Raising crops for dry and saline lands: Challenges and the way forward.

Plant hormones play major roles in abiotic stress. This study shows that gibberellin acid-inefficient transgenic rice (Oryza sativa L.) is more tolerant to drought and salinity stress. We also investigated the underlying molecular mechanism by which plants respond to salinity and drought stress. We previously showed how the HD-Zip family I gene Oshox4 (Homeobox 4) regulates GA deactivation by controlling DELLA subfamily and rice GA 2-oxidase and GA 3-oxidase genes in rice. Here we investigated the role of rice Oshox4 gene for drought and salinity stress tolerance. Phenotypic analysis indicated that Oshox4 transgenic lines are more tolerant to drought and salinity stress relative to non-transgenic IR64 plants. To explain the strong osmotic stress tolerance of Oshox4 transgenic lines, we studied the expression of rice GA 2-oxidase, rice YABBY, rice KRP, and rice GRAS gene families. qRT-PCR results showed that OsGA2ox4, OsGA2ox6, OsGA2ox8, OsGAI and OsSCRL were highly expressed under severe drought in Oshox4 transgenic plants. Oshox4 enhanced the expression of OsGA2ox5, OsKRP2 and OsKRP5 under 50 mM salt treatment and OsSCRL under 200 mM salt treatment. Our findings have confirmed that Oshox4 transgenic rice plants are more tolerant to drought and salinity stress and that Oshox4 plays a role in rice osmatic tolerance. This study provides novel insights into methods of maintaining higher yield amidst a limited supply of water by improving physiological conditions.

The critical roles of cis-regulatory elements (cREs) in the regulation of gene expression in response to environmental stress were reported in previous studies. Although transcription factor families to regulate gene expression in plants are well documented, there is a limited number of cREs related to salinity and drought tolerance in rice to be identified. Therefore, in this study, a comparative analysis and characterization of cREs associated with specific drought and salinity tolerance genes of rice, namely OsNHX1, OsNHX5, OsHKT1;1, OsHKT2;1, and OsSOS1, was performed using the PLACE and PlantPAN 3.0 databases, along with in silico methods. Several cis-elements within the core promoter region, including TATA-box, CAAT-box, G-box, DPE, and Y-Patch were identified. Additionally, eight other cis elements: ABRE, MYBRS, MYCRS, NAC-binding site, ACGTATERD1, GT1GMSCAM4, W-box, and DRE, were discovered and suggested to be potentially involved in drought and salinity tolerance in rice. Comparative analysis revealed that OsNHX1 and OsHKT1;1 exhibit a higher abundance of cREs compared to the other genes studied. The presence of an increased number of cREs suggests a more complex regulatory network, potentially enhancing the ability of these genes to cope with environmental stressors and fine-tune their responses to changing conditions. Furthermore, understanding the distribution and diversity of cREs across different genes can offer practical implications for genetic engineering and crop improvement strategies. Genes with desirable regulatory profiles, especially those associated with specific stress tolerances, may be prime candidates for genetic manipulation.

Soil salinity stress is one of the major bottlenecks for crop production. Although, allantoin is known to be involved in nitrogen metabolism in plants, yet several reports in recent time indicate its involvement in various abiotic stress responses including salinity stress. However, the detail mechanism of allantoin involvement in salinity stress tolerance in plants is not studied well. Moreover, we demonstrated the role of exogenous application of allantoin as well as increased concentration of endogenous allantoin in rendering salinity tolerance in rice and Arabidopsis respectively, via., induction of abscisic acid (ABA) and brassinosteroid (BR) biosynthesis pathways. Exogenous application of allantoin (10µM) provides salt-tolerance to salt-sensitive rice genotype (IR-29). Transcriptomic data after exogenous supplementation of allantoin under salinity stress showed induction of ABA (OsNCED1) and BR (Oscytochrome P450) biosynthesis genes in IR-29. Further, the key gene of allantoin biosynthesis pathway i.e., urate oxidase of the halophytic species Oryza coarctata was also found to induce ABA and BR biosynthesis genes when over-expressed in transgenic Arabidopsis. Thus, indicating that ABA and BR biosynthesis pathways were involved in allantoin mediated salinity tolerance in both rice and Arabidopsis. Additionally, it has been found that several physio-chemical parameters such as biomass, Na+/K+ ratio, MDA, soluble sugar, proline, allantoin and chlorophyll contents were also associated with the allantoin-mediated salinity tolerance in urate oxidase overexpressed lines of Arabidopsis. These findings depicted the functional conservation of allantoin for salinity tolerance in both plant clades.

Abiotic stresses collectively are responsible for crop losses worldwide. Among various abiotic stresses, drought and salinity are the most destructive. Different strategies have been adopted for the management of these stresses. Being complex traits, conventional breeding approaches have shown less success in improving salinity and drought stress tolerance. Roles of compatible solutes in salinity and drought stress tolerance have been studied extensively. At physiological level, osmotic adjustment is an adaptive mechanism involved in drought and/or salinity tolerance and permits the maintenance of turgor pressure under stress conditions. Increasing evidences from series of in vivo and in vitro studies involving physiological, biochemical, genetic, and molecular approaches strongly suggest that osmolytes such as ammonium compounds (polyamines, glycinebetaine, b-alanine betaine, dimethyl-sulfonio propionate and choline-O-sulfate), sugars and sugar alcohols (fructan, trehalose, mannitol, d-ononitol and sorbitol) and amino acids (proline and ectoine) perform important function in adjustment of plants against salinity and drought stresses. Thus, aim of this review is to expose how to osmoprotectants detoxify adverse effect of reactive oxygen species and alleviate drought and salinity stresses. An understanding of the relationship between these two sets of parameters is needed to develop measures for mitigating the damaging impacts of salinity and drought stresses.

Identification of concurrent genomic regions contributing tolerance to salinity at the seedling and reproductive stages were done using 45 quantitative trait loci (QTL) mapping studies reporting 915 individual QTLs. The QTL-data were used to perform a meta-analysis to predict, validate and analyze the Meta-QTLs governing component traits contributing to salinity tolerance. We predicted a total of 65 and 49 Meta-QTLs distributed across the genome governing seedling and reproductive stage salinity tolerance, respectively. Salinity stress (EC ~10.0dSm-1 ) was evaluated in a set of 32 genotypes grown hydroponically, from these eight extreme (highly tolerant and highly susceptible) genotypes were selected for validation of significant Meta-QTLs. Another set of eight previously known and reported (highly tolerant and highly susceptible) genotypes were evaluated under saline micro plot conditions (EC ~8.0dSm-1 ) and used for validation of significant Meta-QTLs for reproductive stage salinity tolerance. The microsatellite marker "RM5635" linked to MSQTL4.2 (~295.43 kb) was able to clearly differentiate contrasting genotypes for seedling stage salinity tolerance, whereas at the reproductive stage, none of the markers were able to validate the predicted Meta-QTL for salinity tolerance. Earlier reported, gene expression studies were used for candidate gene analysis of validated MSQTL4.2, which indicated the down regulation of Os04g0423100, a gene encoding Mono-oxygenase-FAD binding domain containing protein. The traits associated with this Meta-QTL were root and shoot sodium and potassium concentration and leaf chlorophyll content. The identified and validated genomic region assumes a great significant role in seedling stage salinity tolerance in rice, and it can be used for marker-assisted backcross breeding programs.

Drought and salinity stresses badly influence rice plant growth and yield. The rising efforts to advance rice genotypes for drought and salt tolerance necessitate the selection of tolerant genotypes. Here, a total 51 rice genotypes were tested for salinity and moisture stress tolerance at germination and early seedling growth in NaCl (200 mM) and PEG-6000 (20%). Physiological and biochemical parameters viz., seed morphology, seed germination, length, fresh weight, dry weight and proline accumulation were evaluated with respect to salt and drought stress, depicting that rice genotypes performed differentially in their response to these stresses. Allelic variations with reference to the control varieties were found and these alleles can be used as a source of genomic approach and mining of elite alleles. The present study provides the information about the significant variation in salinity and drought tolerance subsisting within tested rice genotypes, which intended to make available with great promise for improving salt and drought tolerance in rice.

Plants experience diverse abiotic stresses, encompassing low or high temperature, drought, water logging and salinity. The challenge of maintaining worldwide crop cultivation and food sustenance becomes particularly serious due to drought and salinity stress. Sustainable agriculture has significant promise with the use of nano-biotechnology. Nanoparticles (NPs) have evolved into remarkable assets to improve agricultural productivity under the robust climate alteration and increasing drought and salinity stress severity. Drought and salinity stress adversely impact plant development, and physiological and metabolic pathways, leading to disturbances in cell membranes, antioxidant activities, photosynthetic system, and nutrient uptake. NPs protect the membrane and photosynthetic apparatus, enhance photosynthetic efficiency, optimize hormone and phenolic levels, boost nutrient intake and antioxidant activities, and regulate gene expression, thereby strengthening plant's resilience to drought and salinity stress. In this paper, we explored the classification of NPs and their biological effects, nanoparticle absorption, plant toxicity, the relationship between NPs and genetic engineering, their molecular pathways, impact of NPs in salinity and drought stress tolerance because the effects of NPs vary with size, shape, structure, and concentration. We emphasized several areas of research that need to be addressed in future investigations. This comprehensive review will be a valuable resource for upcoming researchers who wish to embrace nanotechnology as an environmentally friendly approach for enhancing drought and salinity tolerance.

Abiotic stresses such as salinity and drought greatly impact the growth and production of crops worldwide. Here, a shikimate kinase-like 2 (SKL2) gene was cloned from rice and characterized for its regulatory function in salinity and drought tolerance. OsSKL2 was localized in the chloroplast, and its transcripts were significantly induced by drought and salinity stress as well as H2O2 and abscisic acid (ABA) treatment. Meanwhile, overexpression of OsSKL2 in rice increased tolerance to salinity, drought and oxidative stress by increasing antioxidant enzyme activity, and reducing levels of H2O2, malondialdehyde, and relative electrolyte leakage. In contrast, RNAi-induced suppression of OsSKL2 increased sensitivity to stress treatment. Interestingly, overexpression of OsSKL2 also increased sensitivity to exogenous ABA, with an increase in reactive oxygen species (ROS) accumulation. Moreover, OsSKL2 was found to physically interact with OsASR1, a well-known chaperone-like protein, which also exhibited positive roles in salt and drought tolerance. A reduction in ROS production was also observed in leaves of Nicotiana benthamiana showing transient co-expression of OsSKL2 with OsASR1. Taken together, these findings suggest that OsSKL2 together with OsASR1 act as important regulatory factors that confer salt and drought tolerance in rice via ROS scavenging.

MYB-type transcription factors (TFs) play essential roles in plant growth, development and respond to environmental stresses. Role of MYB-related TFs of rice in drought stress tolerance is not well documented. Here, we report the isolation and characterization of a novel MYB-related TF, OsMYB48-1, of rice. Expression of OsMYB48-1 was strongly induced by polyethylene glycol (PEG), abscisic acid (ABA), H2O2, and dehydration, while being slightly induced by high salinity and cold treatment. The OsMYB48-1 protein was localized in the nucleus with transactivation activity at the C terminus. Overexpression of OsMYB48-1 in rice significantly improved tolerance to simulated drought and salinity stresses caused by mannitol, PEG, and NaCl, respectively, and drought stress was caused by drying the soil. In contrast to wild type plants, the overexpression lines exhibited reduced rate of water loss, lower malondialdehyde (MDA) content and higher proline content under stress conditions. Moreover, overexpression plants were hypersensitive to ABA at both germination and post-germination stages and accumulated more endogenous ABA under drought stress conditions. Further studies demonstrated that overexpression of OsMYB48-1 could regulate the expression of some ABA biosynthesis genes (OsNCED4, OsNCED5), early signaling genes (OsPP2C68, OSRK1) and late responsive genes (RAB21, OsLEA3, RAB16C and RAB16D) under drought stress conditions. Collectively, these results suggested that OsMYB48-1 functions as a novel MYB-related TF which plays a positive role in drought and salinity tolerance by regulating stress-induced ABA synthesis.

Abiotic stresses are emerging as a potential threat to sustainable agriculture worldwide. Soil salinity and drought will be the major limiting factors for rice productivity in years to come. The Salt Overly Sensitive (SOS) pathway plays a key role in salinity tolerance by maintaining the cellular ion homeostasis, with SOS2, a S/T kinase, being a vital component. The present study investigated the role of the OsSOS2, a SOS2 homolog from rice, in improving salinity and drought tolerance. Transgenic plants with either overexpression (OE) or knockdown (KD) of OsSOS2 were raised in one of the high-yielding cultivars of rice-IR64. Using a combined approach based on physiological, biochemical, anatomical, microscopic, molecular, and agronomic assessment, the evidence presented in this study advocates the role of OsSOS2 in improving salinity and drought tolerance in rice. The OE plants were found to have favorable ion and redox homeostasis when grown in the presence of salinity, while the KD plants showed the reverse pattern. Several key stress-responsive genes were found to work in an orchestrated manner to contribute to this phenotype. Notably, the OE plants showed tolerance to stress at both the seedling and the reproductive stages, addressing the two most sensitive stages of the plant. Keeping in mind the importance of developing crops plants with tolerance to multiple stresses, the present study established the potential of OsSOS2 for biotechnological applications to improve salinity and drought stress tolerance in diverse cultivars of rice.

Water scarcity and soil salinization are increasingly becoming limiting factors in food production, including olives, a major fruit crop in several parts of the world. Investigating historical olives, which are the last resort for genetic resources, is essential due to their natural resilience to drought and salinity, making them valuable for breeding stress-tolerant cultivars and ensuring sustainable olive production. In this study, four historic olive cultivars ('Nabali', 'Mehras', 'Frantoio', and 'Manzanillo') were investigated under both drought and salinity stresses. These cultivars also preserve local biodiversity, support traditional agriculture, and offer economic opportunities through unique, heritage-based olive oils. Drought and salt stress in olives are assessed through physiological [the ratio of variable to maximum fluorescence (Fv/Fm), relative water content (RWC)], biochemical (proline content), and molecular (stress-responsive genes) analyses to evaluate stress tolerance. Under salinity and drought stress, RWC decreased in all olive cultivars, with drought having the most severe impact. 'Nabali' exhibited the highest salinity tolerance, while all cultivars showed similar sensitivity to drought. Proline levels remained stable in 'Mehras' but decreased under salinity stress in 'Frantoio', 'Manzanillo', and 'Nabali'. Higher proline accumulation under drought suggested better drought tolerance than salinity in these cultivars. Photosynthetic efficiency (Fv/Fm) declined under salinity and drought stress in all cultivars, with drought causing a more significant reduction. 'Manzanillo' showed the highest sensitivity to drought, while the other cultivars maintained moderate efficiency under stress. 'Manzanillo' and 'Mehras' exhibited the highest number of differentially expressed genes (DEGs) under both drought and salinity stress, with 'Manzanillo' showing 2,934 DEGs under drought and 664 under salinity stress, while 'Mehras' had 2,034 and 2,866 DEGs, respectively. 'Nabali' demonstrated the strongest salinity-specific response, with 3,803 DEGs under salinity stress compared to 1,346 under drought. 'Frantoio' consistently had the lowest number of DEGs, with 345 under drought and 512 under salinity stress, indicating a more stable transcriptional response. Comparative analyses between drought and salinity conditions revealed significant variations, with 'Manzanillo' showing 2,599 unique DEGs under drought relative to salinity stress, while 'Nabali' exhibited 2,666 DEGs under salinity stress relative to drought. The major novel upregulated genes under salinity stress were Xyloglucan endotransglucosylase hydrolase (7 fold in 'Nabali' and 6.9 fold in 'Mehras'). The novel drought genes detected in 'Frantoio' included Phytosulfokines 3 (4.9 fold), while Allene oxide synthase (6.5 fold) and U-box domain-containing (6.4 fold) were detected in 'Manzanillo'. The data revealed both novel and common stress-specific biomarkers under both salinity and drought stress, which can potentially be utilized in olive breeding and genetic improvement programs to mitigate stress.

A causal relationship between salinity and oxidative stress tolerance and a suitability of using root antioxidant activity as a biochemical marker for salinity tolerance in barley was investigated. Net ion fluxes were measured from the mature zone of excised roots of two barley varieties contrasting in their salinity tolerance using non-invasive MIFE technique in response to acute and prolonged salinity treatment. These changes were correlated with activity of major antioxidant enzymes; ascorbate peroxidase, catalase, and superoxide dismutase. It was found that genotypic difference in salinity tolerance was largely independent of root integrity, and observed not only for short-term but also long-term NaCl exposures. Higher K+ retention ability (and, hence, salinity tolerance) positively correlated with oxidative stress tolerance. At the same time, antioxidant activities were constitutively higher in a sensitive but not tolerant variety, and no correlation was found between SOD activity and salinity tolerance index during large-scale screening. Although salinity tolerance in barley correlates with its oxidative stress tolerance, higher antioxidant activity at one particular time does not correlate with salinity tolerance and, as such, cannot be used as a biochemical marker in barley screening programs.

A novel SAPK10-WRKY87-ABF1 biological pathway synergistically enhance abiotic stress tolerance in transgenic rice (Oryza sativa)

Salinity stress is one of the most problematic constraints to significantly reduce rice productivity. The Saltol QTL (quantitative trait locus) has been known as one among many principal genes/QTLs responsible for salinity tolerance in rice. However, the introgression of the Saltol QTL from the donor (male) into the recipient (female) cultivars induces great recessions from the progeny generation, which results in heavy fieldwork and greater cost and time required for breeding. In this study, the F1 generation of the cross TBR1 (female cultivar, salinity tolerant) × KD18 (male cultivar, salinity susceptible) was preliminarily treated with N-methyl-N-nitrosourea (MNU) to induce the mutants M1. Results on physiological traits show that all the M2 (self-pollinated from M1) and M3 (self-pollinated from M2) individuals obtain salinity tolerant levels as the recurrent TBR1. Twelve SSR (simple sequence repeat) markers involved in the Saltol QTL (RM493, RM562, RM10694, RM10720, RM10793, RM10852, RM13197, RM201, RM149, RM508, RM587, and RM589) and other markers related to yield-contributing traits and disease resistance, as well as water and nitrogen use, have efficacy that is polymorphic. The phenotype and genotype analyses indicate that the salinity tolerant Saltol QTL, growth parameter, grain yield and quality, pest resistance, water and nitrogen use efficacy, and beneficial phytochemicals including antioxidants, momilactone A (MA) and momilactone B (MB) are uniparentally inherited from the recurrent (female) TBR1 cultivar and stabilized in the M2 and M3 generations. Further MNU applications should be examined to induce the uniparental inheritance of other salinity tolerant genes such as OsCPK17, OsRMC, OsNHX1, OsHKT1;5 to target rice cultivars. However, the mechanism of inducing this novel uniparental inheritance for salinity tolerance by MNU application needs elaboration.

Context: Desertification of arable lands due to global warming and water shortage mandates use of low-quality water for irrigation. Using low-quality water imposes more stress on plants which are already under stress. Thus, there is an urgent need for finding stress tolerant plant species to survive/sustain under such stressful conditions. Since the native plants are already growing under such conditions and are adapted to these stresses, they are the most suitable candidates to be manipulated under the minimum cultural practices and minimum inputs for use under stress. If stress tolerant species/genotypes of the native plants are identified, there would be a substantial savings in cultural practices and inputs in using them. Aim: This grass has multi usages, including animal feed, soil conservation, saline soils reclamation, use in desert landscaping, and combating desertification. The objectives of this study were to find the most salinity and drought tolerant of various saltgrass genotypes for use in arid regions, where limited water supplies coupled with saline soils result in drought and salinity stresses. Materials and Methods: Various genotypes of saltgrass were studied in a greenhouse either hydroponically in culture solution for salt tolerance or in large galvanized cans contained fritted clay for drought tolerance. For the salinity stress tolerance, twelve inland saltgrass clones were studied in a greenhouse, using hydroponics technique to evaluate their growth responses under salt stress. Four salt treatments (EC 6, 20, 34, and 48 dS/m salinity stress) were replicated 3 times in a randomized complete block design experiment. Grasses were grown under these conditions for 10 weeks. During this period, shoots were clipped bi-weekly, clippings were oven dried at 75°C and dry matter (DM) weights were recorded, shoot and root lengths were also measured. At the last harvest, roots were also harvested, oven dried, and DM weights were determined. Grass quality was weekly evaluated and recorded. Although all the grasses showed a high level of salinity tolerance, there was a wide range of variations observed in salt tolerance of these saltgrass clones. For the drought tolerance study, 21 saltgrass clones were studied to evaluate their growth responses under drought stress. Plants were grown under normal condition for 6 months for complete establishment. Then, they were deprived from water for 4 months. Plant shoots were harvested weekly and oven dried at 75°C for DM weight determination. At each harvest, percentages of plant green covers were also estimated and recorded. Both the shoot dry weights and the percent of plant visual green cover decreased as drought period progressed. Results: Although all the grasses exhibited a high level of drought tolerance, there was a wide range of variations observed in various clones' responses. The superior salinity and drought stress tolerant genotypes were identified to be used for biological salinity control or reclamation of desert saline soils and combating desertification. Conclusion: My investigations at the University of Arizona on saltgrass (Distichlis spicata L.), a halophytic plant species, have indicated that this plant has an excellent drought and salinity tolerance with a great potential to be used under harsh environmental conditions.