Tassel Morphology Research Articles

Genetic Structure and Molecular Mechanisms Underlying the Formation of Tassel, Anther, and Pollen in the Male Inflorescence of Maize (Zea mays L.).

Maize tassel is the male reproductive organ which is located at the plant’s apex; both its morphological structure and fertility have a profound impact on maize grain yield. More than 40 functional genes regulating the complex tassel traits have been cloned up to now. However, the detailed molecular mechanisms underlying the whole process, from male inflorescence meristem initiation to tassel morphogenesis, are seldom discussed. Here, we summarize the male inflorescence developmental genes and construct a molecular regulatory network to further reveal the molecular mechanisms underlying tassel-trait formation in maize. Meanwhile, as one of the most frequently studied quantitative traits, hundreds of quantitative trait loci (QTLs) and thousands of quantitative trait nucleotides (QTNs) related to tassel morphology have been identified so far. To reveal the genetic structure of tassel traits, we constructed a consensus physical map for tassel traits by summarizing the genetic studies conducted over the past 20 years, and identified 97 hotspot intervals (HSIs) that can be repeatedly mapped in different labs, which will be helpful for marker-assisted selection (MAS) in improving maize yield as well as for providing theoretical guidance in the subsequent identification of the functional genes modulating tassel morphology. In addition, maize is one of the most successful crops in utilizing heterosis; mining of the genic male sterility (GMS) genes is crucial in developing biotechnology-based male-sterility (BMS) systems for seed production and hybrid breeding. In maize, more than 30 GMS genes have been isolated and characterized, and at least 15 GMS genes have been promptly validated by CRISPR/Cas9 mutagenesis within the past two years. We thus summarize the maize GMS genes and further update the molecular regulatory networks underlying male fertility in maize. Taken together, the identified HSIs, genes and molecular mechanisms underlying tassel morphological structure and male fertility are useful for guiding the subsequent cloning of functional genes and for molecular design breeding in maize. Finally, the strategies concerning efficient and rapid isolation of genes controlling tassel morphological structure and male fertility and their application in maize molecular breeding are also discussed.

A decision support system for hybrid corn classification

High yielding corn is primarily derived from a cross-pollination among superior appearing male and female plants. Cross-pollination is closely linked at the tasseling/flowering stage, marked by the emergence of tassel for 5-10 days. With the advancement of machine learning, there are opportunities to apply deep learning models to control the purity of plants. The research aims to develop a decision support system based on deep learning to enable earlier identification and removal of contamination/off-type plants during seed production. The datasets containing 1,587 tassel images taken by high resolution camera. The results of the training and the validation sequence indicated a highly correlated accuracy score. A quite contrasting tassel morphology makes it easier for the model to distinguish on and off-type plants. The loss value during the training and the validation stages was 0.05 and 0.1 respectively. A stand-alone graphical user interface (GUI) was deployed to support the early detection of tassels in the field. This tool can be used to support national corn seed production programs.

Selection Signatures Underlying Dramatic Male Inflorescence Transformation During Modern Hybrid Maize Breeding.

Inflorescence capacity plays a crucial role in reproductive fitness in plants, and in production of hybrid crops. Maize is a monoecious species bearing separate male and female flowers (tassel and ear, respectively). The switch from open-pollinated populations of maize to hybrid-based breeding schemes in the early 20th century was accompanied by a dramatic reduction in tassel size, and the trend has continued with modern breeding over the recent decades. The goal of this study was to identify selection signatures in genes that may underlie this dramatic transformation. Using a population of 942 diverse inbred maize accessions and a nested association mapping population comprising three 200-line biparental populations, we measured 15 tassel morphological characteristics by manual and image-based methods. Genome-wide association studies identified 242 single nucleotide polymorphisms significantly associated with measured traits. We compared 41 unselected lines from the Iowa Stiff Stalk Synthetic (BSSS) population to 21 highly selected lines developed by modern commercial breeding programs, and found that tassel size and weight were reduced significantly. We assayed genetic differences between the two groups using three selection statistics: cross population extended haplotype homozogysity, cross-population composite likelihood ratio, and fixation index. All three statistics show evidence of selection at genomic regions associated with tassel morphology relative to genome-wide null distributions. These results support the tremendous effect, both phenotypic and genotypic, that selection has had on maize male inflorescence morphology.

TIPS: a system for automated image-based phenotyping of maize tassels

BackgroundThe maize male inflorescence (tassel) produces pollen necessary for reproduction and commercial grain production of maize. The size of the tassel has been linked to factors affecting grain yield, so understanding the genetic control of tassel architecture is an important goal. Tassels are fragile and deform easily after removal from the plant, necessitating rapid measurement of any shape characteristics that cannot be retained during storage. Some morphological characteristics of tassels such as curvature and compactness are difficult to quantify using traditional methods, but can be quantified by image-based phenotyping tools. These constraints necessitate the development of an efficient method for capturing natural-state tassel morphology and complementary automated analytical methods that can quickly and reproducibly quantify traits of interest such as height, spread, and branch number.ResultsThis paper presents the Tassel Image-based Phenotyping System (TIPS), which provides a platform for imaging tassels in the field immediately following removal from the plant. TIPS consists of custom methods that can quantify morphological traits from profile images of freshly harvested tassels acquired with a standard digital camera in a field-deployable light shelter. Correlations between manually measured traits (tassel weight, tassel length, spike length, and branch number) and image-based measurements ranged from 0.66 to 0.89. Additional tassel characteristics quantified by image analysis included some that cannot be quantified manually, such as curvature, compactness, fractal dimension, skeleton length, and perimeter. TIPS was used to measure tassel phenotypes of 3530 individual tassels from 749 diverse inbred lines that represent the diversity of tassel morphology found in modern breeding and academic research programs. Repeatability ranged from 0.85 to 0.92 for manually measured phenotypes, from 0.77 to 0.83 for the same traits measured by image-based methods, and from 0.49 to 0.81 for traits that can only be measured by image analysis.ConclusionsTIPS allows morphological features of maize tassels to be quantified automatically, with minimal disturbance, at a scale that supports population-level studies. TIPS is expected to accelerate the discovery of associations between genetic loci and tassel morphology characteristics, and can be applied to maize breeding programs to increase productivity with lower resource commitment.

Concerted modification of flowering time and inflorescence architecture by ectopic expression of TFL1-like genes in maize.

TERMINAL FLOWER1 (TFL1)-like genes are highly conserved in plants and are thought to function in the maintenance of meristem indeterminacy. Recently, we described six maize (Zea mays) TFL1-related genes, named ZEA CENTRORADIALIS1 (ZCN1) to ZCN6. To gain insight into their functions, we generated transgenic maize plants overexpressing their respective cDNAs driven by a constitutive promoter. Overall, ectopic expression of the maize TFL1-like genes produced similar phenotypes, including delayed flowering and altered inflorescence architecture. We observed an apparent relationship between the magnitude of the transgenic phenotypes and the degree of homology between the ZCN proteins. ZCN2, -4, and -5 form a monophylogenetic clade, and their overexpression produced the strongest phenotypes. Along with very late flowering, these transgenic plants produced a "bushy" tassel with increased lateral branching and spikelet density compared with nontransgenic siblings. On the other hand, ZCN1, -3, and -6 produced milder effects. Among them, ZCN1 showed moderate effects on flowering time and tassel morphology, whereas ZCN3 and ZCN6 did not change flowering time but still showed effects on tassel morphology. In situ hybridizations of tissue from nontransgenic plants revealed that the expression of all ZCN genes was associated with vascular bundles, but each gene had a specific spatial and temporal pattern. Expression of four ZCN genes localized to the protoxylem, whereas ZCN5 was expressed in the protophloem. Collectively, our findings suggest that ectopic expression of the TFL1-like genes in maize modifies flowering time and inflorescence architecture through maintenance of the indeterminacy of the vegetative and inflorescence meristems.

Profiling expression changes caused by a segmental aneuploid in maize

BackgroundWhile changes in chromosome number that result in aneuploidy are associated with phenotypic consequences such as Down syndrome and cancer, the molecular causes of specific phenotypes and genome-wide expression changes that occur in aneuploids are still being elucidated.ResultsWe employed a segmental aneuploid condition in maize to study phenotypic and gene expression changes associated with aneuploidy. Maize plants that are trisomic for 90% of the short arm of chromosome 5 and monosomic for a small distal portion of the short arm of chromosome 6 exhibited a phenotypic syndrome that includes reduced stature, tassel morphology changes and the presence of knots on the leaves. The knotted-like homeobox gene knox10, which is located on the short arm of chromosome 5, was shown to be ectopically expressed in developing leaves of the aneuploid plants. Expression profiling revealed that ~40% of the expressed genes in the trisomic region exhibited the expected 1.5 fold increased transcript levels while the remaining 60% of genes did not show altered expression even with increased gene dosage.ConclusionWe found that the majority of genes with altered expression levels were located within the chromosomal regions affected by the segmental aneuploidy and exhibits dosage-dependent expression changes. A small number of genes exhibit higher levels of expression change not predicted by the dosage, or display altered expression even though they are not located in the aneuploid regions.

Tassel Morphology as an Indicator of Potential Pollen Production in Maize

Adequate pollen production is an essential prerequisite for achieving high yields in commercial corn (Zea mays L.) production and for insuring high levels of genetic purity in the production of hybrid seed. Documenting the timing and intensity of pollen shed are fundamental to these goals, but methods to describe patterns of pollen release from maize tassels are limited and laborious. Our objective was to explore characteristics of tassel morphology that could be used as simple and indirect measures of pollen production per plant under field conditions. The progress of tassel development was documented using a nine-stage scale based on easily-quantified morphological characteristics. Genetic variation among hybrids and inbreds as well as environmental variation across planting densities and years was correlated with levels of pollen production. This analysis revealed that a change in tassel dry weight during pollen shed was not an accurate measure of pollen production per tassel. Likewise, no single morphological characterization captured all the genetic and environmental variation in pollen production per tassel. But a combination of morphological traits incorporated into a Tassel Area Index (TAI) accounted for up to 89% of the variation in pollen production among hybrids in response to population density, and 64% of the variation in pollen production among inbred heterotic groups. Because data collection is simple, quick, and non-destructive, the Tassel Area Index approach is well-suited for distinguishing genetic variation in pollen production and relative responses to treatments under field conditions. The accuracy of the technique could be increased, if necessary, by incorporating additional information about flower density or pollen production per anther. But this would entail a much greater investment of time and resources.

Quantitative Trait Loci Controlling Leaf and Tassel Traits in a B73 × Mo17 Population of Maize

ABSTRACTLight penetration into the canopy of maize (Zea mays L.) production fields is an important determinant of grain yield. Factors affecting light penetration include agricultural practices such as planting density and plant morphological factors such as leaf angle, leaf size, and tassel size. The objectives of this experiment were to identify genomic regions controlling the inheritance of leaf angle and tassel morphology in a B73 × Mo17 recombinant inbred population. Three quantitative trait loci (QTL) for tassel branch angle were detected which explained 35.6% of the phenotypic variation. Six QTL were detected for tassel branch number with three of these QTL on chromosome 2. Nine QTL were detected for leaf angle in one or more environments. Significant phenotypic correlations were detected between tassel branch angle and tassel branch number and between tassel branch number and leaf angle. Overlapping support intervals were identified between QTL detected for leaf angle and for tassel branch number on chromosome 2 near marker umc53a. Additionally, a QTL near marker bnl6.10 on chromosome 5 identified for tassel branch angle was in the same region as a QTL identified for leaf angle. The results of this study indicate that common genetic relationships exist between tassel traits and leaf angle.

Heterochronic Effects of Teopod 2 on the Growth and Photosensitivity of the Maize Shoot.

Teopod 2 (Tp2) is a semidominant mutation of maize that prolongs the expression of juvenile vegetative traits, increases the total number of leaves produced by the shoot, and transforms reproductive structures into vegetative ones. Here, we show that Tp2 prolongs the duration of vegetative growth without prolonging the overall duration of shoot growth. Mutant shoots produce leaves at the same rate as wild-type plants and continue to produce leaves after wild-type plants have initiated a tassel. Although Tp2/+ plants initiate a tassel later than their wild-type siblings, this mutant tassel ceases differentiation at the same time as, or shortly before, the primary meristem of a wild-type tassel completes its development. To investigate the relationship between the vegetative and reproductive development of the shoot, Tp2/+ and wild-type plants were exposed to floral inductive short day (SD) treatments at various stages of shoot growth. Tassel initiation in wild-type plants (which normally produced 18 to 19 leaves) was maximally sensitive to SD between plastochrons 15 and 16, whereas tassel branching was maximally sensitive to SD between plastochrons 15 and 18. Tassel initiation and tassel morphology in Tp2/+ plants (which normally produced 21 to 26 leaves) were both maximally sensitive to SD between plastochrons 15 and 18. Thus, the constitutive expression of a juvenile vegetative program in Tp2/+ plants does not significantly delay the reproductive maturation of the shoot.

Isoenzymatic Variation in Zea (Gramineae)

An average of 14(7-34) plants each for 61 different collections of Zea (maize and its wild relatives, the teosintes) were studied for 12 enzyme systems coded for by 21 loci. Principal component and cluster analyses based on allele frequencies showed Zea can be divided into two major groups: 1) sect. Luxuriantes, including Z. perennis, Z. diploperennis, and Z. luxurians; and 2) sect. Zea (in part), including Z. mays subsp. mays, var. parviglumis, and subsp. mexicana. Zea mays var. huehuetenangensis (Huehuetenango teosinte) is isoenzymatically distinct from both sections, but shows its closest relationship to Z. mays var. parviglumis of sect. Zea. Populations of Z. mays subsp. mexicana and var. parviglumis grade isoenzymatically from one into the other without any clear break, but without any overlap either. Five populations of Z. mays subsp. mays are all isoenzymat- ically very similar to Z. mays var. parviglumis. The isozyme data are consistent with the theory that Mexican annual teosinte is the ancestor of maize. With the exception of the unusual isoenzymatic nature of Z. mays var. huehuetenangensis, the isoenzymatic data agree with previous studies of tassel morphology and cytoplasm DNAs. A comparison of sympatric populations of maize and teosinte suggests that teosintes are not greatly affected by introgression from maize. Zea mays var. parviglumis and Z. diploperennis have considerable within and among population variation; Z. luxurians has much less. Race Central Plateau of Z. mays subsp. mexicana has greater among population variation than any other taxon in Zea. Zea tends to show greater within population heterozygosity and more variance within species and subspecies than most other plants that have been studied isoenzymat- ically.

Leaf Angle, Tassel Morphology, and the Performance of Maize Hybrids1

The relative importance of leaf orientation, planting densities, planting patterns, and tassel shading on grain production of certain maize (Zea mays L.) genotypes was investigated. Three experiments were conducted over a 4‐year period to ascertain the effect of leaf angle on grain yield and percentage of unproductive plants in several maize hybrids. A plant‐density experiment using normal, liguleless‐l, and liguleless‐2 leaf types at densities of 60,000, 75,000, and 90,000 plants/ha in 50.8 cm row spacings was conducted over a 3‐year period. The liguleless‐2 hybrids produced 6.7% and 12.9% more grain at 75,000 and 90,000 plants/ha, respectively, than the normal leaf type hybrids. A plant‐spacing experiment using 1,290 cm2 and 645 cm2/plant and the same three leaf types was grown over a 2‐year period. The liguleless‐2 hybrids produced 9.7% and 49.7% more grain at 1,290 cm2 and 645 cm2 (25.4 ✕ 25.4 cm) plant spacings, respectively, than the normal leaf type hybrids. In both experiments an increase in unproductive plants was associated with a reduction is grain yields. An additional 1‐year experiment was conducted to determine if light penetration into the leaf canopy and tassel morphology could account for the increased grain yield of liguleless‐2 hybrids. The normal leaf type hybrids had only 89% as much incident light in the leaf canopy as the liguleless‐ 2 hybrids. In addition, tassel removal increased grain yield of the normal leaf type hybrids but not that of the liguleless‐2 hybrids. Based on results of this study, leaf angle and tassel morphology were two characters associated with increased grain yield of liguleless‐2 hybrids under high plant densities and narrow row spacings. Liguleless‐I hybrids have almost a total vertical leaf orientation and did not respond in the same manner in these experiments as liguleless‐2 hybrids, although the leaf angle and light penetration into the canopy were greater than liguleless‐2 hybrids. Responses of liguleless‐1 hybrids were similar to those of normal hybrids.

Inheritance of selected pathotoxin resistance in maize plants regenerated from cell cultures

Texas male-sterile cytoplasm (cms-T) maize is susceptible to Helminthosporium maydis race T and its pathotoxin, whereas nonsterile cytoplasm maize is resistant. Callus cultures initiated from immature embryos of a cms-T genotype, BC(1)A188(T), were susceptible to the toxin and were capable of plant regeneration. Toxin-resistant cell lines were selected by a sublethal enrichment procedure in which cms-T callus was grown for several selection cycles (subculture transfers) in the presence of progressively higher concentrations of toxin. Periodically during the selection process, plants were regenerated from the cms-T cultures to determine their susceptibility or resistance to the toxin. Plants regenerated after four cycles of selection were male-sterile and toxin-susceptible as shown by leaf bioassays. All plants regenerated from cell lines isolated from the fifth selection cycle onward, however, were toxin-resistant and 52 of 65 were fully male-fertile. The remaining 13 "male-sterile" resistant plants did not shed pollen and did not resemble cms-T plants in tassel morphology. Some "male-sterile" plants produced anthers containing a small amount of starch-filled pollen, suggesting that the sterility of these 13 plants was not the result of the cms-T trait. Leaf bioassays on F(1) progeny from regenerated resistant plants indicated that resistance to the toxin was inherited only through the female. The male-fertility trait also was inherited only through the female. After inoculation with H. maydis race T spores, leaf lesion size for progeny from regenerated resistant plants coincided with their reaction to the toxin. This result indicated that plant resistance to the pathogen was closely correlated with the toxin resistance obtained through cell culture selection.