Natural variation in teosinte at the domestication locus teosinte branched1 (tb1)

Molecular characterization of teosinte branched1 gene governing branching architecture in cultivated maize and wild relatives

We sequenced the entire tb1 gene in six maize inbreds and its wild relatives (parviglumis, mexicana, perennis and luxurians) to characterize it at molecular level. Hopscotch and Tourist transposable elements were observed in the upstream of tb1 in all maize inbreds, while they were absent in wild relatives. In maize, tb1 consisted of 431-443 bp 5'UTR, 1101 bp coding sequence and 211-219 bp 3'UTR. In promoter region, mutations in the light response element in mexicana (~ 35 bp and ~ 55 bp upstream of TSS) and perennis (at ~ 35 bp upstream of TSS) were found. A 6 bp insertion at 420 bp downstream of the polyA signal site was present among teosinte accessions, while it was not observed in maize. A codominant marker flanking the 6 bp InDel was developed, and it differentiated the teosintes from maize. In Tb1 protein, alanine (12.7-14.6%) was the most abundant amino acid with tryptophan as the rarest (0.5-0.9%). The molecular weight of Tb1 protein was 38757.15 g/mol except 'Palomero Toluqueno' and HKI1128. R and TCP motifs in Tb1 protein were highly conserved across maize, teosinte and orthologues, while TCP domain differed for tb1 paralogue. Tb1 possessed important role in light-, auxin-, stress-response and meristem identity maintenance. Presence of molecular signal suggested its localization in mitochondria, nucleus and nucleolus. Parviglumis and mexicana shared closer relationship with maize than perennis and luxurians. A highly conserved 59-60 amino acids long bHLH region was observed across genotypes. Information generated here assumes significance in evolution of tb1 gene and breeding for enhancement of prolificacy in maize.

Maize domestication and gene interaction

Contents Summary 395 I. Introduction 395 II. The genetic basis of maize domestication 396 III. The tempo of maize domestication 401 IV. Genetic interactions and selection during maize domestication 401 V. Gene networks of maize domestication alleles 404 VI. Implications of gene interactions on evolution and selection404 VII. Conclusions 405 Acknowledgements 405 References 405 SUMMARY: Domestication is a tractable system for following evolutionary change. Under domestication, wild populations respond to shifting selective pressures, resulting in adaptation to the new ecological niche of cultivation. Owing to the important role of domesticated crops in human nutrition and agriculture, the ancestry and selection pressures transforming a wild plant into a domesticate have been extensively studied. In Zea mays, morphological, genetic and genomic studies have elucidated how a wild plant, the teosinte Z. mays subsp. parviglumis, was transformed into the domesticate Z. mays subsp. mays. Five major morphological differences distinguish these two subspecies, and careful genetic dissection has pinpointed the molecular changes responsible for several of these traits. But maize domestication was a consequence of more than just five genes, and regions throughout the genome contribute. The impacts of these additional regions are contingent on genetic background, both the interactions between alleles of a single gene and among alleles of the multiple genes that modulate phenotypes. Key genetic interactions include dominance relationships, epistatic interactions and pleiotropic constraint, including how these variants are connected in gene networks. Here, we review the role of gene interactions in generating the dramatic phenotypic evolution seen in the transition from teosinte to maize.

Molecular cloning and sequence variance analysis of the TEOSINTE BRANCHED1 (TB1) gene in bermudagrass [Cynodon dactylon (L.) Pers]

TEOSINTE BRANCHED1 (TB1) encodes a TEOSINTE BRANCHED1, CYCLOIDEA, and PROLIFERATING CELL FACTOR (TCP) transcription factor that represses axillary bud outgrowth and lateral branch formation in plants. Previous studies have elucidated the essential tillering regulatory roles of TB1 in many grasses, including maize and rice; however, the functions of TB1 in turf grasses remain unclear. In this study, we cloned the CdTB1 gene from bermudagrass, an important turfgrass species, and characterized the transactivation function of the CdTB1 protein. Sequencing the CdTB1 gene locus in a mini-core germplasm collection of Chinese bermudagrasses led to the successful identification of 66 SNP and 2 indel mutations in the protein-coding region as well as 28 SNP and 11 indel mutations in the promoter region. Interestingly, mutations in the C-terminal transactivation domain of the CdTB1 protein had no significant influence on the transactivation activity, whereas a novel 335-bp insertion mutation located in the promoter region could significantly increase the expression of the CdTB1 gene. Furthermore, wild accessions of bermudagrass harboring the novel insertion mutation were found to have significantly reduced tillers compared with other accessions, suggesting a negative correlation between the mutation and tillering. The results of this study not only expanded our knowledge of TB1 gene expression regulation but also provided possible molecular markers to breed cultivars of turf and forage grasses with specific architectural features.

Identification of cis-regulatory elements by chromatin structure

The systematic identification of cis-regulatory elements (CREs) in plant genomes is critically important in understanding transcriptional regulation during development and in response to environmental cues. Several genome-wide structure-based methods have been successfully applied to plant genomes in the past few years. Here, we review recent results on the identification and characterization of CREs in multiple plant species and in different biological processes and discuss future applications of chromatin accessibility data to understand the mechanism, function and evolution of transcriptional regulation networks.

Genome-wide characterization of non-reference transposable element insertion polymorphisms reveals genetic diversity in tropical and temperate maize

Background

Maize was originally domesticated in a tropical environment but is now widely cultivated at temperate latitudes. Temperate and tropical maize populations have diverged both genotypically and phenotypically. Tropical maize lines grown in temperate environments usually exhibit delayed flowering, pollination, and seed set, which reduces their grain yield relative to temperate adapted maize lines. One potential mechanism by which temperate maize may have adapted to a new environment is novel transposable element insertions, which can influence gene regulation. Recent advances in sequencing technology have made it possible to study variation in transposon content and insertion location in large sets of maize lines.

Results

In total, 274,408 non-redundant TEs (NRTEs) were identified using resequencing data generated from 83 maize inbred lines. The locations of DNA TEs and copia-superfamily retrotransposons showed significant positive correlations with gene density and genetic recombination rates, whereas gypsy-superfamily retrotransposons showed a negative correlation with these two parameters. Compared to tropical maize, temperate maize had fewer unique NRTEs but higher insertion frequency, lower background recombination rates, and higher linkage disequilibrium, with more NRTEs close to flowering and stress-related genes in the genome. Association mapping demonstrated that the presence/absence of 48 NRTEs was associated with flowering time and that expression of neighboring genes differed between haplotypes where a NRTE was present or absent.

Conclusions

This study suggests that NRTEs may have played an important role in creating the variation in gene regulation that enabled the rapid adaptation of maize to diverse environments.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4103-x) contains supplementary material, which is available to authorized users.

A Gene for Genetic Background in Zea mays: Fine-Mapping enhancer of teosinte branched1.2 to a YABBY Class Transcription Factor

The effects of an allelic substitution at a gene often depend critically on genetic background, i.e., the genotypes at other genes in the genome. During the domestication of maize from its wild ancestor (teosinte), an allelic substitution at teosinte branched (tb1) caused changes in both plant and ear architecture. The effects of tb1 on phenotype were shown to depend on multiple background loci, including one called enhancer of tb1.2 (etb1.2). We mapped etb1.2 to a YABBY class transcription factor (ZmYAB2.1) and showed that the maize alleles of ZmYAB2.1 are either expressed at a lower level than teosinte alleles or disrupted by insertions in the sequences. tb1 and etb1.2 interact epistatically to control the length of internodes within the maize ear, which affects how densely the kernels are packed on the ear. The interaction effect is also observed at the level of gene expression, with tb1 acting as a repressor of ZmYAB2.1 expression. Curiously, ZmYAB2.1 was previously identified as a candidate gene for another domestication trait in maize, nonshattering ears. Consistent with this proposed role, ZmYAB2.1 is expressed in a narrow band of cells in immature ears that appears to represent a vestigial abscission (shattering) zone. Expression in this band of cells may also underlie the effect on internode elongation. The identification of ZmYAB2.1 as a background factor interacting with tb1 is a first step toward a gene-level understanding of how tb1 and the background within which it works evolved in concert during maize domestication.

Identification of candidate genes for dissecting complex branch number trait in chickpea

The present study exploited integrated genomics-assisted breeding strategy for genetic dissection of complex branch number quantitative trait in chickpea. Candidate gene-based association analysis in a branch number association panel was performed by utilizing the genotyping data of 401 SNP allelic variants mined from 27 known cloned branch number gene orthologs of chickpea. The genome-wide association study (GWAS) integrating both genome-wide GBS- (4556 SNPs) and candidate gene-based genotyping information of 4957 SNPs in a structured population of 60 sequenced desi and kabuli accessions (with 350-400 kb LD decay), detected 11 significant genomic loci (genes) associated (41% combined PVE) with branch number in chickpea. Of these, seven branch number-associated genes were further validated successfully in two inter (ICC 4958 × ICC 17160)- and intra (ICC 12299 × ICC 8261)-specific mapping populations. The axillary meristem and shoot apical meristem-specific expression, including differential up- and down-regulation (4-5 fold) of the validated seven branch number-associated genes especially in high branch number as compared to the low branch number-containing parental accessions and homozygous individuals of two aforesaid mapping populations was apparent. Collectively, this combinatorial genomic approach delineated diverse naturally occurring novel functional SNP allelic variants in seven potential known/candidate genes [PIN1 (PIN-FORMED protein 1), TB1 (teosinte branched 1), BA1/LAX1 (BARREN STALK1/LIKE AUXIN1), GRAS8 (gibberellic acid insensitive/GAI, Repressor of ga13/RGA and Scarecrow8/SCR8), ERF (ethylene-responsive element-binding factor), MAX2 (more axillary growth 2) and lipase] governing chickpea branch number. The useful information generated from this study have potential to expedite marker-assisted genetic enhancement by developing high-yielding cultivars with more number of productive (pods and seeds) branches in chickpea.