Stacking Tolerance to Drought and Resistance to a Parasitic Weed in Tropical Hybrid Maize for Enhancing Resilience to Stress Combinations

Overexpression of BnMYBL2-1 improves plant drought tolerance via the ABA-dependent pathway

Drought stress is a major environmental challenge that poses considerable threats to crop survival and growth. Previous research has indicated anthocyanins play a crucial role in alleviating oxidative damage, photoprotection, membrane stabilization, and water retention under drought stress. However, the presence of MYBL2 (MYELOBBLASTOSIS LIKE 2), an R3-MYB transcription factor (TF) which known to suppress anthocyanin biosynthesis. In this study, four BnMYBL2 members were cloned from Brassica napus L, and BnMYBL2-1 was overexpressed in Triticum aestivum L (No BnMYBL2 homologous gene was detected in wheat). Subsequently, the transgenic wheat lines were treated with drought, ABA and anthocyanin. Results showed that transgenic lines exhibited greater drought tolerance compared to the wild-type (WT), characterized by improved leaf water content (LWC), elevated levels of soluble sugars and chlorophyll, and increased antioxidant enzyme activity. Notably, transgenic lines also exhibited significant upregulation in abscisic acid (ABA) content, along with the transcriptional levels of key enzymes involved in ABA signalling under drought. Results also demonstrated that BnMYBL2-1 promoted the accumulation of ABA and anthocyanins in wheat. Overall, the study highlights the positive role of BnMYBL2-1 in enhancing crop drought tolerance through ABA signalling and establishes its close association with anthocyanin biosynthesis. These findings offer valuable insights for the development of drought-resistant crop varieties and enhance the understanding of the molecular mechanisms underlying plant responses to drought stress.

Assessment of the impact of crop management strategies on the yield of early-maturing maize varieties in the drylands of Niger Republic: Application of the DSSAT-CERES-Maize model

Maize is increasingly becoming important in Niger for use as food and feed. Production is however, faced with several abiotic and biotic constraints. Researchers have developed early-maturing maize varieties that are tolerant to drought, the parasitic weed Striga hermonthica and diseases that fit into the short growing production environment. The evaluation and deployment of these varieties would, however, involve costly and time-consuming field trials across the maize production zones of the country. The CERES-Maize model was applied to assess the performance of two early-maturing maize varieties under varying planting windows and nitrogen application in three agroecological zones of the country. The model was calibrated with datasets collected from field trials conducted under optimal conditions (supplementary irrigation and full nutrient supply) at three locations in northern Nigeria. The model was validated with independent data set obtained from field trials conducted in 2020 and 2021 at 4 locations in the Republic of Niger under rainfed conditions. For each variety the treatments were five nitrogen (N) rates (0, 30, 60, 90 and 120 kg ha^-1). The results from model calibration and validation revealed that the model accurately reproduced the observed value for days to flowering, physiological maturity, aboveground dry biomass and grain yield with low nRMSE (0.4-12.7%) and high d-index (0.70-0.99) for both varieties. The long-term simulation results (1985-2020) showed that the maize performance was dependent on location, planting window and nitrogen rates. The variety 2014 TZE-Y yielded higher than Brico in all locations for all treatments because it takes longer to mature and accumulate higher dry matter and have higher number of kernels. Simulated yields were generally higher in the Sudan savanna agroecological zone than in the other zones because of higher rainfall and higher clay content of the soil in this zone. The response to N application was influenced by planting window in each agroecological zone. With the exception of two sites, grain yield declined with planting beyond July 14 (PW3) and response to N was not significant beyond this date in the Sudan savanna agroecological zone. Grain yield declined with planting beyond July 7 in the Sahel and Sudan Sahel agroecological zones. There was no further response to N beyond 30 and 60 kg N ha^-1 when planting is delayed beyond July 7 in the Sahel and Sahel-Sudan agroecological zones, respectively.

Estimating genetic gains for tolerance to stress combinations in tropical maize hybrids

Maize is a strategic food crop in sub-Saharan Africa. However, most maize growing tropical savannas particularly in West and Central African experience the occurrence of frequent droughts and Striga infestation, resulting in 30-100% yield losses. This production zones need maize cultivars that combine tolerance to the two stresses. IITA in collaboration with national partners has thus employed a sequential selection scheme to incorporate both drought tolerance and Striga resistance in topical maize hybrids using reliable screening protocols. The main objective of the present study was therefore to use grain yield and other agronomic traits recorded in regional collaborative hybrid trials conducted for 8 years under manged stressful and non-stressful conditions and across rainfed field environments to estimate genetic gains in grain yields using mixed model analyses. The results showed significant (p < 0.05) annual yield gains of 11.89 kg ha^-1 under manged drought stress (MDS) and 86.60 kg ha^-1 under Striga infestation (STRIN) with concomitant yield increases of 62.65 kg ha^-1 under full irrigation (WW), 102.44 kg ha^-1 under Striga non-infested (STRNO) conditions and 53.11 kg ha^-1 across rainfed field environments. Grain yield displayed significant but not strong genetic correlation of 0.41 ± 0.07 between MDS and STRIN, indicating that gene expression was not consistent across the two stress conditions. Furthermore, grain yield recorded in MET had significant moderate genetic correlations of 0.58 ± 0.06 and 0.44 ± 0.07It with MDS and STRIN, respectively. These results emphasize the need to screen inbred linens under both stress conditions to further enhance the rate of genetic gain in grain yield in hybrids for areas where the two stresses co-occur. Nonetheless, this study demonstrated that the sequential selection scheme has been successful in generating hybrids with dependable yields that can reduce chronic food deficits in rural communities experiencing simultaneous presence of drought and S. hermonthica infestation in their production fields.

Paclobutrazol improves surface water use efficiency by regulating allometric trait behavior in maize

Paclobutrazol (PBZ) role in drought management of maize is least understood. In maize, root traits are linked with surface water management. Over three years, early and terminal deficit irrigation (EDI and TDI) with or without PBZ were imposed on DKC-9144 and PG-2475 maize varieties. Several allometric parameters viz. stem height, stem diameter, leaf area and root traits along with physiological processes were measured. Implication of these parameters in the management of soil surface irrigation in terms of water use efficiency (WUE) was demonstrated in maize. Increased number of lateral roots and root number density in DKC-9144 provided more surface area for water absorption for better management of EDI. Root growth rates showed a similar pattern with root length, root surface areas, and root numbers in EDI. Elevated expressions of ZmRTCL, ZmRTCS and ZmARF34 in EDI and EDI plus PBZ were associated with seminal roots and root laterals initiation. Under TDI alone or in combination with PBZ, root lengths (BRL, CRL, SRL) and root surface areas varied in DKC-9144 and PG-2475 over control. Furthermore, correlation analysis showed that decrease in WUE under TDI was significantly associated with a reduction in stem thickness and leaf surface area. For WUE_N in TDI and PBZ plus TDI, structural equation modelling proposed, brace root surface area (BRSA_N) as a positive contributor, while a negative contributor was seminal root surface area (SRSA_N). Present study explained the importance of specific root traits and their association with other allometric parameters for improving WUE in DKC-9144 variety of maize and the crop in general.

Optimizing use of U.S. Ex-PVP inbred lines for enhancing agronomic performance of tropical Striga resistant maize inbred lines

BACKGROUND

Temperate maize inbred lines with expired Plant Variety Protection Act certificates (Ex-PVP) are potential sources of desirable alleles for tropical germplasm improvement. Up to now, the usefulness of the Ex-PVP inbred lines as a potential source of novel beneficial alleles for Striga hermonthica resistance breeding to enhance genetic gain in tropical maize has not been reported.

RESULTS

This study was thus conducted to characterize the combining ability of 24 Ex-PVP inbred lines in crosses with two tropical Striga resistant inbred testers under Striga-infested and non-infested conditions and across three locations for 2 years. Many testcrosses between Ex-PVP inbred lines and the first tester (T1) produced competitive or significantly higher grain yields compared to the hybrid between the two resistant testers under Striga infested and non-infested conditions and across multiple test locations. Also, most of the testcrosses with positive heterosis for grain yield and negative heterosis for Striga damage and emerged Striga count involved T1 as a tester. Our study identified six Ex-PVP inbred lines with positive GCA effects for grain yield under Striga infested and non-infested conditions and across multiple test locations. Amongst these, inbred lines HB8229-1 and WIL900-1 also displayed negative GCA effects for emerged Striga count and Striga damage rating. The inbred line HB8229-1 showed positive SCA effects for grain yield with T2, whereas WIL900-1 had positive SCA effects for grain yield with T1. Over 70% of the Ex-PVP inbred lines were consistently assigned to specific heterotic groups using yield-based classifying methods (mean grain yield and SCA effects).

CONCLUSIONS

These results could facilitate systematic introgression of the Ex-PVP inbred lines into the existing Striga resistant heterotic groups in IITA. The Ex-PVP inbred lines with positive GCA effects and producing high grain yields in hybrid combinations could be useful parents for enhancing Striga resistance and agronomic performance of tropical maize hybrids.

Walnut ethylene response factor JrERF2-2 interact with JrWRKY7 to regulate the GSTs in plant drought tolerance

Juglans regia is a world-famous woody oil plant, whose yield and quality are affected by drought stress. Ethylene-responsive factors (ERFs) play vital role in plant stress response. In current study, to comprehend the walnut molecular mechanism of drought stress response, an ERF transcription factor was clarified from J. regia (JrERF2-2) and its potential function mechanism to drought was clarified. The results showed that JrERF2-2 could be induced significantly by drought. The transgenic Arabidopsis over-expression of JrERF2-2 displayed enhanced growth, antioxidant enzyme vitalities, reactive oxygen species scavenging and proline produce under drought stress. Especial the glutathione-S-transferase (GST) activity and most GST genes' transcription were elevated obviously. Yeast one-hybrid (Y1H) and co-transient expression (CTE) methods revealed that JrERF2-2 could recognize JrGST4, JrGST6, JrGST7, JrGST8, and JrGSTF8 by binding to GCC-box, and recognize JrGST11, JrGST12, and JrGSTN2 by binding to DRE motif. Meanwhile, the binding activity was strengthened by drought stress. Moreover, JrERF2-2 could interact with JrWRKY7 to promote plant drought tolerance; JrWRKY7 could also distinguish JrGST4, JrGST7, JrGST8, JrGST11, JrGST12, and JrGSTF8 via binding to W-Box motif. These results suggested that JrERF2-2 could effectively improve plant drought tolerance through interacting with JrWRKY7 to control the expression of GSTs. JrERF2-2 is a useful plant representative gene for drought response in molecular breeding.

Genotypic Variation in Cultivated and Wild Sorghum Genotypes in Response to Striga hermonthica Infestation

Striga hermonthica is the most important parasitic weed in sub-Saharan Africa and remains one of the most devastating biotic factors affecting sorghum production in the western regions of Kenya. Farmers have traditionally managed Striga using cultural methods, but the most effective and practical solution to poor smallholder farmers is to develop Striga-resistant varieties. This study was undertaken with the aim of identifying new sources of resistance to Striga in comparison with the conventional sources as standard checks. We evaluated 64 sorghum genotypes consisting of wild relatives, landraces, improved varieties, and fourth filial generation (F₄) progenies in both a field trial and a pot trial. Data were collected for days to 50% flowering (DTF), dry panicle weight (DPW, g), plant height (PH, cm), yield (YLD, t ha^-1), 100-grain weight (HGW, g), overall disease score (ODS), overall pest score (OPS), area under Striga number progress curve (ASNPC), maximum above-ground Striga (NS_max), and number of Striga-forming capsules (NSFC) at relevant stages. Genetic diversity and hybridity confirmation was determined using Diversity Arrays Technology sequencing (DArT-seq). Residual heterosis for HGW and NS_max was calculated as the percent increase or decrease in performance of F₄ crossover midparent (MP). The top 10 best yielding genotypes were predominantly F₄ crosses in both experiments, all of which yielded better than resistant checks, except FRAMIDA in the field trial and HAKIKA in the pot trial. Five F₄ progenies (ICSVIII IN × E36-1, LANDIWHITE × B35, B35 × E36-1, F6YQ212 × B35, and ICSVIII IN × LODOKA) recorded some of the highest HGW in both trials revealing their stability in good performance. Three genotypes (F6YQ212, GBK045827, and F6YQ212xB35) and one check (SRN39) were among the most resistant to Striga in both trials. SNPs generated from DArT-seq grouped the genotypes into three major clusters, with all resistant checks grouping in the same cluster except N13. We identified more resistant and high-yielding genotypes than the conventional checks, especially among the F₄ crosses, which should be promoted for adoption by farmers. Future studies will need to look for more diverse sources of Striga resistance and pyramid different mechanisms of resistance into farmer-preferred varieties to enhance the durability of Striga resistance in the fields of farmers.

Genetic dissection of Striga hermonthica (Del.) Benth. resistance via genome-wide association and genomic prediction in tropical maize germplasm

KEY MESSAGE

Genome-wide association revealed that resistance to Striga hermonthica is influenced by multiple genomic regions with moderate effects. It is possible to increase genetic gains from selection for Striga resistance using genomic prediction. Striga hermonthica (Del.) Benth., commonly known as the purple witchweed or giant witchweed, is a serious problem for maize-dependent smallholder farmers in sub-Saharan Africa. Breeding for Striga resistance in maize is complicated due to limited genetic variation, complexity of resistance and challenges with phenotyping. This study was conducted to (i) evaluate a set of diverse tropical maize lines for their responses to Striga under artificial infestation in three environments in Kenya; (ii) detect quantitative trait loci associated with Striga resistance through genome-wide association study (GWAS); and (iii) evaluate the effectiveness of genomic prediction (GP) of Striga-related traits. An association mapping panel of 380 inbred lines was evaluated in three environments under artificial Striga infestation in replicated trials and genotyped with 278,810 single-nucleotide polymorphism (SNP) markers. Genotypic and genotype x environment variations were significant for measured traits associated with Striga resistance. Heritability estimates were moderate (0.42) to high (0.92) for measured traits. GWAS revealed 57 SNPs significantly associated with Striga resistance indicator traits and grain yield (GY) under artificial Striga infestation with low to moderate effect. A set of 32 candidate genes physically near the significant SNPs with roles in plant defense against biotic stresses were identified. GP with different cross-validations revealed that prediction of performance of lines in new environments is better than prediction of performance of new lines for all traits. Predictions across environments revealed high accuracy for all the traits, while inclusion of GWAS-detected SNPs led to slight increase in the accuracy. The item-based collaborative filtering approach that incorporates related traits evaluated in different environments to predict GY and Striga-related traits outperformed GP for Striga resistance indicator traits. The results demonstrated the polygenic nature of resistance to S. hermonthica, and that implementation of GP in Striga resistance breeding could potentially aid in increasing genetic gain for this important trait.

Evolutionary Plant Breeding as a Response to the Complexity of Climate Change

Climate change is one of the processes that have already overstepped the safe planetary boundaries, together with the rate of biodiversity loss and human interference with the nitrogen and phosphorus cycles. The three processes are related to agriculture and, as such, to both food safety and food security, and ultimately to human health. Adaptation to climate change is a difficult breeding objective because of its complexity, its unpredictability, and its location specificity. However, one strategy exists, which is based on a more dynamic use of agrobiodiversity in agriculture through the cultivation of evolutionary populations. In this review, we show how the translation into agricultural practice of nearly 100 years of research on evolutionary populations and mixtures is able to address the complexity of climate change while stabilizing yield, decreasing the use of most agrochemicals, thus reducing emissions and producing healthy food.