Genomic resources for breeding crops with enhanced abiotic stress tolerance

To meet the challenges of climate change, exploring natural diversity in the existing plant genetic resource pool as well as creation of new mutants through chemical mutagenesis and molecular biology is needed for developing climate‐resilient elite genotypes. Ever‐increasing area under existing abiotic stresses as well as emerging abiotic stress factors and their combinations have further added to the problems of the current crop improvement programmes. However, with the advancement in modern techniques such as next‐generation sequencing technologies, it is now possible to generate on a whole‐genome scale, genomic resources for crop species at a much faster pace with considerably less efforts and money. The genomic resources thus generated will be useful for various plant breeding applications such as marker‐assisted breeding for gene introgression, mapping QTLs or identifying new or rare alleles associated with a particular trait. In this article, we discuss various aspects of generation of genomic resources and their utilization for developing abiotic stress‐tolerant crops to ensure sustainable agricultural production and food security in the backdrop of rapid climate change.     

Evaluation of wild Arachis species for abiotic stress tolerance: I. Thermal stress and leaf water relations

The Indian groundnut cultivars have a narrow genetic base. Hence, it was of interest to investigate the genetic variability among wild Arachis species and their accessions for tolerance to thermal stress. A wide variation was observed in leaf morphological characters such as colour, shape, hairiness, length and width and thickness (SLA). The temperature and time required for 50% leaf injury was worked out with limited number of genotypes and was found to be 54°C for 50 min. Among 36 genotypes (having SLA in the range of 66 and 161 cm² g⁻¹) screened, the inherent potential for cold as well as heat tolerance in terms of relative leaf injury (RI) was observed. Thus, based on RI-values, A. glabrata 11824 and A. paraguariensis 12042 were identified as heat-tolerant and cold-tolerant genotypes, respectively while A. appresipila 11786 was found to be susceptible to both heat and cold. Correlation between SLA and RI values for heat (r = 0.38, P < 0.05) and cold (r = 0.52, P < 0.05) tolerance was positive, indicating that thicker the leaf the lower the injury or higher the tolerance. Among six species and 13 accessions, comprising both heat-tolerant and heat- susceptible genotypes, the concentrations of various leaf chemical constituents such as total protein, phenols, sugars, reducing sugar, amino acids, proline, epicuticular wax load and chlorophyll varied significantly. The epicuticular wax load ranged between 1.1 and 2.5 mg dm² among 13 A.glabrata accessions. These accessions were categorized into two groups, i.e. high-wax (range: 2.0–2.5 mg dm²) and low-wax types (range: 1.1–1.6 mg dm²). The high-wax type showed a higher diffusion resistance (dr) as compared to low-wax type; though the transpiration rate (tr) in high-wax type was moderate (between 9.5 and 11.6 μg cm⁻² s⁻¹). Genetic variability in parameters such as canopy temperature, dr and tr was also distinct. The fully turgid leaves with relative water content ≥91%, showed leaf water potential (ψ_leaf) between −0.7 and −1.2 MPa. Results indicated that the plants with thicker leaves are better protected from heat injuries. Further, epicuticular wax load seems to help in maintaining stomatal regulation and leaf water relations, thus affording adaptation to wild Arachis species to thrive under water-limited environments. The sources of tolerance, as identified in this study, could be utilized to improve thermal tolerance of the groundnut cultivars by intra-specific hybridization, following either conventional breeding using embryo rescue techniques, if required or utilizing biotechnological tools.     

Associations of simple sequence repeats with quantitative trait variation including biotic and abiotic stress tolerance in Hordeum spontaneum

A total of 33 simple sequence repeats (SSRs) was analyzed in 52 genotypes of Hordeum spontaneum originally collected from two different soil types (Terra rossa and Basalt) at Tabigha in Israel. Data on the performance of developmental, morphological, and yield‐related traits under well‐watered control and water‐stress conditions were available from previous experimentation, and powdery mildew susceptibility was scored. Regression analyses based on SSR allele class differences were performed. Highly significant associations were detected at the SSR loci Bmac181 (on chromosome 4H) and Bmac316 (6H) for water ‐stress tolerance and powdery mildew resistance, respectively. The study shows that association mapping using SSRs and genetically diverse germplasm provides an effective means of relating genotypes to complex quantitative phenotypes.     

Impact of wheat-Leymus racemosus added chromosomes on wheat adaptation and tolerance to heat stress

Adaptation of wheat (Triticum aestivum L.) to high temperatures could be improved by introducing alien genes from wild relatives. We evaluated the responses of wheat-Leymus racemosus chromosome introgression lines to high temperature to determine their potentiality for developing improved wheat cultivars. Introgression lines and their parent Chinese Spring were evaluated in a growth chamber at the seedling stage and in the field at the reproductive stage in two heat-stressed environments in Sudan. Optimum and late planting were used to ensure exposure of the plants to heat stress at the reproductive stage. The results revealed the impact of several Leymus chromosomes in improving wheat adaptation and tolerance to heat. Three lines possessed enhanced adaptation, whereas two showed high heat tolerance. Two addition lines showed a large number of kernels per spike, while one possessed high yield potential. Grain yield was correlated negatively with the heat susceptibility index, days to heading and maturity and positively with kernel number per spike and triphenyl tetrazolium chloride assay under late planting. The findings suggest that these genetic stocks could be used as a bridge to introduce the valuable Leymus traits into a superior wheat genetic background, thus helping maximize wheat yield in heat-stressed environments.     

Application of biotechnology in breeding lentil for resistance to biotic and abiotic stress

Summary Lentil is a self-pollinating diploid (2n = 14 chromosomes) annual cool season legume crop that is produced throughout the world and is highly valued as a high protein food. Several abiotic stresses are important to lentil yields world wide and include drought, heat, salt susceptibility and iron deficiency. The biotic stresses are numerous and include: susceptibility to Ascochyta blight, caused by Ascochyta lentis; Anthracnose, caused by Colletotrichum truncatum; Fusarium wilt, caused by Fusarium oxysporum; Sclerotinia white mold, caused by Sclerotinia sclerotiorum; rust, caused by Uromyces fabae; and numerous aphid transmitted viruses. Lentil is also highly susceptible to several species of Orabanche prevalent in the Mediterranean region, for which there does not appear to be much resistance in the germplasm. Plant breeders and geneticists have addressed these stresses by identifying resistant/tolerant germplasm, determining the genetics involved and the genetic map positions of the resistant genes. To this end progress has been made in mapping the lentil genome and several genetic maps are available that eventually will lead to the development of a consensus map for lentil. Marker density has been limited in the published genetic maps and there is a distinct lack of co-dominant markers that would facilitate comparisons of the available genetic maps and efficient identification of markers closely linked to genes of interest. Molecular breeding of lentil for disease resistance genes using marker assisted selection, particularly for resistance to Ascochyta blight and Anthracnose, is underway in Australia and Canada and promising results have been obtained. Comparative genomics and synteny analyses with closely related legumes promises to further advance the knowledge of the lentil genome and provide lentil breeders with additional genes and selectable markers for use in marker assisted selection. Genomic tools such as macro and micro arrays, reverse genetics and genetic transformation are emerging technologies that may eventually be available for use in lentil crop improvement.     

Meta-analysis of QTL associated with tolerance to abiotic stresses in barley

A meta-analysis of quantitative trait loci (QTL) associated with tolerance to abiotic stresses in barley was carried out using results from 35 different experiments. ??MetaQTL?? software was used to project QTL positions on a reference map. Three hundred and thirty-seven QTL for traits associated with tolerance to abiotic stresses were included in this analysis which identified 79 metaQTL (MQTL) including 26 for drought, 11 for low temperature, 22 for salinity, 17 for water-logging, and 3 for mineral toxicity and deficiency. The distribution of MQTL was similar to that of the initial QTL. Many of these MQTL were located on chromosomes 2H (mainly for water-logging and drought) and 5H (mainly for salinity and low temperature). It inferred that chromosomes 2H and 5H were important for researches on barley abiotic tolerance, and the genes associated with abiotic stresses were concentrated relatively. As expected from trait correlations, 22.8?% of these MQTL displayed overlapping CIs. These overlapping regions were mainly on chromosomes 1H, 2H and 4H. The results indicated that the tolerance to diverse abiotic stresses were associated with each other in barley. Additionally, 67 candidate genes responsive to abiotic stresses were co-located with the abiotic-stress MQTL. Of them, a total of 55 had different conserved motifs. It inferred that the tolerance to abiotic stresses was contributed by multi-genes with diverse functions, though there might be some important genes associated with the tolerance to abiotic stresses in barley. Additionally, of these candidate genes, scsnp02622, scsnp01644 and scsnp19641 could be better for further studies of abiotic stresses tolerance.     

Biotechnology approaches to overcome biotic and abiotic stress constraints in legumes

Summary Biotic and abiotic stresses cause significant yield losses in legumes and can significantly affect their productivity. Biotechnology tools such as marker-assisted breeding, tissue culture, in vitro mutagenesis and genetic transformation can contribute to solve or reduce some of these constraints. However, only limited success has been achieved so far. The emergence of “omic” technologies and the establishment of model legume plants such as Medicago truncatula and Lotus japonicus are promising strategies for understanding the molecular genetic basis of stress resistance, which is an important bottleneck for molecular breeding. Understanding the mechanisms that regulate the expression of stress-related genes is a fundamental issue in plant biology and will be necessary for the genetic improvement of legumes. In this review, we describe the current status of biotechnology approaches in relation to biotic and abiotic stresses in legumes and how these useful tools could be used to improve resistance to important constraints affecting legume crops.     

Breeding methods and selection indices for improved tolerance to biotic and abiotic stresses in cool season food legumes

The objective of breeding for stress tolerance is to improve productivity for a target level of stress. If tolerance is viewed as resistance to change in productivity with increasing stress, productivity under stress depends not only on stress tolerance, but also on maximum productivity. Index selection theory indicates that selection in non-stress environments will be more effective than direct selection for productivity under stress whenever the correlation between the two types of environments exceeds the heritability of productivity under stress. With high genetic correlation, selection should be conducted within a level of stress that maximizes heritability. In cases where heritability under non-stress is much higher than under stress, an index combining data from stress and non-stress environments is expected to be more efficient than selection based on evaluation only within stress environments.Secondary traits will be useful in breeding for productivity under stress whenever they have high heritability and high genetic correlation with productivity under stress. For some abiotic stresses and many biotic stresses, heritability will be highest in the presence of stress and indirect or index selection will be of limited value.     

Heat stress in crop plants: its nature,impacts and integrated breeding strategies to improve heat tolerance

Increasing severity of high temperature worldwide presents an alarming threat to the humankind. As evident by massive yield losses in various food crops, the escalating adverse impacts of heat stress (HS) are putting the global food as well as nutritional security at great risk. Intrinsically, plants respond to high temperature stress by triggering a cascade of events and adapt by switching on numerous stress‐responsive genes. However, the complex and poorly understood mechanism of heat tolerance (HT), limited access to the precise phenotyping techniques, and above all, the substantial G × E effects offer major bottlenecks to the progress of breeding for improving HT. Therefore, focus should be given to assess the crop diversity, and targeting the adaptive/morpho‐physiological traits while making selections. Equally important is the rapid and precise introgression of the HT‐related gene(s)/QTLs to the heat‐susceptible cultivars to recover the genotypes with enhanced HT. Therefore, the progressive tailoring of the heat‐tolerant genotypes demands a rational integration of molecular breeding, functional genomics and transgenic technologies reinforced with the next‐generation phenomics facilities.     

Oxidative stress indicators as selection tools for salt tolerance

Oxidative stress indicators, malondialdehyde (MDA) accumulation and chlorophyll retention, were tested as tools for salt‐tolerance screening in Chloris gayana (Kunth), a forage grass exhibiting inter‐ and intra‐cultivar variability in the response to salt stress. Three types of experimental system were compared, salt shock, gradual salt treatment and leaf segments floated on control and saline solutions. Results followed the same trend in the three systems, but leaf segments or gradually salinized plants are the most convenient. Lower mean MDA and higher mean chlorophyll content were found under salinity in cv. ‘Katambora’, which is considered, from previous field trials, to be more salt tolerant than cv. ‘Boma’. Nevertheless, chlorophyll content did not give consistent results in other tests and it is not recommended for selection purposes. Within cv. ‘Boma’, clones rating higher in a salt tolerance evaluation under greenhouse conditions had lower MDA content under salinity and lower mean MDA ratios between salt‐treated and control samples. The survival of 80‘Boma’ plants under increasing NaCl concentration was assessed and MDA was measured in the leaves of very plant before the final NaCl level was reached, to assess whether it was predictive of survival capability. MDA values were significantly lower in the group surviving longer, thus, a selection based on low MDA values would have enriched the original population with more tolerant individuals.     

铝胁迫对小麦的影响及小麦的抗铝机理

简要概述了铝胁迫对小麦的影响及小麦的抗铝机理。小麦根主要吸收单核铝且主要积累于0～5mm根尖处。在铝胁迫下，小麦细胞对Ca^2＋和Mg^2＋等元素的净吸收量减少，细胞中游离脯氨酸含量增加，液泡膜中Ca^2＋-ATP酶、H^＋-ATP酶活性和磷脂的含量下降。线粒体中Ca^2＋-ATP酶、H^＋-ATP酶和焦磷酸酶活性也下降。小麦通过增加有机酸、磷酸、蛋白质、已糖、糖醛酸的分泌，增加钙、磷的含量，提高根际pH覆保持低水平的渗透势来抵抗铝毒。     

Breeding white clover for tolerance to low temperature and grazing stress

Summary Low temperature and grazing are the two major stresses limiting white clover yield in mixed swards grown in the marginal areas of the UK. White clover has traditionally been used to improve productivity in such areas but is increasingly being used to reduce input costs in more productive areas. Considerable genetic variation exists in characters associated with cold tolerance, low temperature growth and grazing tolerance. This paper describes recent progress in developing varieties tolerant to three major stresses and which give greater and more reliable production.     

红麻非生物逆境胁迫响应基因HCWRKY71表达分析及转化拟南芥

WRKY转录因子在植物响应非生物逆境胁迫过程中具有重要的调控作用。本研究根据红麻转录组unigene序列(CL3883.Contig4),设计引物进行PCR扩增,经sanger测序获得全长为957 bp的HcWRKY71基因cDNA序列。该基因开放读码框为957 bp,编码1个含有318个氨基酸的蛋白,具有1个WRKY功能保守结构域,属于II类WRKY转录因子。在盐胁迫下,HcWRKY71基因表达量随NaCl溶液浓度升高而增加;在干旱胁迫下,HcWRKY71基因表达量随干旱胁迫时间的增加呈现先下降再上升然后再下降的趋势;在重金属镉胁迫下,HcWRKY71基因表达量随CdCl2溶液浓度的增加而降低。表明该基因表达受盐、干旱和重金属镉胁迫的诱导。利用农杆菌介导花序浸染法将该基因转化拟南芥发现,HcWRKY71基因提高了转基因拟南芥幼苗的耐盐性,这为进一步研究HcWRKY71基因的耐逆机制奠定了坚实的基础。     

谷子PEPC基因的鉴定及其对非生物逆境的响应特性

磷酸烯醇式丙酮酸羧化酶(phosphoenolpyruvatecarboxylase,PEPC)是C4植物光合作用关键酶,并在植物多种代谢途径及逆境信号应答过程中起重要作用。本研究通过序列比对,从谷子基因组中筛选出6个SiPEPC候选基因。SiPEPC蛋白特征参数相似度很高,序列非常保守,都含有PEPC基因特征功能域PEPcase Motif。SiPEPC成员主要被定位在细胞质、细胞核和线粒体。在SiPEPC成员启动子序列中含大量有光、激素、逆境以及其他生长调控相关的顺式应答元件。苗期逆境qRT-PCR表达谱分析表明,5个SiPEPC基因(SiPEPC1、SiPEPC2、SiPEPC3、SiPEPC5、SiPEPC6)不同程度受ABA、PEG、高盐和低温诱导表达,表明其参与了苗期对非生物逆境的响应。5个SiPEPC基因表达量在正常生长条件下随着谷子的生长呈增强趋势,且在不同生育时期干旱胁迫下明显增加,表明其参与了拔节、抽穗、灌浆期的干旱胁迫应答。拔节期弱光可诱导5个SiPEPC基因的表达,而在拔节期中等强度光照以及抽穗期和灌浆期的中等光照和弱光照下表达量均急剧降低,揭示光照强度严重影响...     

Salt stress tolerance in heterogeneous populations of barley

Summary Four highly heterogeneous populations of barley were assayed for salt stress tolerance at the time of seed germination. Three of these, Davis, Dryland and Irrigated, were derived originally from a population called Composite Cross XXI and propagated until F₁₇ under contrasting conditions. The fourth, Composite Cross XXX-C, was of relatively recent (F₅) origin. This population showed the highest germinability in nutrient solution salinated with sodium chloride. Davis was poorer in salt tolerance than CC XXX-C but better than Dryland and Irrigated. Dryland and Irrigated, propagated for 14 successive genrations under contrasting levels of soil moisture, showed no significant difference in salt stress tolerance. In several hundred random samples of lines developed from these two populations, ear row number, lemma awn texture, seed weight and yield showed non-random associations with salt tolerance. These associations might be gainfully utilized in barley breeding for salt stress tolerance.     

Selection of young barley plants for tolerance to high soil acidity in relation with some agronomic characteristics of mature plants

Summary In four segregating populations of barley, grown in a naturally highly acid soil good plants and less vigorous ones were marked in a 3–4 leaf stage of development.Some agronomic characters of mature plants were studied in relation with the level of tolerance to high soil acidity of their progenies when tested in a seedling stage in a laboratory method.As compared with the good groups the reductions proved highly significant viz. 60% for mean number of tillers per plant, 25% for average culm length, 35% for mean number of kernels per spike and about 12% for 1000-grain weight.On the average more than 95% of the good plants showed a high level of tolerance to high soil acidity, indicating a close correlation between low-pH tolerance and agronomic performance.Although a rather high percentage (40%) of the progenies among the plants marked as bad was found to be tolerant, those lines showed a poor general performance.This goes to show that selection for tolerance to high soil acidity before flowering is an advisable procedure, without the risk of including sensitive lines or losing otherwise valuable genotypes.This study was carried out during a fellowship of the Spanish Ministry of Agriculture.     

Expression and inheritance of tolerance to waterlogging stress in wheat

Approximately 10 million hectares of wheat (Triticum aestivum L.) globally experience medium toserious waterlogging. The inheritance of waterloggingtolerance was determined in reciprocal crosses ofthree tolerant (Prl/Sara, Ducula and Vee/Myna), andtwo sensitive (Seri-82 and Kite/Glen) spring breadwheat lines. Parents, F1, F2, F3, and backcrossgenerations were studied under field conditions in Cd.Obregon, State of Sonora, Mexico. Flooding was appliedwhen plants were at the three-leaf and first-internodestages. Basins were drained after 40 days of flooding.Leaf chlorosis was used as a measure of waterloggingtolerance. The sensitive by sensitive cross, Seri-82 × Kite/Glen, showed the highest mean values forpercentage leaf chlorosis and area under chlorosisprogress curve (AUCPC), and the lowest mean values forplant height, biomass, grain yield, and kernel weight.The F3 of the cross between the two tolerant parentsDucula and Vee/Myna had the lowest mean values forpercentage leaf chlorosis and AUCPC, and the highestmean values for plant height, biomass, and grainyield. The expression of waterlogging tolerance wasnot influenced by a maternal effect. The F1 hybridswere intermediate for leaf chlorosis, indicating thattolerance was additive. Quantitative analysis alsoindicated that additive gene effects mainly controlledwaterlogging tolerance in these crosses. Segregationratios of F3 lines indicated that up to four genescontrolled waterlogging tolerance in these crosses,with two genes adequate to provide significanttolerance. Early-generation selection for tolerancewould be effective in these populations.