Phytohormones and their metabolic engineering for abiotic stress tolerance in crop plants

This perspective paper explores the utilization of abiotic stress-responsive transcription factors (TFs) from crassulacean acid metabolism (CAM) plants to improve abiotic stress tolerance in crop plants. CAM is a specialized type of photosynthetic adaptation that enhances water-use efficiency (WUE) by shifting CO2 uptake to all or part of the nighttime when evaporative water losses are minimal. Recent studies have shown that TF-based genetic engineering could be a useful approach for improving plant abiotic stress tolerance because of the role of TFs as master regulators of clusters of stress-responsive genes. Here, we explore the use of abiotic stress-responsive TFs from CAM plants to improve abiotic stress tolerance and WUE in crops by controlling the expression of gene cohorts that mediate drought-responsive adaptations. Recent research has revealed several TF families including AP2/ERF, MYB, WRKY, NAC, NF-Y, and bZIP that might regulate water-deficit stress responses and CAM in the inducible CAM plant Mesembryanthemum crystallinum under water-deficit stress-induced CAM and in the obligate CAM plant Kalanchoe fedtschenkoi. Overexpression of genes from these families in Arabidopsis thaliana can improve abiotic stress tolerance in A. thaliana in some instances. Therefore, we propose that TF-based genetic engineering with a small number of CAM abiotic stress-responsive TFs will be a promising strategy for improving abiotic stress tolerance and WUE in crop plants in a projected hotter and drier landscape in the 21st-century and beyond.

Grafting improves drought tolerance by regulating antioxidant enzyme activities and stress-responsive gene expression in tobacco

Light constitutes one of the most important environmental factors for plant growth and development. It determines the photosynthetic rate and accumulate-assimilation besides its regulatory roles in plant growth and productivity. However, plants are frequently exposed to excess or inadequate light intensities and these fluctuations, collectively known as light stress, affect the agronomic traits in plants via inhibiting their physiological metabolic processes including photosynthesis, antioxidant machinery, and their abilities to fix atmospheric carbon and nitrogen. Within the photosynthetic machinery, photosystem II (PSII) and its reaction centers are particularly sensitive to these perturbations and have therefore been characterized as primary targets of light stress at physiological, biochemical, and molecular levels including microRNA (miRNA)-mediated post-transcriptional modifications. Through this review, we are presenting herein the current knowledge and recent updates on light stress and its significance for plant growth and crop yields, plant responses, and multilevel adaptation strategies to cope up with light stress including excess and low light. The review highlights and assesses the utilization of biotechnological tools for engineering light stress tolerance in major crops and model plants including omics and transgenic approaches and exploration of molecular markers and quantitative trait loci. The roles of miRNAs in regulation of light stress responses and adaptive mechanisms in plants have been discussed besides emphasizing on possible exploration of light-regulated miRNAs as potential targets for engineering light stress tolerance in crop plants.

Abiotic stresses such as salinity, drought, nutrient defi ciency or toxicity, and fl ooding limit crop productivity world-wide. However, this situation becomes more problematic in developing countries, where they cause food insecurity for large populations and poverty, particularly in rural areas. For example, drought stress has affected more than 70 million hectares of rice-growing land world-wide. While salt stress and nutrient stress render more than 100 million hectares of agricultural land uncultivable thereby resulting in low outputs, poor human nutrition and reduced educational and employment opportunities. Thus, abiotic stresses are the major factors of poverty for millions of people. In this scenario, it is widely urged that strategies should be adopted which may be used to get maximum crop stand and economic returns from stressful environments. Major strategies include breeding of new crop varieties, screening and selection of the existing germ-plasm of potential crops, production of genetically modifi ed (GM) crops, exogenous use of osmoprotec-tants etc. In the last century, conventional selection and breeding program proved to be highly effective in improving crops against abiotic stresses. Therefore, breeding for abiotic stress tolerance in crop plants (for food supply) should be given high research priority. However, extent and rate of progress in improving stress tolerance in crops through conventional breeding program is limited. This is due to complex mechanism of abiotic stress tolerance, which is controlled by the expression of several minor genes. Furthermore, techniques employed for selecting tolerant plants are time consumable and consequently expensive. During the last decade, using advanced molecular biology techniques different researchers showed some promising results in understanding molecular mechanisms of abiotic stress tolerance as well as in inducing stress tolerance in some potential crops. These fi ndings emphasized that future research should focus on molecular, physiological and metabolic aspects of stress tolerance to facilitate the development of crops with an inherent capacity to withstand abiotic stresses. This would help stabilize the crop production, and signifi cantly contribute to food and nutritional security in developing countries and semi-arid tropical regions.

Abiotic stress, including extreme temperature, salinity, drought, and other environmental pollution with excessive heavy metals, is regarded to be a global issue in agricultural systems that results in considerable yield and quality losses for crops (Waadt et al., 2022) . With the continued rise of the world's population, it is crucial for sustainable agriculture and food security to develop advanced breeding strategies that effectively mitigate abiotic stress (Zhang et al., 2022) . Additionally, a multifaceted strategy is required for crops to enhance their ability to adapt to abiotic stress, including hormone modulation, plant enzymatic system activation, and stress gene expression (Gong et al., 2020) . Therefore, understanding how cereal crops react to abiotic stress is crucial (Mittler et al., 2022) . Figure out the important characteristics of abiotic stress and their underlying physiological, biochemical, and molecular bases (e.g., genetic, epigenetic, transcriptomic, and metabolomic) will extend our knowledge in breeding efforts to create abiotic stressresistant crops (Chang et al., 2020) . In this editorial, we set up a Research Topic of Advanced Breeding for Abiotic Stress Tolerance in Crops, which covers up-to-date scientific evidence and the potential for future research to improve our knowledge of the mechanisms that control the development of abiotic stress tolerance in the world's major crop species. The following themes are included in this Research Topic: (a) Advanced crop breeding applications for increasing abiotic stress resistance in crops; (b) Novel plant growth regulators for enhancing abiotic stress tolerance in plants; (c) Metabolomic and molecular strategies to improve abiotic stress-resistance in crops; (d) Genetic mechanisms related to abiotic stress tolerance in plants and their related traits in plants by quantitative trait loci (QTL) mapping, genomewide association (GWAS) investigation, or QTL-sequencing; (e) Epigenetic bases of abiotic stress resistance and their applications in crop breeding. Despite significant advances in understanding the underlying mechanism of abiotic stress, there remain knowledge gaps in these areas, and our Research Topic aims to address these gaps. In the end, we accepted and published 20 articles (16 Original Research, 3 Review, and 1 Perspective) written by 158 researchers from seven different countries, e.g.

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.

Stress-tolerant Wild Plants: a Source of Knowledge and Biotechnological Tools for the Genetic Improvement of Stress Tolerance in Crop Plants

Abiotic stresses such as drought, salinity, high temperature, chilling, and heavy metals have caused alterations in plant growth and development, threatening crop yield and quality, and leading to global food insecurity. In this aspect, plant breeders have developed many genetic engineering approaches to enhance crop productivity, which are not able to meet the demand of food production as the inheritance of abiotic stress tolerance is so complex. To overcome the limitations of genetic engineering techniques, plant breeders are now focusing on recent availability of genome editing because of its simplicity, high efficiency, and precise target modification at genomic loci for developing abiotic stress-tolerant crops. Advancements in genome editing technologies such as zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats (CRISPR/Cas9) have made it possible for molecular biologists to more precisely target any gene of interest. However, ZFNs and TALENs are costly and protracted as they involve intricate steps that require protein engineering. Among these techniques, CRISPR/Cas9 is widely used for reasons of its simplicity, low cost, and ease of genome editing. This chapter focuses on the application of recent genome editing tools in advancing abiotic stress tolerance in different crop plants.

Abiotic stresses adversely affect plant growth and agricultural productivity. According to the current climate prediction models, crop plants will face a greater number of environmental stresses, which are likely to occur simultaneously in the future. So it is very urgent to breed broad-spectrum tolerant crops in order to meet an increasing demand for food productivity due to global population increase. As one of the largest families of transcription factors (TFs) in plants, NAC TFs play vital roles in regulating plant growth and development processes including abiotic stress responses. Lots of studies indicated that many stress-responsive NAC TFs had been used to improve stress tolerance in crop plants by genetic engineering. In this review, the recent progress in NAC TFs was summarized, and the potential utilization of NAC TFs in breeding abiotic stress tolerant transgenic crops was also be discussed. In view of the complexity of field conditions and the specificity in multiple stress responses, we suggest that the NAC TFs commonly induced by multiple stresses should be promising candidates to produce plants with enhanced multiple stress tolerance. Furthermore, the field evaluation of transgenic crops harboring NAC genes, as well as the suitable promoters for minimizing the negative effects caused by over-expressing some NAC genes, should be considered.

Agriculture production was effectively decreased by abiotic and biotic stresses, which affect the plant growth by ion toxicity, hormonal and nutritional imbalance, and physiological and metabolic changes. Plant growth-promoting rhizobacteria (PGPR) are the root-colonizing non-pathogenic bacterium, which helps in plant growth promotion and alleviation of the stress-induced changes to result in the maintenance of agricultural productivity. Plants inoculated with the PGPR provide resistance to various abiotic stresses such as salt, drought, and heavy metal toxicity. Some PGPR strains protect both the biotic and abiotic stresses. In addition, several PGPR contribute to multiple abiotic stress tolerance in plants. PGPR produce phytohormones, siderophores, organic acids, and stress-induced metabolites such as osmotic solutes, prolines, and antioxidant enzymes and up- and downregulates the expression of various stress-responsive genes that provide resistance to the plants under stressful conditions. The use of PGPR is a simple and effective alternative approach to genetic engineering and breeding methods for crop improvement, since breeding and genetic engineering are time-consuming, expensive, and laborious procedures. In this chapter, we described the potential role of PGPR in the abiotic stress tolerance in plants. Moreover, the mechanism of PGPR in drought, salt, and heavy metal stress alleviation was described briefly.

The document is an updated review, starting from the Special Issue “Molecular Breeding for Abiotic Stress Tolerance in Crops” published in the Int. J. Mol. Sci. It reviews molecular breeding strategies to enhance abiotic stress tolerance in crops, addressing challenges like drought, salinity, temperature extremes, and waterlogging, which threaten global food security. Climate change intensifies these stresses, making it critical to develop resilient crop varieties. Plants adapt to stress through mechanisms such as hormonal regulation (e.g., ABA, ethylene), antioxidant defense (e.g., SOD, CAT), osmotic adjustment (e.g., proline accumulation), and gene expression regulation via transcription factors like MYB and WRKY. Advanced tools, such as CRISPR/Cas9 genome editing, enable precise modifications of stress-related genes, improving tolerance without compromising yield. Examples include rice (OsRR22, OsDST) and wheat (TaERF3, TaHKT1;5). Epigenetic regulation, including DNA methylation and histone modifications, also plays a role in stress adaptation. Specific studies focused on polyamine seed priming for improved germination and stress resistance, cadmium detoxification mechanisms, and genome-wide association studies (GWAS) to identify genetic markers for salt tolerance and yield. Research on salinity tolerance in wheat emphasizes sodium exclusion and tissue tolerance mechanisms. Future perspectives focus on genetic engineering, molecular markers, epigenetic studies, and functional validation to address environmental stress challenges, including the use of AI and machine learning to manage the large amount of data. The review underscores the importance of translating molecular findings into practical applications to ensure sustainable crop production under changing climates.

Exacerbation of plant growth and productivity due to a wide range of stresses has significantly affected global food security, agricultural productivity, and quality worldwide. In order to bridge the gap between the supply and demand of the ever-increasing global population, it is indispensable to foster a new breed of stress-tolerant crops with refined traits and higher yields against several abiotic and biotic stresses. The transgenic approach of conventional breeding, owing to the limited and time-consuming success due to the complex nature of genes involved in stress tolerance, is now being widely adopted to breed crop plants with enhanced stress tolerance. Thus, identification and characterization of critical genes involved in abiotic and biotic stress tolerance are an important requisite to develop stress-tolerant crops. Genetic engineering of crop plants employs two strategies (i) either manipulating single functional gene or (ii) by editing those regulatory genes which modulate the expression of other stress-responsive genes. Genome editing using artificial nucleases such as zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENS), and Clustered Regulatory Interspaced Short Palindromic Repeat (CRISPR), CRISPR-associated protein 9 (Cas9), has significantly impacted basic as well as applied research including plant breeding by accelerating the editing of target genome in precise and predictable manner. Here, in this chapter, we are not going to discuss the past transgenic development approaches; mostly we will review some of the recent advancement made in the field of transgenic plants and the potential exploitation of genome-editing tools such as in conferring environmental stress tolerance in crops under field condition.

Use of multiple ‘omics techniques to accelerate the breeding of abiotic stress tolerant crops

Plants are constantly exposed to environmental challenges, namely the biotic and abiotic stresses which adversely affect plant growth and agriculture productivity worldwide. To adapt to ever changing climatic conditions, plants have evolved complex molecular and physiological mechanisms to counteract such adversities. Several plant signaling pathways operate which include production of phytohormones, transcription factors, and reactive oxygen species, among others. Plant–microbe associations define a significant mutualistic interaction which enhances plant growth and development, increases nutritional uptake as well as confers abiotic stress tolerance to agricultural crops. Recent researches have extensively studied the production of phytohormones by fungal communities and genetic engineering of these phytohormones for enhancing abiotic stress tolerance in plants. The chapter provides a comprehensive analysis of fungal species producing phytohormones and their signaling mechanisms. Moreover, the chapter further discusses the major contributions of these plant–microbe associations in plant growth promotion and in conferring abiotic stress tolerance in plants. Highlighting the significance of genetic engineering of crops for improvement and production of better plant varieties as well as the associated challenges with crop production are discussed.

Plants are constantly challenged by various abiotic stresses that negatively affect growth and productivity worldwide. During the course of their evolution, plants have developed sophisticated mechanisms to recognize external signals allowing them to respond appropriately to environmental conditions, although the degree of adjustability or tolerance to specific stresses differs from species to species. Overproduction of reactive oxygen species (ROS; hydrogen peroxide, H2O2; superoxide, ; hydroxyl radical, OH⋅ and singlet oxygen, 1O2) is enhanced under abiotic and/or biotic stresses, which can cause oxidative damage to plant macromolecules and cell structures, leading to inhibition of plant growth and development, or to death. Among the various ROS, freely diffusible and relatively long-lived H2O2 acts as a central player in stress signal transduction pathways. These pathways can then activate multiple acclamatory responses that reinforce resistance to various abiotic and biotic stressors. To utilize H2O2 as a signaling molecule, non-toxic levels must be maintained in a delicate balancing act between H2O2 production and scavenging. Several recent studies have demonstrated that the H2O2-priming can enhance abiotic stress tolerance by modulating ROS detoxification and by regulating multiple stress-responsive pathways and gene expression. Despite the importance of the H2O2-priming, little is known about how this process improves the tolerance of plants to stress. Understanding the mechanisms of H2O2-priming-induced abiotic stress tolerance will be valuable for identifying biotechnological strategies to improve abiotic stress tolerance in crop plants. This review is an overview of our current knowledge of the possible mechanisms associated with H2O2-induced abiotic oxidative stress tolerance in plants, with special reference to antioxidant metabolism.