves as complementary tools to confer plant drought tolerance by altering polyamine, ethylene, and protein levels.

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.