Population genomic analysis reveals key genetic variations and the driving force for embryonic callus induction capability in maize

Genetic transformation has been an effective technology for improving the agronomic traits of maize.However,it is highly reliant on the use of embryonic callus(EC)and shows a serious genotype dependence.In this study,we performed genomic sequencing for 80 core maize germplasms and constructed a high-density genomic variation map using our newly developed pipeline(MQ2Gpipe).Based on the induction rate of EC(REC),these inbred lines were categorized into three subpopulations.The low-REC germplasms displayed more abundant genetic diversity than the high-REC germplasms.By integrating a genome-wide selective signature screen and region-based association analysis,we revealed 95.23 Mb of selective regions and 43 REC-associated variants.These variants had phenotypic variance explained values ranging between 21.46 and 49.46%.In total,103 candidate genes were identified within the linkage disequilibrium regions of these REC-associated loci.These genes mainly participate in regulation of the cell cycle,regulation of cytokinesis,and other functions,among which MYB15 and EMB2745 were located within the previously reported QTL for EC induction.Numerous leaf area-associated variants with large effects were closely linked to several REC-related loci,implying a potential synergistic selection of REC and leaf size during modern maize breeding.

Genome assembly of the maize inbred line A188 provides a new reference genome for functional genomics

The current assembled maize genomes cannot represent the broad genetic diversity of maize germplasms.Acquiring more genome sequences is critical for constructing a pan-genome and elucidating the linkage between genotype and phenotype in maize.Here we describe the genome sequence and annotation of A188,a maize inbred line with high phenotypic variation relative to other lines,acquired by single-molecule sequencing and optical genome mapping.We assembled a 2210-Mb genome with a scaffold N50 size of 11.61 million bases(Mb),compared to 9.73 Mb for B73 and 10.2 Mb for Mo17.Based on the B73_Ref Gen_V4 genome,295 scaffolds(2084.35 Mb,94.30%of the final genome assembly)were anchored and oriented on ten chromosomes.Comparative analysis revealed that~30%of the predicted A188 genes showed large structural divergence from B73,Mo17,and W22 genomes,which causes high protein divergence and may lead to phenotypic variation among the four inbred lines.As a line with high embryonic callus(EC)induction capacity,A188 provides a convenient tool for elucidating the molecular mechanism underlying the formation of EC in maize.Combining our new A188 genome with previously reported QTL and RNA sequencing data revealed eight genes with large structural variation and two differentially expressed genes playing potential roles in maize EC induction.

Transcriptomic responses in resistant and susceptible maize infected with Fusarium graminearum

Gibberella ear rot(GER)caused by Fusarium graminearum(teleomorph Gibberella zeae)is a common maize disease that not only severely reduces grain yield but also contaminates maize grain with mycotoxins.We investigated the molecular mechanism underlying the host defense responses against pathogen infection using comparative transcriptomic analysis.We injected F.graminearum spore suspensions into plants of resistant(IBM-81)and a susceptible(IBM-85)maize inbred line after pollination and performed RNA-seq 48,72,and 96 h after inoculation.Respectively 487 and 410 differentially expressed genes(DEGs)were induced in the resistant and susceptible lines across three time points,indicating that a stronger defense response was activated in the resistant than in the susceptible line.Among them,198 genes commonly induced in the two lines were subjected to pathway analysis,revealing that most of the DEGs were closely associated with defense and a wide range of metabolic activities.DEGs associated with pathogenesis-related protein 1(PR1)and regulation of salicylic acid were significantly enriched during F.graminearum infection,suggesting that these DEGs play dominant roles in maize resistance to GER.Our results provide a resource for future gene discovery and facilitate elucidation of the complex defense mechanisms involved in resistance to GER.

Identification and functional analysis of miRNAs in developing kernels of a viviparous mutant in maize

Given the important roles of miRNAs in post-transcriptional gene regulation, identification of differentially expressed miRNAs will facilitate the elucidation of molecular mechanisms underlying kernel development. In this study, we constructed a small RNA library to comprehensively represent the full complement of individual small RNAs and to characterize miRNA expression profiles in pooled ears of maize(Zea mays L.) at 10, 15,20, 22, 25 and 30 days after pollination(DAP). At least 21 miRNAs were differentially expressed. The differential expression of three of these miRNAs, i.e., miR528a, miR167a and miR160b, at each stage was verified by qRT-PCR. The results indicated that these miRNAs might be involved in kernel development. In addition, the predicted functions of target genes indicated that most of the target genes are involved in signal transduction and cell communication pathways, particularly the auxin signaling pathway. The expression of candidate germination-associated miRNAs was analyzed by hybridization to a maize genome microarray, and revealed differential expression of genes involved in plant hormone signaling pathways. This finding suggests that phytohormones play a critical role in the development of maize kernels. We found that in combination with other miRNAs, miR528a regulated a putative laccase, a Ring-H2 zinc finger protein and a MADS box-like protein, whereas miR167a and miR160b regulated multiple target genes,including ARF(auxin response factor), a member of the B3 transcription factor family. All three miRNAs are important for ear germination, development and physiology. The small RNA transcriptomes and mRNA obtained in this study will help us gain a betterunderstanding of the expression and function of small RNAs in the development of maize kernel.

The genetic variation of the backcross modified lines developed from the maize line 08-641 selected by different directions

In this study, two donaors CN962 and 8065 were used to improve the resistance to northern leaf blight of the recurrent parent inbred line 08-641 (R08). A total of 79 lines (BC2F4) were developed by a bidirectional selection based on the similarity and dissimilarity in the shape and color of seeds to R08.The genetic variation of these lines were analyzed by 44 pairs of SSR molecular markers, the result showed that a total of 272 alleles were detected in the improved lines and R08, 123 out of them were detected in the modified lines but discarded in R08. The modified line selected based on dissimialry in the shape and corlor of seeds to R08 have lower genetic similary between R08 than that between the lines selected based on similary in the shape and color of seeds and R08, and the genetic variation of these lines were wider. It concluded that when the backcross breeding were used to improve the maize inbred lines, multidirectional selection based on phenotypic value were contribute to create and keep genetic variation.

The Genetic Architecture of Flowering Time and Photoperiod Sensitivity in Maize as Revealed by QTL Review and Meta Analysis

The control of flowering is not only important for reproduction, but also plays a key role in the processes of domestication and adaptation. To reveal the genetic architecture for flowering time and photoperiod sensitivity, a comprehensive evaluation of the relevant literature was performed and followed by meta analysis. A total of 25 synthetic con- sensus quantitative trait loci （QTL） and four hot-spot genomic regions were identified for photoperiod sensitivity including 11 genes related to photoperiod response or flower morphogenesis and development. Besides, a comparative analysis of the QTL for flowering time and photoperiod sensitivity highlighted the regions containing shared and unique QTL for the two traits. Candidate genes associated with maize flowering were identified through integrated analysis of the homologous genes for flowering time in plants and the consensus QTL regions for photoperiod sensitivity in maize （Zea mays L.）. Our results suggest that the combination of literature review, meta-analysis and homologous blast is an efficient approach to identify new candidate genes and create a global view of the genetic architecture for maize photoperiodic flowering. Sequences of candidate genes can be used to develop molecular markers for various models of marker-assisted selection, such as marker-assisted recurrent selection and genomic selection that can contribute significantly to crop environmental adaptation.

Detumbling strategy based on friction control of dual-arm space robot for capturing tumbling target

The rotational motion of a tumbling target brings great challenges to space robot on successfully capturing the tumbling target.Therefore,it is necessary to reduce the target's rotation to a rate at which capture can be accomplished by the space robot.In this paper,a detumbling strategy based on friction control of dual-arm space robot for capturing tumbling target is proposed.This strategy can reduce the target's rotational velocity while maintaining base attitude stability through the establishment of the rotation attenuation controller and base attitude adjustment controller.The rotation attenuation controller adopts the multi-space hybrid impedance control method to control the friction precisely.The base attitude adjustment controller applies the dual-arm extended Jacobian matrix to stabilize the base attitude.The main contributions of this paper are as follows:(1)The compliant control method is adopted to achieve a precise friction control,which can reduce the target angular velocity steadily;(2)The dual-arm extended Jacobian matrix is applied to stabilize the base attitude without affecting the target capture task;(3)The detumbling strategy of dualarm space robot is designed considering base attitude stabilization,realizing coordinated planning of the base attitude and the arms.The strategy is verified by a dual-arm space robot with two 7-DOF(degrees of freedom)arms.Simulation results show that,target with a rotation velocity of 20(°)/s can be effectively controlled to stop within 30 s,and the final deflection of the base attitude is less than 0.15°without affecting the target capture task,verifying the correctness and effectiveness of the strategy.Except to the tumbling target capture task,the control strategy can also be applied to other typical on-orbit operation tasks such as space debris removal and spacecraft maintenance.

Auxin Binding Protein 1 Reinforces Resistance to Sugarcane Mosaic Virus in Maize

Dear Editor,Sugarcane mosaic virus （SCMV） causes severe viral diseases in maize worldwide （Fuchs and Gruntzig, 1995）, resulting in significant losses in grain and forage yield in susceptible cultivars of maize and related crops. The most promising solution is to cultivate resistant varieties, which contribute to sustainable crop production. Two epistatically interacting major SCMV resistance loci （Scmvl and Scmv2） are required to confer complete resistance against SCMV in the resistant nearisogenic line F7RPJRR （the letters left of the slash refer to the genotype at Scmv2 on chromosome 3 and those on the right refer to the genotype at Scmvl on chromosome 6, with R indicating a resistance allele and S a susceptibility allele） （Xing et al., 2006）.