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Breeding wheat for drought tolerance: Progress and technologies

Abstract Recurrent drought associated with climate change is among the principal constraints to global productivity of wheat ( Triticum aestivum (L.) and T. turgidum (L.)). Numerous efforts to mitigate drought through breeding resilient varieties are underway across the world. Progress is, however, hampered because drought tolerance is a complex trait that is controlled by many genes and its full expression is affected by the environment. Furthermore, wheat has a structurally intricate and large genome. Consequently, breeding for drought tolerance requires the integration of various knowledge systems and methodologies from multiple disciplines in plant sciences. This review summarizes the progress made in dry land wheat improvement, advances in knowledge, complementary methodologies, and perspectives towards breeding for drought tolerance in the crop to create a coherent overview. Phenotypic, biochemical and genomics-assisted selection methodologies are discussed as leading research components used to exploit genetic variation. Advances in phenomic and genomic technologies are highlighted as options to circumvent existing bottlenecks in phenotypic and genomic selection, and gene transfer. The prospects of further integration of these technologies with other omics technologies are also provided.

Screening of Bread Wheat Genotypes for Drought Tolerance Using Phenotypic and Proline Analyses

Drought stress is one of the leading constraints to wheat (Triticum aestivum L.) production globally. Breeding for drought tolerance using novel genetic resources is an important mitigation strategy. This study aimed to determine the level of drought tolerance among diverse bread wheat genotypes using agronomic traits and proline analyses and to establish correlation of proline content and agronomic traits under drought-stress conditions in order to select promising wheat lines for breeding. Ninety-six diverse genotypes including 88 lines from the International Maize and Wheat Improvement Center (CIMMYT)s heat and drought nurseries, and eight local checks were evaluated under greenhouse and field conditions during 2014/15 and 2015/16 making four testing environments. The following phenotypic traits were collected after stress imposed during the heading to anthesis period: the number of days to heading (DTH), days to maturity (DTM), productive tiller number (TN), plant height (PH), spike length (SL), spikelet per spike (SPS), kernels per spike (KPS), thousand kernel weight (TKW) and grain yield (GY) and proline content (PC). Analysis of variance, Pearsons correlation coefficient, principal component and stress tolerance index were calculated. Genotypes with high yield performance under stressed and optimum conditions maintained high values for yield components. Proline content significantly increased under stress, but weakly correlated with agronomic traits under both optimal and water limited conditions. The positive correlation observed between grain yield and proline content under-drought stress conditions provides evidence that proline accumulation might ultimately be considered as a tool for effective selection of drought tolerant genotypes. The study selected 12 genotypes with high grain yields under drought stressed conditions and favorable adaptive traits useful for breeding.

Genome-wide association analysis of agronomic traits in wheat under drought-stressed and non-stressed conditions

This study determined the population structure and genome-wide marker-trait association of agronomic traits of wheat for drought-tolerance breeding. Ninety-three diverse bread wheat genotypes were genotyped using the Diversity Arrays Technology sequencing (DArTseq) protocol. The number of days-to-heading (DTH), number of days-to-maturity (DTM), plant height (PHT), spike length (SPL), number of kernels per spike (KPS), thousand kernel weight (TKW) and grain yield (GYLD), assessed under drought-stressed and non-stressed conditions, were considered for the study. Population structure analysis and genome-wide association mapping were undertaken based on 16,383 silico DArTs loci with < 10% missing data. The population evaluated was grouped into nine distinct genetic structures. Inter-chromosomal linkage disequilibrium showed the existence of linkage decay as physical distance increased. A total of 62 significant (P < 0.001) marker-trait associations (MTAs) were detected explaining more than 20% of the phenotypic variation observed under both drought-stressed and non-stressed conditions. Significant (P < 0.001) MTA event(s) were observed for DTH, PHT, SPL, SPS, and KPS; under both stressed and non-stressed conditions, while additional significant (P < 0.05) associations were observed for TKW, DTM and GYLD under non-stressed condition. The MTAs reported in this population could be useful to initiate marker-assisted selection (MAS) and targeted trait introgression of wheat under drought-stressed and non-stressed conditions, and for fine mapping and cloning of the underlying genes and QTL.

Resistance breeding and biocontrol of Striga asiatica (L.) Kuntze in maize: a review

ABSTRACT Purpose: The aims of this article are to highlight pre-breeding procedures for identifying primary sources of Striga-resistance genes and to summarize complimentary breeding techniques that enhance partial resistance of maize varieties against Striga species. Materials and methods: The paper presented a comprehensive account of Striga screening and controlling techniques and highlighted the potential of integrating partial resistance with FOS to boost maize production and productivity in SSA. Results: Striga infestation is a major constraint to maize production and productivity in Sub-Saharan Africa (SSA). A lack of Striga-resistant maize varieties and the limited adoption of other control methods hinder effective and integrated control of the parasitic weed in maize and related cereal crops globally. Genetic resistance of maize should be complemented with the use of Fusarium oxysporum f.sp. strigea (FOS), a biocontrol agent known to suppress Striga. Conclusions: A combined use of genetic resistance and FOS has remained largely unutilized in controlling Striga in Africa. A combination of conventional and molecular Striga-resistance breeding tools as well as the use of FOS are promising methods to effectively control Striga in SSA.

Genetic resources and breeding methodologies for improving drought tolerance in wheat

ABSTRACT Yield gains from rain-fed wheat (Triticum aestivum L.) production, particularly in areas experiencing intermittent and terminal dry spells, can be realized through integrated breeding with promising genetic and genomic resources using appropriate methodologies. This enables targeted recombination of novel genes for drought tolerance and selection of desirable genotypes. Continuous exploration of new sources of genetic variation and introgression of suitable genes into elite drought-susceptible genotypes, including via transgenic approaches, and the use of genome editing could offer exciting future prospects in acquiring drought-tolerant wheat genotypes. This review highlights the available genetic resources, the major wheat genebanks and databases, as well as the breeding methodologies for drought tolerance in wheat, including prebreeding, conventional breeding, hybrid breeding, and genomics-assisted breeding. The potential of genetic modification through the transgenic and genome-editing approaches is also discussed. Emphasis is placed on how best these breeding methods can be brought together to develop strategies aimed at improving drought tolerance in wheat.

Sugarcane production under smallholder farming systems: Farmers preferred traits, constraints and genetic resources

Abstract Smallholder sugarcane production sector is under researched and underdeveloped with limited industrial link and support. The objectives of this study were to assess the current state of sugarcane production, farmers’ perceived production constraints and preferred traits, and to collect germplasm grown by smallholder farmers in southern Ethiopia for strategic breeding and conservation. The study was conducted across 16 administrative zones, 28 districts and 56 peasant associations involving 560 smallholder sugarcane growers in southern Ethiopia using a participatory rural appraisal approach. Sugarcane genetic resources were collected through structured sampling. Findings from this study indicated that monocropping was identified as the predominant sugarcane farming system. Respondent farmers prioritized drought tolerance (21%), increased cane yield (20%), early maturity (18%), marketability (17%), and high biomass (14%) as the top preferred traits of sugarcane. Ninety diverse sugarcane landraces were collected from homesteads of smallholder farmers. Findings from this study would serve as baseline information towards sugarcane research and development emphasizing the constraints and preferences of smallholder sugarcane growers in Ethiopia or similar agro-ecologies. This is the first study to report farmers preferred traits and constraints, and genetic resources of sugarcane under smallholder farming systems in Ethiopia.

Agro-morphological characterisation and selection of sorghum landraces

ABSTRACT Sorghum [Sorghum bicolor (L.) Moench] grown under rain-fed conditions is frequently affected by drought stress at different stages, resulting in reduced grain and biomass yield. The aim of this study was to characterise sorghum landraces and to select farmer-preferred medium-maturing genotypes under rainfed and irrigated conditions. Hundred and ninety-six sorghum accessions were evaluated at Kobo site of the Sirinka Agricultural Research Center in 2014/2015 in Ethiopia. Data collected from 14 traits were subjected to analysis of variance, cluster analysis, Pearson’s correlation coefficient analysis, path coefficient analysis and principal component analysis (PCA). Significant genotypic differences (p < 0.05) were observed. Medium-maturing and drought tolerant sorghum genotypes including E-72457, E-72438, E-72435, E-206214, E-72449, E-75460 and E-75458 with superior agronomic performance were recommend for large-scale production or for further breeding. The genotypes evaluated under rain-fed and irrigated conditions were grouped into five and six clusters, respectively, representing varied different heterotic groups. Grain yield had significant and positive correlation with yield-related traits assessed under the two test conditions. Further, path coefficient analysis revealed that days to maturity under rainfed condition and harvest index under irrigated condition had the highest positive direct effects on grain yield, therefore can be targets for direct selection. Overall, there was marked genetic diversity among the tested genotypes. Suitable medium-maturing farmers-preferred accessions selected from the study will be useful for effective breeding for drought tolerance and medium-maturity.