Mapping the 'early salinity response' triggered proteome adaptation in contrasting rice genotypes using iTRAQ approach

A glutathione-independent DJ-1/Pfp1 domain containing glyoxalase III, OsDJ-1C, functions in abiotic stress adaptation in rice

MAIN CONCLUSION

Overexpression of OsDJ-1C in rice improves root architecture, photosynthesis, yield and abiotic stress tolerance through modulating methylglyoxal levels, antioxidant defense, and redox homeostasis. Exposure to abiotic stresses leads to elevated methylglyoxal (MG) levels in plants, impacting seed germination and root growth. In response, the activation of NADPH-dependent aldo-keto reductase and glutathione (GSH)-dependent glyoxalase enzymes helps to regulate MG levels and reduce its toxic effects. However, detoxification may not be carried out effectively due to the limitation of GSH and NADPH in plants under stress. Recently, a novel enzyme called glyoxalase III (GLY III) has been discovered which can detoxify MG in a single step without needing GSH. To understand the physiological importance of this pathway in rice, we overexpressed the gene encoding GLYIII enzyme (OsDJ-1C) in rice. It was observed that OsDJ-1C overexpression in rice regulated MG levels under stress conditions thus, linked well with plants' abiotic stress tolerance potential. The OsDJ-1C overexpression lines displayed better root architecture, improved photosynthesis, and reduced yield penalty compared to the WT plants under salinity, and drought stress conditions. These plants demonstrated an improved GSH/GSSG ratio, reduced level of reactive oxygen species, increased antioxidant capacity, and higher anti-glycation activity thereby indicating that the GLYIII mediated MG detoxification plays a significant role in plants' ability to reduce the impact of abiotic stress. Furthermore, these findings imply the potential of OsDJ-1C in crop improvement programs.

Pokkali: A Naturally Evolved Salt-Tolerant Rice Shows a Distinguished Set of lncRNAs Possibly Contributing to the Tolerant Phenotype

Pokkali is a strong representation of how stress-tolerant genotypes have evolved due to natural selection pressure. Numerous omics-based investigations have indicated different categories of stress-related genes and proteins, possibly contributing to salinity tolerance in this wild rice. However, a comprehensive study towards understanding the role of long-noncoding RNAs (lncRNAs) in the salinity response of Pokkali has not been done to date. We have identified salt-responsive lncRNAs from contrasting rice genotypes IR64 and Pokkali. A total of 63 and 81 salinity-responsive lncRNAs were differentially expressed in IR64 and Pokkali, respectively. Molecular characterization of lncRNAs and lncRNA-miRNA-mRNA interaction networks helps to explore the role of lncRNAs in the stress response. Functional annotation revealed that identified lncRNAs modulate various cellular processes, including transcriptional regulation, ion homeostasis, and secondary metabolite production. Additionally, lncRNAs were predicted to bind stress-responsive transcription factors, namely ERF, DOF, and WRKY. In addition to salinity, expression profiling was also performed under other abiotic stresses and phytohormone treatments. A positive modulation in TCONS_00035411, TCONS_00059828, and TCONS_00096512 under both abiotic stress and phytohormone treatments could be considered as being of potential interest for the further functional characterization of IncRNA. Thus, extensive analysis of lncRNAs under various treatments helps to delineate stress tolerance mechanisms and possible cross-talk.

Susceptibility of Rice Crop to Salt Threat: Proteomic, Metabolomic, and Physiological Inspections

Rice is a staple food crop worldwide; however, salinity stress is estimated to reduce its global production by 50%. Knowledge about initial molecular signaling and proteins associated with sensing salinity among crop plants is limited. We characterized early salt effects on the proteome and metabolome of rice tissues. Omics results were validated by western blotting and multiple reaction monitoring assays and integrated with physiological changes. We identified 8160 proteins and 2045 metabolites in rice tissues. Numerous signaling pathways were induced rapidly or partially by salinity. Combined data showed the most susceptible proteins or metabolites in each pathway that likely affected the sensitivity of rice to salinity, such as PLA1, BON3 (involved in sensing stress), SnRK2, pro-resilin, GDT1, G-proteins, calmodulin activators (Ca²⁺ and abscisic acid signaling), MAPK3/5, MAPKK1/3 (MAPK pathway), SOS1, ABC F/D, PIP2-7, and K⁺ transporter-23 (transporters), OPR1, JAR1, COL1, ABA2, and MAPKK3 (phytohormones). Additionally, our results expanded the stress-sensing function of receptor-like kinases, phosphatidylinositols, and Na⁺ sensing proteins (IPUT1). Combined analyses revealed the most sensitive components of signaling pathways causing salt-susceptibility in rice and suggested potential targets for crop improvement.

Proteomic Approaches to Uncover Salt Stress Response Mechanisms in Crops

Salt stress is an unfavorable outcome of global climate change, adversely affecting crop growth and yield. It is the second-biggest abiotic factor damaging the morphological, physio-biochemical, and molecular processes during seed germination and plant development. Salt responses include modulation of hormonal biosynthesis, ionic homeostasis, the antioxidant defense system, and osmoprotectants to mitigate salt stress. Plants trigger salt-responsive genes, proteins, and metabolites to cope with the damaging effects of a high salt concentration. Enhancing salt tolerance among crop plants is direly needed for sustainable global agriculture. Novel protein markers, which are used for crop improvement against salt stress, are identified using proteomic techniques. As compared to single-technique approaches, the integration of genomic tools and exogenously applied chemicals offers great potential in addressing salt-stress-induced challenges. The interplay of salt-responsive proteins and genes is the missing key of salt tolerance. The development of salt-tolerant crop varieties can be achieved by integrated approaches encompassing proteomics, metabolomics, genomics, and genome-editing tools. In this review, the current information about the morphological, physiological, and molecular mechanisms of salt response/tolerance in crops is summarized. The significance of proteomic approaches to improve salt tolerance in various crops is highlighted, and an integrated omics approach to achieve global food security is discussed. Novel proteins that respond to salt stress are potential candidates for future breeding of salt tolerance.

Conventional and Omics Approaches for Understanding the Abiotic Stress Response in Cereal Crops-An Updated Overview

Cereals have evolved various tolerance mechanisms to cope with abiotic stress. Understanding the abiotic stress response mechanism of cereal crops at the molecular level offers a path to high-yielding and stress-tolerant cultivars to sustain food and nutritional security. In this regard, enormous progress has been made in the omics field in the areas of genomics, transcriptomics, and proteomics. Omics approaches generate a massive amount of data, and adequate advancements in computational tools have been achieved for effective analysis. The combination of integrated omics and bioinformatics approaches has been recognized as vital to generating insights into genome-wide stress-regulation mechanisms. In this review, we have described the self-driven drought, heat, and salt stress-responsive mechanisms that are highlighted by the integration of stress-manipulating components, including transcription factors, co-expressed genes, proteins, etc. This review also provides a comprehensive catalog of available online omics resources for cereal crops and their effective utilization. Thus, the details provided in the review will enable us to choose the appropriate tools and techniques to reduce the negative impacts and limit the failures in the intensive crop improvement study.

Integrated Multi-Omics Perspective to Strengthen the Understanding of Salt Tolerance in Rice

Salt stress is one of the major constraints to rice cultivation worldwide. Thus, the development of salt-tolerant rice cultivars becomes a hotspot of current rice breeding. Achieving this goal depends in part on understanding how rice responds to salt stress and uncovering the molecular mechanism underlying this trait. Over the past decade, great efforts have been made to understand the mechanism of salt tolerance in rice through genomics, transcriptomics, proteomics, metabolomics, and epigenetics. However, there are few reviews on this aspect. Therefore, we review the research progress of omics related to salt tolerance in rice and discuss how these advances will promote the innovations of salt-tolerant rice breeding. In the future, we expect that the integration of multi-omics salt tolerance data can accelerate the solution of the response mechanism of rice to salt stress, and lay a molecular foundation for precise breeding of salt tolerance.

Identifying Novel Genes and Proteins Involved in Salt Stress Perception and Signaling of Rice Seedlings

Rice is one of the most important crops worldwide. Crop production is constrained markedly, however, by abiotic stresses such as salinity. To elucidate early stress response signaling networks involved in rice, we report in this study an original quantitative proteomic analysis of the rice seedlings subjected to short-term salt stress. We detected 570 differentially regulated proteins (DRPs) in the root sample. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis demonstrated that DRPs of the root were mainly involved in membrane trafficking, kinase activity, and ion toxicity responses. Interactome analysis revealed the central role of root proteins involved in membrane trafficking in the early response to salinity, such as cell surface receptor-like kinases (RLKs), phosphatidylinositols (PIs), calcium-dependent protein kinases 1 and 5, calcineurin B-like protein-interacting proteins, protein phosphatase 2C (PP2C) inhibitors, and abscisic acid receptors (PYL5/10), indicating activation of S-type anion channel. Furthermore, the proteogenomic analysis revealed 128 unique genome search-specific peptides with high-quality mass spectromety (MS/MS) spectra. We identified 38 novel protein-coding genes, refined the annotation of 17 existing gene models, and suggested several novel stress-responsive proteins, such as RLK5, peroxidase 27, and growth-regulating factor 2. Novel peptides had an ortholog match in the curated protein sequence set of other plant species. In conclusion, this study identifies novel stress-responsive proteins and genes of rice, thus warrant future consideration as candidates for molecular breeding of stress-tolerant crop varieties.

Label-free proteomics approach reveals candidate proteins in rice (Oryza sativa L.) important for ACC deaminase producing bacteria-mediated tolerance against salt stress

The omics-based studies are important for identifying characteristic proteins in plants to elucidate the mechanism of ACC deaminase producing bacteria-mediated salt tolerance. This study evaluates the changes in the proteome of rice inoculated with ACC deaminase producing bacteria under salt stress conditions. Salt stress resulted in a significant decrease in photosynthetic pigments, whereas inoculation of Methylobacterium oryzae CBMB20 had significantly increased pigment contents under normal and salt stress conditions. A total of 76, 51 and 33 differentially abundant proteins (DAPs) were identified in non-inoculated salt stressed plants, bacteria inoculated plants under normal and salt stress conditions, respectively. The abundances of proteins responsible for ethylene emission and programmed cell death were increased, and that of photosynthesis-related proteins were decreased in non-inoculated plants under salt stress. Whereas, bacteria-inoculated plants had shown higher abundance of antioxidant proteins, RuBisCo and ribosomal proteins that are important for enhancing stress tolerance and improving plant physiological traits. Collectively, salt stress might affect plant physiological traits by impairing photosynthetic machinery and accelerating apoptosis leading to a decline in biomass. However, inoculation of plants with bacteria can assist in enhancing photosynthetic activity, antioxidant activities and ethylene regulation related proteins for attenuating salt induced apoptosis and sustaining growth and development. This article is protected by copyright. All rights reserved.

Integrated physiological and comparative proteomics analysis of contrasting genotypes of pearl millet reveals underlying salt-responsive mechanisms

Salinity stress poses a significant risk to plant development and agricultural yield. Therefore, elucidation of stress-response mechanisms has become essential to identify salt-tolerance genes in plants. In the present study, two genotypes of pearl millet (Pennisetum glaucum L.) with contrasting tolerance for salinity exhibited differential morpho-physiological and proteomic responses under 150 mM NaCl. The genotype IC 325825 was shown to withstand the stress better than IP 17224. The salt-tolerance potential of IC 325825 was associated with its ability to maintain intracellular osmotic, ionic, and redox homeostasis and membrane integrity under stress. The IC 325825 genotype exhibited a higher abundance of C4 photosynthesis enzymes, efficient enzymatic and non-enzymatic antioxidant system, and lower Na⁺ /K⁺ ratio compared with IP 17224. Comparative proteomics analysis revealed greater metabolic perturbation in IP 17224 under salinity, in contrast to IC 325825 that harbored pro-active stress-responsive machinery, allowing its survival and better adaptability under salt stress. The differentially abundant proteins were in silico characterized for their functions, subcellular-localization, associated pathways, and protein-protein interaction. These proteins were mainly involved in photosynthesis/response to light stimulus, carbohydrate and energy metabolism, and stress responses. Proteomics data were validated through expression profiling of the selected genes, revealing a poor correlation between protein abundance and their relative transcript levels. This study has provided novel insights into salt adaptive mechanisms in P. glaucum, demonstrating the power of proteomics-based approaches. The critical proteins identified in the present study could be further explored as potential objects for engineering stress tolerance in salt-sensitive major crops.

Plant proteomic research for improvement of food crops under stresses: a review

Crop improvement approaches have been changed due to technological advancements in traditional plant-breeding methods. Abiotic and biotic stresses limit plant growth and development, which ultimately lead to reduced crop yield. Proteins encoded by genomes have a considerable role in the endurance and adaptation of plants to different environmental conditions. Biotechnological applications in plant breeding depend upon the information generated from proteomic studies. Proteomics has a specific advantage to contemplate post-translational modifications, which indicate the functional effects of protein modifications on crop production. Subcellular proteomics helps in exploring the precise cellular responses and investigating the networking among subcellular compartments during plant development and biotic/abiotic stress responses. Large-scale mass spectrometry-based plant proteomic studies with a more comprehensive overview are now possible due to dramatic improvements in mass spectrometry, sample preparation procedures, analytical software, and strengthened availability of genomes for numerous plant species. Development of stress-tolerant or resilient crops is essential to improve crop productivity and growth. Use of high throughput techniques with advanced instrumentation giving efficient results made this possible. In this review, the role of proteomic studies in identifying the stress-response processes in different crops is summarized. Advanced techniques and their possible utilization on plants are discussed in detail. Proteomic studies accelerate marker-assisted genetic augmentation studies on crops for developing high yielding stress-tolerant lines or varieties under stresses.

How do rice seedlings of landrace Pokkali survive in saline fields after transplantation? Physiology, biochemistry, and photosynthesis

Rice, one of the most important staple food crops in the world, is highly sensitive to soil salinity at the seedling stage. The ultimate yield of this crop is a function of the number of seedlings surviving after transplantation in saline water. Oryza sativa cv. IR64 is a high-yielding salinity-sensitive variety, while Pokkali is a landrace traditionally cultivated by the local farmers in the coastal regions in India. However, the machinery responsible for the seedling-stage tolerance in Pokkali is not understood. To bridge this gap, we subjected young seedlings of these contrasting genotypes to salinity and performed detailed investigations about their growth parameters, ion homeostasis, biochemical composition, and photosynthetic parameters after every 24 h of salinity for three days. Taken together, all the physiological and biochemical indicators, such as proline accumulation, K⁺/Na⁺ ratio, lipid peroxidation, and electrolyte leakage, clearly revealed significant differences between IR64 and Pokkali under salinity, establishing their contrasting nature at this stage. In response to salinity, the F_v/F_m ratio (maximum quantum efficiency of Photosystem II as inferred from Chl a fluorescence) and the energy conserved for the electron transport after the reduction of Q_A (the primary electron acceptor of PSII), to Q_A^-, and reduction of the end electron acceptor molecules towards the PSI (Photosystem I) electron acceptor side was higher in Pokkali than IR64 plants. These observations reflect a direct contribution of photosynthesis towards seedling-stage salinity tolerance in rice. These findings will help to breed high-yielding crops for salinity prone agricultural lands.

Physiological evaluation for salt tolerance in green and purple leaf color rice cultivars at seedling stage

Anthocyanin, a water-soluble pigment found in plants, has been reported to be associated with abiotic stress tolerance including salt stress. For a better understanding of the role of anthocyanin in response to salt stress, two salt-tolerant rice genotypes having different leaf anthocyanin content, one having green ('Pokkali'; PK) and the other purple leaves ('Niew Dam 019'; ND 019), were used in this study. After being subjected to salt stress (150 mM NaCl) for 5 d, the 3-week-old rice genotypes PK and ND 019 exhibited significant physiological responses (water content, Na⁺/K⁺ ratio, osmolyte accumulation, osmotic adjustment, antioxidant capacity, membrane damage and chlorophyll) and expression of ion transporter genes, indicating overall salt tolerance ability. However, the green-leaved rice variety, PK, had better root-to-shoot Na⁺ exclusion mechanism than the purple-leaved variety, ND 019 as evidenced by lower Na⁺ accumulation in leaves compared to ND 019 despite the fact that they accumulated the similar level of Na⁺ in roots. On the other hand, ND 019 accumulated higher concentration of osmolytes leading to more enhanced osmotic adjustment. These results revealed that Na⁺ ion exclusion was the prominent salt tolerance mechanism in the green-leaved PK whereas in the purple-leaved ND 019 osmotic adjustment was the more significant strategy. Under salt stress, there was no remarkable change in anthocyanin in PK while a reduction was found in ND 019. Thus, it could be proposed that anthocyanin did not play a vital role in protecting the purple-leaved rice, ND 019 from salt stress during seedling stage.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01114-y.

Toward Integrated Multi-Omics Intervention: Rice Trait Improvement and Stress Management

Rice (Oryza sativa) is an imperative staple crop for nearly half of the world's population. Challenging environmental conditions encompassing abiotic and biotic stresses negatively impact the quality and yield of rice. To assure food supply for the unprecedented ever-growing world population, the improvement of rice as a crop is of utmost importance. In this era, "omics" techniques have been comprehensively utilized to decipher the regulatory mechanisms and cellular intricacies in rice. Advancements in omics technologies have provided a strong platform for the reliable exploration of genetic resources involved in rice trait development. Omics disciplines like genomics, transcriptomics, proteomics, and metabolomics have significantly contributed toward the achievement of desired improvements in rice under optimal and stressful environments. The present review recapitulates the basic and applied multi-omics technologies in providing new orchestration toward the improvement of rice desirable traits. The article also provides a catalog of current scenario of omics applications in comprehending this imperative crop in relation to yield enhancement and various environmental stresses. Further, the appropriate databases in the field of data science to analyze big data, and retrieve relevant information vis-à-vis rice trait improvement and stress management are described.

iTRAQ-based proteomic analysis provides insights into the molecular mechanisms of rice formyl tetrahydrofolate deformylase in salt response

A new molecular mechanism of tetrahydrofolate deformylase involved in the salt response presumably affects mitochondrial and chloroplast function by regulating energy metabolism and accumulation of reactive oxygen species. High salinity severely restrains plant growth and development, consequently leading to a reduction in grain yield. It is therefore critical to identify the components involved in plant salt resistance. In our previous study, we identified a rice leaf early-senescence mutant hpa1, which encodes a formyl tetrahydrofolate deformylase (Xiong et al. in Sci China Life Sci 64(5):720-738, 2021). Here, we report that HPA1 also plays a role in the salt response. To explore the molecular mechanism of HPA1 in salt resistance, we attempted to identify the differentially expressed proteins between wild type and hpa1 mutant for salinity treatment using an iTRAQ-based comparative protein quantification approach. A total of 4598 proteins were identified, of which 279 were significantly altered, including 177 up- and 102 down-regulated proteins. A functional analysis suggested that the 279 differentially expressed proteins are involved mainly in the regulation of oxidative phosphorylation, phenylpropanoid biosynthesis, photosynthesis, posttranslational modifications, protein turnover and energy metabolism. Moreover, a deficiency in HPA1 impaired chlorophyll metabolism and photosynthesis in chloroplasts and affected the electron flow of the electron transport chain in mitochondria. These changes led to abnormal energy metabolism and accumulation of reactive oxygen species, which may affect the permeability and integrity of cell membranes, leading to cell death. In addition, the results were verified by transcriptional or physiological experiments. Our results provide an insight into a new molecular mechanism of the tetrahydrofolate cycle protein formyl tetrahydrofolate deformylase, which is involved in the salt response, presumably by affecting mitochondrial and chloroplast function regulating energy metabolism and accumulation of reactive oxygen species under salt stress.

Comparative ribosome profiling reveals distinct translational landscapes of salt-sensitive and -tolerant rice

BACKGROUND

Soil salinization represents a serious threat to global rice production. Although significant research has been conducted to understand salt stress at the genomic, transcriptomic and proteomic levels, few studies have focused on the translatomic responses to this stress. Recent studies have suggested that transcriptional and translational responses to salt stress can often operate independently.

RESULTS

We sequenced RNA and ribosome-protected fragments (RPFs) from the salt-sensitive rice (O. sativa L.) cultivar 'Nipponbare' (NB) and the salt-tolerant cultivar 'Sea Rice 86' (SR86) under normal and salt stress conditions. A large discordance between salt-induced transcriptomic and translatomic alterations was found in both cultivars, with more translationally regulated genes being observed in SR86 in comparison to NB. A biased ribosome occupancy, wherein RPF depth gradually increased from the 5' ends to the 3' ends of coding regions, was revealed in NB and SR86. This pattern was strengthened by salt stress, particularly in SR86. On the contrary, the strength of ribosome stalling was accelerated in salt-stressed NB but decreased in SR86.

CONCLUSIONS

This study revealed that translational reprogramming represents an important layer of salt stress responses in rice, and the salt-tolerant cultivar SR86 adopts a more flexible translationally adaptive strategy to cope with salt stress compared to the salt susceptible cultivar NB. The differences in translational dynamics between NB and SR86 may derive from their differing levels of ribosome stalling under salt stress.

Genome-Wide Association Mapping of Salinity Tolerance at the Seedling Stage in a Panel of Vietnamese Landraces Reveals New Valuable QTLs for Salinity Stress Tolerance Breeding in Rice

Rice tolerance to salinity stress involves diverse and complementary mechanisms, such as the regulation of genome expression, activation of specific ion-transport systems to manage excess sodium at the cell or plant level, and anatomical changes that avoid sodium penetration into the inner tissues of the plant. These complementary mechanisms can act synergistically to improve salinity tolerance in the plant, which is then interesting in breeding programs to pyramidize complementary QTLs (quantitative trait loci), to improve salinity stress tolerance of the plant at different developmental stages and in different environments. This approach presupposes the identification of salinity tolerance QTLs associated with different mechanisms involved in salinity tolerance, which requires the greatest possible genetic diversity to be explored. To contribute to this goal, we screened an original panel of 179 Vietnamese rice landraces genotyped with 21,623 SNP markers for salinity stress tolerance under 100 mM NaCl treatment, at the seedling stage, with the aim of identifying new QTLs involved in the salinity stress tolerance via a genome-wide association study (GWAS). Nine salinity tolerance-related traits, including the salt injury score, chlorophyll and water content, and K⁺ and Na⁺ contents were measured in leaves. GWAS analysis allowed the identification of 26 QTLs. Interestingly, ten of them were associated with several different traits, which indicates that these QTLs act pleiotropically to control the different levels of plant responses to salinity stress. Twenty-one identified QTLs colocalized with known QTLs. Several genes within these QTLs have functions related to salinity stress tolerance and are mainly involved in gene regulation, signal transduction or hormone signaling. Our study provides promising QTLs for breeding programs to enhance salinity tolerance and identifies candidate genes that should be further functionally studied to better understand salinity tolerance mechanisms in rice.

Genome-wide discovery of natural variation in pre-mRNA splicing and prioritising causal alternative splicing to salt stress response in rice

Pre-mRNA splicing is an essential step for the regulation of gene expression. In order to specifically capture splicing variants in plants for genome-wide association studies (GWAS), we developed a software tool to quantify and visualise Variations of Splicing in Population (VaSP). VaSP can quantify splicing variants from short-read RNA-seq datasets and discover genotype-specific splicing (GSS) events, which can be used to prioritise causal pre-mRNA splicing events in GWAS. We applied our method to an RNA-seq dataset with 328 samples from 82 genotypes from a rice diversity panel exposed to optimal and saline growing conditions. In total, 764 significant GSS events were identified in salt stress conditions. GSS events were used as markers for a GWAS with the shoot Na⁺ accumulation, which identified six GSS events in five genes significantly associated with the shoot Na⁺ content. Two of these genes, OsNUC1 and OsRAD23 emerged as top candidate genes with splice variants that exhibited significant divergence between the variants for shoot growth under salt stress conditions. VaSP is a versatile tool for alternative splicing analysis in plants and a powerful tool for prioritising candidate causal pre-mRNA splicing and corresponding genomic variations in GWAS.

Understanding Rice-Magnaporthe Oryzae Interaction in Resistant and Susceptible Cultivars of Rice under Panicle Blast Infection Using a Time-Course Transcriptome Analysis

Rice blast is a global threat to food security with up to 50% yield losses. Panicle blast is a more severe form of rice blast and the response of rice plant to leaf and panicle blast is distinct in different genotypes. To understand the specific response of rice in panicle blast, transcriptome analysis of blast resistant cultivar Tetep, and susceptible cultivar HP2216 was carried out using RNA-Seq approach after 48, 72 and 96 h of infection with Magnaporthe oryzae along with mock inoculation. Transcriptome data analysis of infected panicle tissues revealed that 3553 genes differentially expressed in HP2216 and 2491 genes in Tetep, which must be the responsible factor behind the differential disease response. The defense responsive genes are involved mainly in defense pathways namely, hormonal regulation, synthesis of reactive oxygen species, secondary metabolites and cell wall modification. The common differentially expressed genes in both the cultivars were defense responsive transcription factors, NBS-LRR genes, kinases, pathogenesis related genes and peroxidases. In Tetep, cell wall strengthening pathway represented by PMR5, dirigent, tubulin, cell wall proteins, chitinases, and proteases was found to be specifically enriched. Additionally, many novel genes having DOMON, VWF, and PCaP1 domains which are specific to cell membrane were highly expressed only in Tetep post infection, suggesting their role in panicle blast resistance. Thus, our study shows that panicle blast resistance is a complex phenomenon contributed by early defense response through ROS production and detoxification, MAPK and LRR signaling, accumulation of antimicrobial compounds and secondary metabolites, and cell wall strengthening to prevent the entry and spread of the fungi. The present investigation provided valuable candidate genes that can unravel the mechanisms of panicle blast resistance and help in the rice blast breeding program.

Genome-Wide Association Study (GWAS) to Identify Salt-Tolerance QTLs Carrying Novel Candidate Genes in Rice During Early Vegetative Stage

BACKGROUND

Rice is considered as a salt-sensitive plant, particularly at early vegetative stage, and its production is suffered from salinity due to expansion of salt affected land in areas under cultivation. Hence, significant increase of rice productivity on salinized lands is really necessary. Today genome-wide association study (GWAS) is a method of choice for fine mapping of QTLs involved in plant responses to abiotic stresses including salinity stress at early vegetative stage. In this study using > 33,000 SNP markers we identified rice genomic regions associated to early stage salinity tolerance. Eight salinity-related traits including shoot length (SL), root length (RL), root dry weight (RDW), root fresh weight (RFW), shoot fresh weight (SFW), shoot dry weight (SDW), relative water content (RWC) and TW, and 4 derived traits including SL-R, RL-R, RDW-R and RFW-R in a diverse panel of rice were evaluated under salinity (100 mM NaCl) and normal conditions in growth chamber. Genome-wide association study (GWAS) was applied based on MLM(+Q + K) model.

RESULTS

Under stress conditions 151 trait-marker associations were identified that were scattered on 10 chromosomes of rice that arranged in 29 genomic regions. A genomic region on chromosome 1 (11.26 Mbp) was identified which co-located with a known QTL region SalTol1 for salinity tolerance at vegetative stage. A candidate gene (Os01g0304100) was identified in this region which encodes a cation chloride cotransporter. Furthermore, on this chromosome two other candidate genes, Os01g0624700 (24.95 Mbp) and Os01g0812000 (34.51 Mbp), were identified that encode a WRKY transcription factor (WRKY 12) and a transcriptional activator of gibberellin-dependent alpha-amylase expression (GAMyb), respectively. Also, a narrow interval on the same chromosome (40.79-42.98 Mbp) carries 12 candidate genes, some of them were not so far reported for salinity tolerance at seedling stage. Two of more interesting genes are Os01g0966000 and Os01g0963000, encoding a plasma membrane (PM) H⁺-ATPase and a peroxidase BP1 protein. A candidate gene was identified on chromosome 2 (Os02g0730300 at 30.4 Mbp) encoding a high affinity K⁺ transporter (HAK). On chromosome 6 a DnaJ-encoding gene and pseudouridine synthase gene were identified. Two novel genes on chromosome 8 including the ABI/VP1 transcription factor and retinoblastoma-related protein (RBR), and 3 novel genes on chromosome 11 including a Lox, F-box and Na⁺/H⁺ antiporter, were also identified.

CONCLUSION

Known or novel candidate genes in this research were identified that can be used for improvement of salinity tolerance in molecular breeding programmes of rice. Further study and identification of effective genes on salinity tolerance by the use of candidate gene-association analysis can help to precisely uncover the mechanisms of salinity tolerance at molecular level. A time dependent relationship between salt tolerance and expression level of candidate genes could be recognized.

Diverse Roles of MAX1 Homologues in Rice

Cytochrome P450 enzymes encoded by MORE AXILLARY GROWTH1 (MAX1)-like genes produce most of the structural diversity of strigolactones during the final steps of strigolactone biosynthesis. The diverse copies of MAX1 in Oryza sativa provide a resource to investigate why plants produce such a wide range of strigolactones. Here we performed in silico analyses of transcription factors and microRNAs that may regulate each rice MAX1, and compared the results with available data about MAX1 expression profiles and genes co-expressed with MAX1 genes. Data suggest that distinct mechanisms regulate the expression of each MAX1. Moreover, there may be novel functions for MAX1 homologues, such as the regulation of flower development or responses to heavy metals. In addition, individual MAX1s could be involved in specific functions, such as the regulation of seed development or wax synthesis in rice. Our analysis reveals potential new avenues of strigolactone research that may otherwise not be obvious.

Expression levels of the Na+/K+ transporter OsHKT2;1 and vacuolar Na+/H+ exchanger OsNHX1, Na enrichment, maintaining the photosynthetic abilities and growth performances of indica rice seedlings under salt stress

Salt affected soil inhibits plant growth, development and productivity, especially in case of rice crop. Ion homeostasis is a candidate defense mechanism in the salt tolerant plants or halophyte species, where the salt toxic ions are stored in the vacuoles. The aim of this investigation was to determine the OsNHX1 (a vacuolar Na⁺/H⁺ exchanger) and OsHKT2;1 (Na⁺/K⁺ transporter) regulation by salt stress (200 mM NaCl) in two rice cultivars, i.e. Pokkali (salt tolerant) and IR29 (salt susceptible), the accumulation of Na⁺ in the root and leaf tissues using CoroNa Green^® staining dye and the associated physiological changes in test plants. Na⁺ content was largely increased in the root tissues of rice seedlings cv. Pokkali (15 min after salt stress) due to the higher expression of OsHKT2;1 gene (by 2.5 folds) in the root tissues. The expression of OsNHX1 gene in the leaf tissues was evidently increased in salt stressed seedlings of Pokkali, whereas it was unchanged in salt stressed seedlings of IR29. Na⁺ in the root tissues of both Pokkali and IR29 was enriched, when subjected to 200 mM NaCl for 12 h and easily detected in the leaf tissues of salt stressed plants exposed for 24 h, especially in cv. Pokkali. Moreover, the overexpression of OsNHX1 gene regulated the translocation of Na⁺ from root to leaf tissues, and compartmentation of Na⁺ into vacuoles, thereby maintaining the photosynthetic abilities in cv. Pokkali. Overall growth performance, maximum quantum yield (F_v/F_m), photon yield of PSII (Φ_PSII) and net photosynthetic rate (P_n) was improved in salt stressed leaves of Pokkali than those in salt stressed IR29.

The quest for osmosensors in plants

Osmotic stress has severe effects on crop productivity. Since climate change is predicted to exacerbate this problem, the development of new crops that are tolerant to osmotic stresses, especially drought and salinity stress, is required. However, only limited success has been achieved to date, primarily because of the lack of a clear understanding of the mechanisms that facilitate osmosensing. Here, we discuss the potential mechanisms of osmosensing in plants. We highlight the roles of proteins such as receptor-like kinases, which sense stress-induced cell wall damage, mechanosensitive calcium channels, which initiate a calcium-induced stress response, and phospholipase C, a membrane-bound enzyme that is integral to osmotic stress perception. We also discuss the roles of aquaporins and membrane-bound histidine kinases, which could potentially detect changes in extracellular osmolarity in plants, as they do in prokaryotes and lower eukaryotes. These putative osmosensors have the potential to serve as master regulators of the osmotic stress response in plants and could prove to be useful targets for the selection of osmotic stress-tolerant crops.

The Saltol QTL-localized transcription factor OsGATA8 plays an important role in stress tolerance and seed development in Arabidopsis and rice

GATA represents a highly conserved family of transcription factors reported in organisms ranging from fungi to angiosperms. A member of this family, OsGATA8, localized within the Saltol QTL in rice, has been reported to be induced by salinity, drought, and ABA. However, its precise role in stress tolerance has not yet been elucidated. Using genetic, molecular, and physiological analyses, in this study we show that OsGATA8 increases seed size and tolerance to abiotic stresses in both Arabidopsis and rice. Transgenic lines of rice were generated with 3-fold overexpression of OsGATA8 compared to the wild-type together with knockdown lines with 2-fold lower expression. The overexpressing lines showed higher biomass accumulation and higher photosynthetic efficiency in seedlings compared to the wild-type and knockdown lines under both normal and salinity-stress conditions. OsGATA8 appeared to be an integrator of diverse cellular processes, including K+/Na+ content, photosynthetic efficiency, relative water content, Fv/Fm ratio, and the stability to sub-cellular organelles. It also contributed to maintaining yield under stress, which was ~46% higher in overexpression plants compared with the wild-type. OsGATA8 produced these effects by regulating the expression of critical genes involved in stress tolerance, scavenging of reactive oxygen species, and chlorophyll biosynthesis.