Metabolic engineering of osmoprotectants to elucidate the mechanism(s) of salt stress tolerance in crop plants

Advancing crop resilience through nucleic acid innovations: rhizosphere engineering for food security and climate adaptation

Rhizosphere engineering has emerged as a transformative strategy to address the pressing challenges of climate change, food security, and environmental sustainability. By harnessing the dynamic interactions between plants and microbes, and environmental processes, this approach offers innovative solutions for enhancing crop production, protecting against pests and diseases, and remediating contaminated environments. This review explores how rhizosphere engineering, both plant-based and microbe-based, can be leveraged to enhance crop productivity, manage pests and diseases, and remediate contaminated environments under shifting climate conditions. We examine the effects of climate change drivers such as elevated CO2, increased N deposition, rising temperatures, and altered precipitation patterns, on plant-microbe interactions and rhizosphere processes. We show that climate change impacts key functions, including respiration, decomposition and stabilization of soil organic matter, nutrient cycling, greenhouse gas emissions, and microbial community dynamics. Despite these challenges, engineered rhizospheres can mitigate adverse effects of climate change by improving rhizodeposition, nitrogen fixation, root architecture modification, selective microbe recruitment, and pathogen control, while enhancing carbon allocation and stabilization in soil. However, the deployment of these technologies is not without challenges. Ecological risks, such as unintended gene transfer and disruption of native microbial communities, as well as socioeconomic barriers, must be carefully addressed to ensure safe and scalable implementation. We identify critical research gaps such as the limited understanding of multi-taxon cooperation and scalability in engineered rhizosphere systems, and how mechanistic understanding of designer plants and microbes can advance crop production, protection, and environmental remediation in agriculture and agroforestry under global changes.

Proline Tagging for Stress Tolerance in Plants

In environments with high levels of stress conditions, plants accumulate various metabolic products under stress conditions. Among these products, amino acids have a cardinal role in supporting and maintaining plant developmental processes. The increase in proline content and stress tolerance in plants has been found optimistic, suggesting the importance of proline in mitigating stress through osmotic adjustments. Exogenous application and pretreatment of plants with proline increase growth and development under various stressful conditions, but excessive proline has negative influence on growth. Proline has two biosynthetic routes: glutamate or the ornithine pathway, and whether plants synthesize proline by glutamate or ornithine precursors is still debatable as relatively little is known about it. Plants have the innate machinery to synthesize proline from both pathways, but the switch of a particular pathway under which it can be activated and deactivated depends upon various factors. Therefore, in this review, we elucidate the importance of proline in stress mitigation; the optimal amount of proline required for maximum benefit; levels at which it inhibits the growth, conditions, and factors that regulate proline biosynthesis; and lastly, how we can benefit from all these answers to obtain better stress tolerance in plants.

Osmoprotectants play a major role in the Portulaca oleracea resistance to high levels of salinity stress-insights from a metabolomics and proteomics integrated approach

Introduction

Purslane (Portulaca oleracea L.) is a non-conventional food plant used extensively in folk medicine and classified as a multipurpose plant species, serving as a source of features of direct importance to the agricultural and agri-industrial sectors. This species is considered a suitable model to study the mechanisms behind resistance to several abiotic stresses including salinity. The recently achieved technological developments in high-throughput biology opened a new window of opportunity to gain additional insights on purslane resistance to salinity stress-a complex, multigenic, and still not well-understood trait. Only a few reports on single-omics analysis (SOA) of purslane are available, and only one multi-omics integration (MOI) analysis exists so far integrating distinct omics platforms (transcriptomics and metabolomics) to characterize the response of purslane plants to salinity stress.

Methods

The present study is a second step in building a robust database on the morpho-physiological and molecular responses purslane to salinity stress and its subsequent use in attempting to decode the genetics behind its resistance to this abiotic stress. Here, the characterization of the morpho-physiological responses of adult purslane plants to salinity stress and a metabolomics and proteomics integrative approach to study the changes at the molecular level in their leaves and roots is presented.

Results and discussion

Adult plants of the B1 purslane accession lost approximately 50% of the fresh and dry weight (from shoots and roots) whensubmitted to very high salinity stress (2.0 g of NaCl/100 g of the substrate). The resistance to very high levels of salinity stress increases as the purslane plant matures, and most of the absorbed sodium remains in the roots, with only a part (~12%) reaching the shoots. Crystal-like structures, constituted mainly by Na⁺, Cl^-, and K⁺, were found in the leaf veins and intercellular space near the stoma, indicating that this species has a mechanism of salt exclusion operating on the leaves, which has its role in salt tolerance. The MOI approach showed that 41 metabolites were statistically significant on the leaves and 65 metabolites on the roots of adult purslane plants. The combination of the mummichog algorithm and metabolomics database comparison revealed that the glycine, serine, and threonine, amino sugar and nucleotide sugar, and glycolysis/gluconeogenesis pathways were the most significantly enriched pathways when considering the total number of occurrences in the leaves (with 14, 13, and 13, respectively) and roots (all with eight) of adult plants; and that purslane plants employ the adaptive mechanism of osmoprotection to mitigate the negative effect of very high levels of salinity stress; and that this mechanism is prevalent in the leaves. The multi-omics database built by our group underwent a screen for salt-responsive genes, which are now under further characterization for their potential to promote resistance to salinity stress when heterologously overexpressed in salt-sensitive plants.

Biotechnological Advances to Improve Abiotic Stress Tolerance in Crops

The major challenges that agriculture is facing in the twenty-first century are increasing droughts, water scarcity, flooding, poorer soils, and extreme temperatures due to climate change. However, most crops are not tolerant to extreme climatic environments. The aim in the near future, in a world with hunger and an increasing population, is to breed and/or engineer crops to tolerate abiotic stress with a higher yield. Some crop varieties display a certain degree of tolerance, which has been exploited by plant breeders to develop varieties that thrive under stress conditions. Moreover, a long list of genes involved in abiotic stress tolerance have been identified and characterized by molecular techniques and overexpressed individually in plant transformation experiments. Nevertheless, stress tolerance phenotypes are polygenetic traits, which current genomic tools are dissecting to exploit their use by accelerating genetic introgression using molecular markers or site-directed mutagenesis such as CRISPR-Cas9. In this review, we describe plant mechanisms to sense and tolerate adverse climate conditions and examine and discuss classic and new molecular tools to select and improve abiotic stress tolerance in major crops.

Contribution of Exogenous Proline to Abiotic Stresses Tolerance in Plants: A Review

Abiotic stresses are the major environmental factors that play a significant role in decreasing plant yield and production potential by influencing physiological, biochemical, and molecular processes. Abiotic stresses and global population growth have prompted scientists to use beneficial strategies to ensure food security. The use of organic compounds to improve tolerance to abiotic stresses has been considered for many years. For example, the application of potential external osmotic protective compounds such as proline is one of the approaches to counteract the adverse effects of abiotic stresses on plants. Proline level increases in plants in response to environmental stress. Proline accumulation is not just a signal of tension. Rather, according to research discussed in this article, this biomolecule improves plant resistance to abiotic stress by rising photosynthesis, enzymatic and non-enzymatic antioxidant activity, regulating osmolyte concentration, and sodium and potassium homeostasis. In this review, we discuss the biosynthesis, sensing, signaling, and transport of proline and its role in the development of various plant tissues, including seeds, floral components, and vegetative tissues. Further, the impacts of exogenous proline utilization under various non-living stresses such as drought, salinity, high and low temperatures, and heavy metals have been extensively studied. Numerous various studies have shown that exogenous proline can improve plant growth, yield, and stress tolerance under adverse environmental factors.

Ethanol Treatment Enhances Physiological and Biochemical Responses to Mitigate Saline Toxicity in Soybean

Soil salinity, a major environmental concern, significantly reduces plant growth and production all around the world. Finding solutions to reduce the salinity impacts on plants is critical for global food security. In recent years, the priming of plants with organic chemicals has shown to be a viable approach for the alleviation of salinity effects in plants. The current study examined the effects of exogenous ethanol in triggering salinity acclimatization responses in soybean by investigating growth responses, and numerous physiological and biochemical features. Foliar ethanol application to saline water-treated soybean plants resulted in an enhancement of biomass, leaf area, photosynthetic pigment contents, net photosynthetic rate, shoot relative water content, water use efficiency, and K⁺ and Mg²⁺ contents, leading to improved growth performance under salinity. Salt stress significantly enhanced the contents of reactive oxygen species (ROS), malondialdehyde, and electrolyte leakage in the leaves, suggesting salt-induced oxidative stress and membrane damage in soybean plants. In contrast, ethanol treatment of salt-treated soybean plants boosted ROS-detoxification mechanisms by enhancing the activities of antioxidant enzymes, including peroxidase, ascorbate peroxidase, catalase, and glutathione S-transferase. Ethanol application also augmented the levels of proline and total free amino acids in salt-exposed plants, implying a role of ethanol in maintaining osmotic adjustment in response to salt stress. Notably, exogenous ethanol decreased Na⁺ uptake while increasing K⁺ and Mg²⁺ uptake and their partitioning to leaves and roots in salt-stressed plants. Overall, our findings reveal the protective roles of ethanol against salinity in soybean and suggest that the use of this cost-effective and easily accessible ethanol in salinity mitigation could be an effective approach to increase soybean production in salt-affected areas.

Cross-Tolerance and Autoimmunity as Missing Links in Abiotic and Biotic Stress Responses in Plants: A Perspective toward Secondary Metabolic Engineering

Plants employ a diversified array of defense activities when they encounter stress. Continuous activation of defense pathways that were induced by mutation or altered expression of disease resistance genes and mRNA surveillance mechanisms develop abnormal phenotypes. These plants show continuous defense genes' expression, reduced growth, and also manifest tissue damage by apoptosis. These macroscopic abrasions appear even in the absence of the pathogen and can be attributed to a condition known as autoimmunity. The question is whether it is possible to develop an autoimmune mutant that does not fetch yield and growth penalty and provides enhanced protection against various biotic and abiotic stresses via secondary metabolic pathways' engineering. This review is a discussion about the common stress-fighting mechanisms, how the concept of cross-tolerance instigates propitious or protective autoimmunity, and how it can be achieved by engineering secondary metabolic pathways.