Natural Genetic Resources from Diverse Plants to Improve Abiotic Stress Tolerance in Plants.

The current agricultural system is biased for the yield increase at the cost of biodiversity. However, due to the loss of precious genetic diversity during domestication and artificial selection, modern cultivars have lost the adaptability to cope with unfavorable environments. There are many reports on variations such as single nucleotide polymorphisms (SNPs) and indels in the stress-tolerant gene alleles that are associated with higher stress tolerance in wild progenitors, natural accessions, and extremophiles in comparison with domesticated crops or model plants. Therefore, to gain a better understanding of stress-tolerant traits in naturally stress-resistant plants, more comparative studies between the modern crops/model plants and crop progenitors/natural accessions/extremophiles are required. In this review, we discussed and summarized recent progress on natural variations associated with enhanced abiotic stress tolerance in various plants. By applying the recent biotechniques such as the CRISPR/Cas9 gene editing tool, natural genetic resources (i.e., stress-tolerant gene alleles) from diverse plants could be introduced to the modern crop in a non-genetically modified way to improve stress-tolerant traits.

Genome wide association study of Arabidopsis seed mucilage layers at a regional scale.

The myxospermous species Arabidopsis thaliana extrudes a polysaccharidic mucilage from the seed coat epidermis during imbibition. The whole seed mucilage can be divided into a seed-adherent layer and a fully soluble layer, both layers presenting natural genetic variations. The adherent mucilage is variable in size and composition, while the soluble mucilage is variable in composition and physical properties. Studies reporting both the genetic architecture and the putative selective agents acting on this natural genetic variation are scarce. In this study, we set up a Genome Wide Association study (GWAS) based on 424 natural accessions collected from 166 natural populations of A. thaliana located south-west of France and previously characterized for a very important number of abiotic and biotic factors. We identified an extensive genetic variation for both mucilage layers. The adherent mucilage was mainly related to precipitation and temperature whereas the non-adherent mucilage was unrelated to any environmental factors. By combining a hierarchical Bayesian model with a local score approach, we identified 55 and 28 candidate genes, corresponding to 26 and 10 QTLs for the adherent and non-adherent mucilages, respectively. Putative or characterized function and expression data available in the literature were used to filter the candidate genes. Only one gene among our set of candidate genes was already described as a seed mucilage actor, leaving a large set of new candidates putatively implicated inseed mucilage synthesis or release. The present study lay out foundation to understand the influence of regional ecological factors acting on seed mucilage in A. thaliana.

Genome-Wide Association Analysis for Salt-Induced Phenotypic and Physiologic Responses in Rice at Seedling and Reproductive Stages.

Salinity is one of the main adverse environmental factors severely inhibiting rice growth and decreasing grain productivity. Developing rice varieties with salt tolerance (ST) is one of the most economical approaches to cope with salinity stress. In this study, the salt tolerance of 220 rice accessions from rice diversity panel l (RDP1), representing five subpopulations, were evaluated based on 16 ST indices at both seedling and reproductive stages under salt stress. An apparent inconsistency was found for ST between the two stages. Through a gene-based/tightly linked genome-wide association study with 201,332 single nucleotide polymorphisms (SNPs) located within genes and their flanking regions were used, a total of 214 SNPs related to 251 genes, significantly associated with 16 ST-related indices, were detected at both stages. Eighty-two SNPs with low frequency favorable (LFF) alleles in the population were proposed to hold high breeding potential in improving rice ST. Fifty-four rice accessions collectively containing all these LFF alleles were identified as donors of these alleles. Through the integration of meta-quantitative trait locus (QTL) for ST and the response patterns of differential expression genes to salt stress, thirty-eight candidate genes were suggested to be involved in the regulation of rice ST. In total, the present study provides valuable information for further characterizing ST-related genes and for breeding ST varieties across whole developmental stages through marker-assisted selection (MAS).

Study of natural diversity in response to a key pathogenicity regulator of Ralstonia solanacearum reveals new susceptibility genes in Arabidopsis thaliana.

Ralstonia solanacearum gram-negative phytopathogenic bacterium exerts its virulence through a type III secretion system (T3SS) that translocates type III effectors (T3Es) directly into the host cells. T3E secretion is finely controlled at the posttranslational level by helper proteins, T3SS control proteins, and type III chaperones. The HpaP protein, one of the type III secretion substrate specificity switch (T3S4) proteins, was previously highlighted as a virulence factor on Arabidopsis thaliana Col-0 accession. In this study, we set up a genome-wide association analysis to explore the natural diversity of response to the hpaP mutant of two A. thaliana mapping populations: a worldwide collection and a local population. Quantitative genetic variation revealed different genetic architectures in both mapping populations, with a global delayed response to the hpaP mutant compared to the GMI1000 wild-type strain. We have identified several quantitative trait loci (QTLs) associated with the hpaP mutant inoculation. The genes underlying these QTLs are involved in different and specific biological processes, some of which were demonstrated important for R. solanacearum virulence. We focused our study on four candidate genes, RKL1, IRE3, RACK1B, and PEX3, identified using the worldwide collection, and validated three of them as susceptibility factors. Our findings demonstrate that the study of the natural diversity of plant response to a R. solanacearum mutant in a key regulator of virulence is an original and powerful strategy to identify genes directly or indirectly targeted by the pathogen.

A complex network of additive and epistatic quantitative trait loci underlies natural variation of Arabidopsis thaliana quantitative disease resistance to Ralstonia solanacearum under heat stress.

Plant immunity is often negatively impacted by heat stress. However, the underlying molecular mechanisms remain poorly characterized. Based on a genome-wide association mapping approach, this study aims to identify in Arabidopsis thaliana the genetic bases of robust resistance mechanisms to the devastating pathogen Ralstonia solanacearum under heat stress. A local mapping population was phenotyped against the R. solanacearum GMI1000 strain at 27 and 30 °C. To obtain a precise description of the genetic architecture underlying natural variation of quantitative disease resistance (QDR), we applied a genome-wide local score analysis. Alongside an extensive genetic variation found in this local population at both temperatures, we observed a playful dynamics of quantitative trait loci along the infection stages. In addition, a complex genetic network of interacting loci could be detected at 30 °C. As a first step to investigate the underlying molecular mechanisms, the atypical meiotic cyclin SOLO DANCERS gene was validated by a reverse genetic approach as involved in QDR to R. solanacearum at 30 °C. In the context of climate change, the complex genetic architecture underlying QDR under heat stress in a local mapping population revealed candidate genes with diverse molecular functions.

Accession-specific life strategies affect responses in leaves of Arabidopsis thaliana plants exposed to excess Cu and Cd.

The natural accession Columbia (Col-0) is considered as the reference genome of the model plant Arabidopsis thaliana. Nonetheless, Col-0 plants are more sensitive to excess copper (Cu) and cadmium (Cd) than other widely used accessions such as Wassilewskija (Ws) plants. In the current study, this accession-specific metal sensitivity is further explored by comparing the responses in leaves of Col-0 and Ws plants exposed to excess Cu and Cd. Our results suggest that different life strategies favored by both accessions under physiological conditions affect their response to metal exposure. While Col-0 plants mainly invest in metal detoxification, Ws plants center on nutrient homeostasis. In particular, the higher expression of genes related to Cu homeostasis genes in non-exposed conditions indicates that Ws plants possess a constitutively efficient metal homeostasis. On the other hand, oxidative stress-related MAPK signaling appears to be boosted in leaves of Col-0 plants exposed to excess Cu. Furthermore, the upregulation of the glutathione (GSH) biosynthesis GSH2 gene and the increased GSH concentration after Cd exposure suggest the activation of detoxification mechanisms, such as phytochelatin production, to counteract the more severe Cd-induced oxidative stress in leaves of Col-0 plants. Exposure to Cd also led to a more pronounced ethylene signaling response in leaves of Col-0 as compared to Ws plants, which could be related to Cd-induced GSH metabolism. In conclusion, accession-specific life strategies clearly affect the way in which leaves of A. thaliana plants cope with excess Cu and Cd.

Transposon-associated polymorphisms of stress-responsive gene promoters in selected accessions of Arabidopsis thaliana.

Genetic diversity caused by transposable element movement can play an important role in plant adaptation to local environments. Regarding genes, transposon-induced alleles were mostly related to gene bodies and a few of them to promoter regions. In this study, promoter regions of 9 stress-related genes were searched for transposable element insertions in 12 natural accessions of Arabidopsis thaliana. The promoter screening was performed via PCR amplification with primers designed to flank transposable element insertions in the promoter regions of the reference accession Col-0. Transposable element-associated insertion/deletion (indel) polymorphisms were identified in 7 of the 12 promoter loci across studied accessions that can be developed further as molecular markers. The transposable element absence in the promoter regions of orthologous genes in A. lyrata indicated that the insertion of these transposable elements in A. thaliana lineage had occurred after its divergence from A. lyrata. Sequence analysis of the promoter regions of CML41 (Calmodulin-like protein 41) and CHAP (chaperone protein dnaJ-related) confirmed the indel polymorphic sites in four accessions - Col-0, Wassilewskija, Shahdara, and Pirin. The observed indel polymorphism of the CHAP promoter region was associated with specific gene expression profiles in the different accessions grown at a normal and elevated temperature in a plant growth chamber. The collected data can be a starting point for gene expression profiling studies under conditions resembling the natural habitats of accessions.

Natural variation in folate levels among tomato (Solanum lycopersicum) accessions.

Folate content was estimated in tomato (Solanum lycopersicum) accessions using microbiological assay (MA) and by LC-MS. The MA revealed that in red-ripe fruits folate levels ranged from 4 to 60µg/100g fresh weight. The LC-MS estimation of red-ripe fruits detected three folate forms, 5-CH3-THF, 5-CHO-THF, 5,10-CH(+)THF and folate levels ranged from 14 to 46µg/100g fresh weight. In mature green and red ripe fruit, 5-CH3-THF was the most abundant folate form. Comparison of LC-MS with MA revealed that MA inaccurately estimates folate levels. The accumulation of folate forms and their distribution varied among accessions. The single nucleotide polymorphism was examined in the key genes of the folate pathway to understand its linkage with folate levels. Despite the significant variation in folate levels among tomato accessions, little polymorphism was found in folate biosynthesis genes. Our results indicate that variation in folate level is governed by a more complex regulation at cellular homeostasis level.

Chlorophyll a fluorescence, under half of the adaptive growth-irradiance, for high-throughput sensing of leaf-water deficit in Arabidopsis thaliana accessions.

BACKGROUND: Non-invasive and high-throughput monitoring of drought in plants from its initiation to visible symptoms is essential to quest drought tolerant varieties. Among the existing methods, chlorophyll a fluorescence (ChlF) imaging has the potential to probe systematic changes in photosynthetic reactions; however, prerequisite of dark-adaptation limits its use for high-throughput screening. RESULTS: To improve the throughput monitoring of plants, we have exploited their light-adaptive strategy, and investigated possibilities of measuring ChlF transients under low ambient irradiance. We found that the ChlF transients and associated parameters of two contrasting Arabidopsis thaliana accessions, Rsch and Co, give almost similar information, when measured either after ~20 min dark-adaptation or in the presence of half of the adaptive growth-irradiance. The fluorescence parameters, effective quantum yield of PSII photochemistry (ΦPSII) and fluorescence decrease ratio (RFD) resulting from this approach enabled us to differentiate accessions that is often not possible by well-established dark-adapted fluorescence parameter maximum quantum efficiency of PSII photochemistry (FV/FM). Further, we screened ChlF transients in rosettes of well-watered and drought-stressed six A. thaliana accessions, under half of the adaptive growth-irradiance, without any prior dark-adaptation. Relative water content (RWC) in leaves was also assayed and compared to the ChlF parameters. As expected, the RWC was significantly different in drought-stressed from that in well-watered plants in all the six investigated accessions on day-10 of induced drought; the maximum reduction in the RWC was obtained for Rsch (16%), whereas the minimum reduction was for Co (~7%). Drought induced changes were reflected in several features of ChlF transients; combinatorial images obtained from pattern recognition algorithms, trained on pixels of image sequence, improved the contrast among drought-stressed accessions, and the derived images were well-correlated with their RWC. CONCLUSIONS: We demonstrate here that ChlF transients and associated parameters measured even in the presence of low ambient irradiance preserved its features comparable to that of measured after dark-adaptation and discriminated the accessions having differential geographical origin; further, in combination with combinatorial image analysis tools, these data may be readily employed for early sensing and mapping effects of drought on plant's physiology via easy and fully non-invasive means.