Nitrogen reduction combined with ETc irrigation maintained summer maize yield and increased water and nitrogen use efficiency.

Introduction: High rainfall and excessive urea application are counterproductive to summer maize growth requirements and lower grain yield and water/nitrogen (N) use efficiency. The objective of this study was to determine whether ETc irrigation based on summer maize demand and reduced nitrogen rate in the Huang Huai Hai Plain increased water and nitrogen use efficiency without sacrificing yield. Methods: To achieve this, we conducted an experiment with four irrigation levels [ambient rainfall (I0) and 50% (I1), 75% (I2), and 100% (I3) of actual crop evapotranspiration (ETc)] and four nitrogen rates [no nitrogen fertilizer (N0), recommended nitrogen rate of urea (NU), recommended nitrogen rate of blending controlled-release urea with conventional urea fertilizer (BCRF) (NC), and reduced nitrogen rate of BCRF (NR)] in 2016-2018. Results: The results show that reduced irrigation and nitrogen rate reduced Fv/Fm, 13C-photosynthate, and nitrogen accumulation both in the kernel and plant. I3NC and I3NU accumulated higher 13C-photosynthate, nitrogen, and dry matter. However, 13C-photosynthate and nitrogen distribution to the kernel was decreased from I2 to I3 and was higher in BCRF than in urea. I2NC and I2NR promoted their distribution to the kernel, resulting in a higher harvest index. Compared with I3NU, I2NR increased root length density by 32.8% on average, maintaining considerable leaf Fv/Fm and obtaining similar kernel number and kernel weight. The higher root length density of I2NR of 40-60 cm promoted 13C-photosynthate and nitrogen distribution to the kernel and increased the harvest index. As a result, the water use efficiency (WUE) and nitrogen agronomic use efficiency (NAUE) in I2NR increased by 20.5%-31.9% and 11.0%-38.0% than that in I3NU, respectively. Discussion: Therefore, 75%ETc deficit irrigation and BCRF fertilizer with 80% nitrogen rate improved root length density, maintained leaf Fv/Fm in the milking stage, promoted 13C-photosynthate, and distributed nitrogen to the kernel, ultimately providing a higher WUE and NAUE without significantly reducing grain yield.

Comparative transcriptomic reveals the molecular mechanism of maize hybrid Zhengdan538 in response to water deficit.

Heterosis, known as one of the most successful strategies for increasing grain yield and abiotic/biotic stress tolerance, has been widely exploited in maize breeding. However, the underlying molecular processes are still to be elucidated. The maize hybrid "Zhengdan538" shows high tolerance to drought stress. The transcriptomes of the seedling leaves of its parents, "ZhengA88" and "ZhengT22" and their reciprocal F1 hybrid under well-watered and water deficit conditions, were analyzed by RNA sequencing (RNA-Seq). Transcriptome profiling of the reciprocal hybrid revealed 2994-4692 differentially expressed genes (DEGs) under well-watered and water-deficit conditions, which were identified by comparing with their parents. The reciprocal hybrid was more closely related to the parental line "ZhengT22" than to the parental line "ZhengA88" in terms of gene expression patterns under water-deficit condition. Furthermore, genes showed expression level dominance (ELD), especially the high-parental ELD (Class 3 and 5), accounted for the largest proportion of DEGs between the reciprocal F1 hybrid and their parental lines under water deficit. These ELD genes mainly participated in photosynthesis, energy biosynthesis, and metabolism processes. The results indicated that ELD genes played important roles in hybrid tolerance to water deficit. Moreover, a set of important drought-responsive transcription factors were found to be encoded by the identified ELD genes and are thought to function in improving drought tolerance in maize hybrid plants. Our results provide a better understanding of the molecular mechanism of drought tolerance in hybrid maize.

Molecular mapping of quantitative trait loci for 3 husk traits using genotyping by sequencing in maize (Zea mays L.).

The maize (Zea mays L.) husk consists of multiple leaf layers and plays an important role in grain growth and development. Despite significant achievements in physiological and morphological research, few studies have focused on the detection of genetic loci underlying husk-related traits due to the lack of efficient tools. In this study, we constructed an ultra-high-density linkage map using genotyping by sequencing based on a recombinant inbred line population to estimate the genetic variance and heritability of 3 husk traits, i.e. husk length, husk width, and husk layer number in 3 field environments and the combined environment. The 3 husk traits showed broad phenotypic variation and high heritability; the broad-sense heritability (H2) was 0.92, 0.84, and 0.86. Twenty quantitative trait loci were consistently detected more than 1 environment, including 9 for husk length, 6 for husk width, and 5 for husk layer number. These loci were considered as stable quantitative trait loci. Based on the quantitative trait loci mapping in the recombinant inbred line population, qHL6 and qHN4 were detected across all environments and inferred to be reliable and major-effect quantitative trait loci for husk length and husk layer number, respectively. In addition, several predicted candidate genes were identified in the region of qHL6 and qHN4, of which 17 candidate genes potentially play a role in biological processes related to development process and energy metabolism. These results will be as a useful resource for performing functional studies aimed at understanding the molecular pathways involved in husk growth and development.

Bulk analysis by resequencing and RNA-seq identifies candidate genes for maintaining leaf water content under water deficit in maize.

Drought is one of the main abiotic stresses adversely affecting maize growth and grain yield. Identifying drought tolerance-related genes and breeding varieties with enhanced tolerance are effective strategies for minimizing the effects of drought stress. In this study, the leaf relative water content (LRWC) was used for evaluating drought tolerance. QTL-seq analysis of 419 F2 individuals from a cross between ZhengT22 (the drought-tolerant line with high LRWC) and ZhengA88 (the drought-sensitive line with low LRWC) revealed four LRWC-related QTLs (qLRWC2, qLRWC10a, qLRWC10b, and qLRWC10c) in maize seedlings under water deficit. Of these QTLs, qLRWC2 was located in a 2.03-Mb interval on chromosome 2, whereas qLRWC10a, qLRWC10b, and qLRWC10c were located in 2.85-, 3.99-, and 2.05-Mb intervals, respectively, on chromosome 10, and the 93 genes contained the variation loci locating in the four QTLs regions. To identify the candidate genes within the QTLs, an RNA-seq analysis was performed for the parents exposed to water deficit. Seven genes with effective variation loci showed significant difference in expression either in ZhengA88 or ZhengT22 in response to water deficit. Moreover, among the genes, ZmPrx64, ZmCIPK, HSP90, and ABCG34 have all been shown to be related to water stress in the previous studies. Thus, they are primary considered as the potential candidate genes controlling LRWC under water deficit at the seeding stage of maize in this study. These findings will help clarify the molecular basis of drought tolerance in maize seedlings and may be relevant for future functional analysis and for breeding drought-tolerant maize varieties.

Molecular mapping of quantitative trait loci for grain moisture at harvest and field grain drying rate in maize (Zea mays L.).

Maize (Zea mays L.) grain moisture (GM) at harvest is an important trait that affects seed preservation during storage, grain quality and artificial drying costs. To date, most of the work on understanding GM dynamics in maize has focused on the grain filling period, while the period of postmaturity grain drying remains unexplored. The field grain drying rate (FDR) is one of the most important factors in determining GM at harvest. Therefore, understanding the genetic basis of FDR will be useful for obtaining low-GM varieties. In this study, a single-cross population (330 F2:3 -generation plants) derived from a cross of two divergent inbred lines was evaluated in two planting environments with a measurement method - Area under the Dry Down Curve (AUDDC). A high-density genetic linkage map of 2491 single nucleotide polymorphism (SNP) loci covering 2415.56 cM was constructed. Using composite interval mapping, four quantitative trait loci (QTL), q45dGM1-1, qHTGM2-2, qAUDDC2-1 and qAUDDC10-1, which were detected on chromosomes 1, 2 and 10, were stable across environments and could explain more than 10% of phenotypic variance. These may be the major QTLs, with non-significant environmental interactions for GM at 45 days, GM at harvest and FDR, respectively. Additionally, several predicted candidate genes for FDR were identified, including several transcription factors, hormone responsive genes, energy-related and DNA replication-related genes. These results will provide useful information for our understanding of the genetic basis of FDR, as well as providing tools for marker-assisted selection in maize breeding.

Transcriptome Analysis of Cadmium-Treated Roots in Maize (Zea mays L.).

Cadmium (Cd) is a heavy metal and is highly toxic to all plant species. However, the underlying molecular mechanism controlling the effects of auxin on the Cd stress response in maize is largely unknown. In this study, the transcriptome produced by maize 'Zheng 58' root responses to Cd stress was sequenced using Illumina sequencing technology. In our study, six RNA-seq libraries yielded a total of 244 million clean short reads and 30.37 Gb of sequence data. A total of 6342 differentially expressed genes (DEGs) were grouped into 908 Gene Ontology (GO) categories and 198 Kyoto Encyclopedia of Genes and Genomes terms. GO term enrichment analysis indicated that various auxin signaling pathway-related GO terms were significantly enriched in DEGs. Comparison of the transcript abundances for auxin biosynthesis, transport, and downstream response genes revealed a universal expression response under Cd treatment. Furthermore, our data showed that free indole-3-acetic acid (IAA) levels were significantly reduced; but IAA oxidase activity was up-regulated after Cd treatment in maize roots. The analysis of Cd activity in maize roots under different Cd and auxin conditions confirmed that auxin affected Cd accumulation in maize seedlings. These results will improve our understanding of the complex molecular mechanisms underlying the response to Cd stress in maize roots.

Identification and expression profiling analysis of calmodulin-binding transcription activator genes in maize (Zea mays L.) under abiotic and biotic stresses.

The calmodulin-binding transcription activators (CAMTA) play critical roles in plant growth and responses to environmental stimuli. However, how CAMTAs function in responses to abiotic and biotic stresses in maize (Zea mays L.) is largely unknown. In this study, we first identified all the CAMTA homologous genes in the whole genome of maize. The results showed that nine ZmCAMTA genes showed highly diversified gene structures and tissue-specific expression patterns. Many ZmCAMTA genes displayed high expression levels in the roots. We then surveyed the distribution of stress-related cis-regulatory elements in the -1.5 kb promoter regions of ZmCAMTA genes. Notably, a large number of stress-related elements present in the promoter regions of some ZmCAMTA genes, indicating a genetic basis of stress expression regulation of these genes. Quantitative real-time PCR was used to test the expression of ZmCAMTA genes under several abiotic stresses (drought, salt, and cold), various stress-related hormones [abscisic acid, auxin, salicylic acid (SA), and jasmonic acid] and biotic stress [rice black-streaked dwarf virus (RBSDV) infection]. Furthermore, the expression pattern of ZmCAMTA genes under RBSDV infection was analyzed to investigate their potential roles in responses of different maize cultivated varieties to RBSDV. The expression of most ZmCAMTA genes responded to both abiotic and biotic stresses. The data will help us to understand the roles of CAMTA-mediated Ca(2+) signaling in maize tolerance to environmental stresses.

Genome-wide identification and expression profiling analysis of ZmPIN, ZmPILS, ZmLAX and ZmABCB auxin transporter gene families in maize (Zea mays L.) under various abiotic stresses.

The auxin influx carriers auxin resistant 1/like aux 1 (AUX/LAX), efflux carriers pin-formed (PIN) (together with PIN-like proteins) and efflux/conditional P-glycoprotein (ABCB) are major protein families involved in auxin polar transport. However, how they function in responses to exogenous auxin and abiotic stresses in maize is largely unknown. In this work, the latest updated maize (Zea mays L.) reference genome sequence was used to characterize and analyze the ZmLAX, ZmPIN, ZmPILS and ZmABCB family genes from maize. The results showed that five ZmLAXs, fifteen ZmPINs, nine ZmPILSs and thirty-five ZmABCBs were mapped on all ten maize chromosomes. Highly diversified gene structures, nonconservative transmembrane helices and tissue-specific expression patterns suggested the possibility of function diversification for these genes. Quantitative real-time polymerase chain reaction (qRT-PCR) was used to analyze the expression patterns of ZmLAX, ZmPIN, ZmPILS and ZmABCB genes under exogenous auxin and different environmental stresses. The expression levels of most ZmPIN, ZmPILS, ZmLAX and ZmABCB genes were induced in shoots and were reduced in roots by various abiotic stresses (drought, salt and cold stresses). The opposite expression response patterns indicated the dynamic auxin transport between shoots and roots under abiotic stresses. Analysis of the expression patterns of ZmPIN, ZmPILS, ZmLAX and ZmABCB genes under drought, salt and cold treatment may help us to understand the possible roles of maize auxin transporter genes in responses and tolerance to environmental stresses.

Genetic analysis and major quantitative trait locus mapping of leaf widths at different positions in multiple populations.

BACKGROUND: Leaf width is an important agricultural trait in maize. Leaf development is dependent on cell proliferation and expansion, and these processes exhibit polarity with respect to the longitudinal and transverse axes of the leaf. However, the molecular mechanism of the genetic control of seed vigor remains unknown in maize, and a better understanding of this mechanism is required. METHODOLOGY/PRINCIPAL FINDINGS: To reveal the genetic architecture of leaf width, a comprehensive evaluation using four RIL populations was performed, followed by a meta-analysis. Forty-six QTLs associated with the widths of leaves at different positions above the uppermost ear were detected in the four RIL populations in three environments. The individual effects of the QTLs ranged from 4.33% to 18.01% of the observed phenotypic variation, with 14 QTLs showing effects of over 10%. We identified three common QTLs associated with leaf width at all of the examined positions, in addition to one common QTL associated with leaf width at three of the positions and six common QTLs associated with leaf width at two of the positions. The results indicate that leaf width at different leaf positions may be affected by one QTL or several of the same QTLs. Such traits may also be regulated by many different QTLs. Thirty-one of the forty-six initial QTLs were integrated into eight mQTLs through a meta-analysis, and 10 of the 14 initial QTLs presenting an R2>10% were integrated into six mQTLs. CONCLUSIONS/SIGNIFICANCE: mQTL1-2, mQTL3-1, mQTL7, and mQTL8 were composed of the initial QTLs showing an R2>10% and included four to six of the initial QTLs that were associated with two to four positions in a single population. Therefore, these four chromosome regions may be hot spots for important QTLs for these traits. Thus, they warrant further studies and may be useful for marker-assisted breeding.