A brittle-2 transgene increases maize yield by acting in maternal tissues to increase seed number

Excess iron accumulation affects maize endosperm development by inhibiting starch synthesis and inducing DNA damage

Iron (Fe) storage in cereal seeds is the principal source of dietary Fe for humans. In maize (Zea mays), the accumulation of Fe in seeds is known to be negatively correlated with crop yield. Hence, it is essential to understand the underlying mechanism, which is crucial for developing and breeding maize cultivars with high yields and high Fe concentrations in the kernels. Here, through the successful application of in vitro kernel culture, we demonstrated that excess Fe supply in the medium caused the kernel to become collapsed and lighter in color, consistent with those found in yellow strip like 2 (ysl2, a small kernel mutant), implicated a crucial role of Fe concentration in kernel development. Indeed, over-accumulation of Fe in endosperm inhibited the abundance and activity of ADP-glucose pyrophosphorylase (AGPase) and the kernel development defect was alleviated by overexpression of Briittle 2 (Bt2, encoding a small subunit of AGPase) in ysl2 mutant. Imaging and quantitative analyses of reactive oxygen species (ROS) and cell death showed that Fe stress-induced ROS burst and severe DNA damage in endosperm cells. In addition, we have successfully identified candidate genes that are associated with iron homeostasis within the kernel, as well as upstream transcription factors that regulate ZmYSL2 by yeast one-hybrid screening. Collectively, our study will provide insights into the molecular mechanism of Fe accumulation-regulated seed development and promote the future efficient application of Fe element in corn improvement.

The novel ZmTCP7 transcription factor targets AGPase-encoding gene ZmBt2 to regulate storage starch accumulation in maize

The process of starch biosynthesis is a major developmental event that affects the final grain yield and quality in maize (Zea mays L.), and transcriptional regulation plays a key role in modulating the expression of the main players in the pathway. ZmBt2, which encodes the small subunits of AGPase, is a rate-controlling gene of the pathway; however, much remains unknown about its transcriptional regulation. Our earlier study identifies a short functional fragment of ZmBt2 promoter (394-bp), and further shows it contains multiple putative cis-acting regulatory elements, demonstrating that several transcription factors may govern ZmBt2 expression. Here, we identified a novel TCP transcription factor (TF), ZmTCP7, that interacted with the functional fragment of the ZmBt2 promoter in a yeast one hybrid screening system. We further showed that ZmTCP7 is a non-autonomous TF targeted to the nucleus and predominantly expressed in maize endosperm. Using promoter deletion analyzes by transient expression in maize endosperm protoplasts combined with electrophoretic mobility shift assays, we found that ZmTCP7 bound to GAACCCCAC elements on the ZmBt2 promoter to suppress its expression. Transgenic overexpression of ZmTCP7 in maize caused a significant repression of ZmBt2 transcription by ~77.58%, resulting in a 21.51% decrease in AGPase activity and a 9.58% reduction in the endosperm starch content of transgenic maize. Moreover, the expressions of ZmBt1, ZmSSI, ZmSSIIa, and ZmSSIIIa were increased, while those of ZmSh2 and ZmSSIV reduced significantly in the endosperm of the transgenic maize. Overall, this study shows that ZmTCP7 functions as a transcriptional repressor of ZmBt2 and a negative regulator of endosperm starch accumulation, providing new insights into the regulatory networks that govern ZmBt2 expression and starch biosynthesis pathway in maize.

Temperature-induced dynamics of plant carbohydrate metabolism

Carbohydrates are direct products of photosynthetic CO₂ assimilation. Within a changing temperature regime, both photosynthesis and carbohydrate metabolism need tight regulation to prevent irreversible damage of plant tissue and to sustain energy metabolism, growth and development. Due to climate change, plants are and will be exposed to both long-term and short-term temperature changes with increasing amplitude. Particularly sudden fluctuations, which might comprise a large temperature amplitude from low to high temperature, pose a challenge for plants from the cellular to the ecosystem level. A detailed understanding of fundamental regulatory processes, which link photosynthesis and carbohydrate metabolism under such fluctuating environmental conditions, is essential for an estimate of climate change consequences. Further, understanding these processes is important for biotechnological application, breeding and engineering. Environmental light and temperature regimes are sensed by a molecular network that comprises photoreceptors and molecular components of the circadian clock. Photosynthetic efficiency and plant productivity then critically depend on enzymatic regulation and regulatory circuits connecting plant cells with their environment and re-stabilising photosynthetic efficiency and carbohydrate metabolism after temperature-induced deflection. This review summarises and integrates current knowledge about re-stabilisation of photosynthesis and carbohydrate metabolism after perturbation by changing temperature (heat and cold).

Maize Endosperm Development: Tissues, Cells, Molecular Regulation and Grain Quality Improvement

Maize endosperm plays important roles in human diet, animal feed and industrial applications. Knowing the mechanisms that regulate maize endosperm development could facilitate the improvement of grain quality. This review provides a detailed account of maize endosperm development at the cellular and histological levels. It features the stages of early development as well as developmental patterns of the various individual tissues and cell types. It then covers molecular genetics, gene expression networks, and current understanding of key regulators as they affect the development of each tissue. The article then briefly considers key changes that have occurred in endosperm development during maize domestication. Finally, it considers prospects for how knowledge of the regulation of endosperm development could be utilized to enhance maize grain quality to improve agronomic performance, nutrition and economic value.

Introduction of glucan synthase into the cytosol in wheat endosperm causes massive maltose accumulation and represses starch synthesis

We expressed a bacterial glucan synthase (Agrobacterium GlgA) in the cytosol of developing endosperm cells in wheat grains, to discover whether it could generate a glucan from cytosolic ADP-glucose. Transgenic lines had high glucan synthase activity during grain filling, but did not accumulate glucan. Instead, grains accumulated very high concentrations of maltose. They had large volumes during development due to high water content, and very shrivelled grains at maturity. Starch synthesis was severely reduced. We propose that cytosolic glucan synthesized by the glucan synthase was immediately hydrolysed to maltose by cytosolic β-amylase(s). Maltose accumulation resulted in a high osmotic potential in developing grain, drawing in excess water that stretched the seed coat and pericarp. Loss of water during grain maturation then led to shrinkage when the grains matured. Maltose accumulation is likely to account for the reduced starch synthesis in transgenic grains, through signalling and toxic effects. Using bioinformatics, we identify an isoform of β-amylase likely to be responsible for maltose accumulation. Removal of this isoform through identification of TILLING mutants or genome editing, combined with co-expression of heterologous glucan synthase and a glucan branching enzyme, may in future enable elevated yields of carbohydrate through simultaneous accumulation of starch and cytosolic glucan.

Genetic and molecular control of grain yield in maize

Understanding the genetic and molecular basis of grain yield is important for maize improvement. Here, we identified 49 consensus quantitative trait loci (cQTL) controlling maize yield-related traits using QTL meta-analysis. Then, we collected yield-related traits associated SNPs detected by association mapping and identified 17 consensus significant loci. Comparing the physical positions of cQTL with those of significant SNPs revealed that 47 significant SNPs were located within 20 cQTL regions. Furthermore, intensive reviews of 31 genes regulating maize yield-related traits found that the functions of many genes were conservative in maize and other plant species. The functional conservation indicated that some of the 575 maize genes (orthologous to 247 genes controlling yield or seed traits in other plant species) might be functionally related to maize yield-related traits, especially the 49 maize orthologous genes in cQTL regions, and 41 orthologous genes close to the physical positions of significant SNPs. In the end, we prospected on the integration of the public sources for exploring the genetic and molecular mechanisms of maize yield-related traits, and on the utilization of genetic and molecular mechanisms for maize improvement.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-021-01214-3.

Combating Dual Challenges in Maize Under High Planting Density: Stem Lodging and Kernel Abortion

High plant density is considered a proficient approach to increase maize production in countries with limited agricultural land; however, this creates a high risk of stem lodging and kernel abortion by reducing the ratio of biomass to the development of the stem and ear. Stem lodging and kernel abortion are major constraints in maize yield production for high plant density cropping; therefore, it is very important to overcome stem lodging and kernel abortion in maize. In this review, we discuss various morphophysiological and genetic characteristics of maize that may reduce the risk of stem lodging and kernel abortion, with a focus on carbohydrate metabolism and partitioning in maize. These characteristics illustrate a strong relationship between stem lodging resistance and kernel abortion. Previous studies have focused on targeting lignin and cellulose accumulation to improve lodging resistance. Nonetheless, a critical analysis of the literature showed that considering sugar metabolism and examining its effects on lodging resistance and kernel abortion in maize may provide considerable results to improve maize productivity. A constructive summary of management approaches that could be used to efficiently control the effects of stem lodging and kernel abortion is also included. The preferred management choice is based on the genotype of maize; nevertheless, various genetic and physiological approaches can control stem lodging and kernel abortion. However, plant growth regulators and nutrient application can also help reduce the risk for stem lodging and kernel abortion in maize.

Maize kernel development

Maize (Zea mays) is a leading cereal crop in the world. The maize kernel is the storage organ and the harvest portion of this crop and is closely related to its yield and quality. The development of maize kernel is initiated by the double fertilization event, leading to the formation of a diploid embryo and a triploid endosperm. The embryo and endosperm are then undergone independent developmental programs, resulting in a mature maize kernel which is comprised of a persistent endosperm, a large embryo, and a maternal pericarp. Due to the well-characterized morphogenesis and powerful genetics, maize kernel has long been an excellent model for the study of cereal kernel development. In recent years, with the release of the maize reference genome and the development of new genomic technologies, there has been an explosive expansion of new knowledge for maize kernel development. In this review, we overviewed recent progress in the study of maize kernel development, with an emphasis on genetic mapping of kernel traits, transcriptome analysis during kernel development, functional gene cloning of kernel mutants, and genetic engineering of kernel traits.

Engineering 6-phosphogluconate dehydrogenase improves grain yield in heat-stressed maize

Endosperm starch synthesis is a primary determinant of grain yield and is sensitive to high-temperature stress. The maize chloroplast-localized 6-phosphogluconate dehydrogenase (6PGDH), PGD3, is critical for endosperm starch accumulation. Maize also has two cytosolic isozymes, PGD1 and PGD2, that are not required for kernel development. We found that cytosolic PGD1 and PGD2 isozymes have heat-stable activity, while amyloplast-localized PGD3 activity is labile under heat stress conditions. We targeted heat-stable 6PGDH to endosperm amyloplasts by fusing the Waxy1 chloroplast targeting the peptide coding sequence to the Pgd1 and Pgd2 open reading frames (ORFs). These WPGD1 and WPGD2 fusion proteins import into isolated chloroplasts, demonstrating a functional targeting sequence. Transgenic maize plants expressing WPGD1 and WPGD2 with an endosperm-specific promoter increased 6PGDH activity with enhanced heat stability in vitro. WPGD1 and WPGD2 transgenes complement the pgd3-defective kernel phenotype, indicating the fusion proteins are targeted to the amyloplast. In the field, the WPGD1 and WPGD2 transgenes can mitigate grain yield losses in high-nighttime-temperature conditions by increasing kernel number. These results provide insight into the subcellular distribution of metabolic activities in the endosperm and suggest the amyloplast pentose phosphate pathway is a heat-sensitive step in maize kernel metabolism that contributes to yield loss during heat stress.

Engineering 6-phosphogluconate dehydrogenase improves grain yield in heat-stressed maize

Endosperm starch synthesis is a primary determinant of grain yield and is sensitive to high-temperature stress. The maize chloroplast-localized 6-phosphogluconate dehydrogenase (6PGDH), PGD3, is critical for endosperm starch accumulation. Maize also has two cytosolic isozymes, PGD1 and PGD2, that are not required for kernel development. We found that cytosolic PGD1 and PGD2 isozymes have heat-stable activity, while amyloplast-localized PGD3 activity is labile under heat stress conditions. We targeted heat-stable 6PGDH to endosperm amyloplasts by fusing the Waxy1 chloroplast targeting the peptide coding sequence to the Pgd1 and Pgd2 open reading frames (ORFs). These WPGD1 and WPGD2 fusion proteins import into isolated chloroplasts, demonstrating a functional targeting sequence. Transgenic maize plants expressing WPGD1 and WPGD2 with an endosperm-specific promoter increased 6PGDH activity with enhanced heat stability in vitro. WPGD1 and WPGD2 transgenes complement the pgd3-defective kernel phenotype, indicating the fusion proteins are targeted to the amyloplast. In the field, the WPGD1 and WPGD2 transgenes can mitigate grain yield losses in high-nighttime-temperature conditions by increasing kernel number. These results provide insight into the subcellular distribution of metabolic activities in the endosperm and suggest the amyloplast pentose phosphate pathway is a heat-sensitive step in maize kernel metabolism that contributes to yield loss during heat stress.

Breeding Maize Maternal Haploid Inducers

Maize doubled haploid (DH) lines are usually created in vivo, through crosses with maternal haploid inducers. These inducers have the inherent ability of generating seeds with haploid embryos when used to pollinate other genotypes. The resulting haploid plants are treated with a doubling agent and self-pollinated, producing completely homozygous seeds. This rapid method of inbred line production reduces the length of breeding cycles and, consequently, increases genetic gain. Such advantages explain the wide adoption of this technique by large, well-established maize breeding programs. However, a slower rate of adoption was observed in medium to small-scale breeding programs. The high price and/or lack of environmental adaptation of inducers available for licensing, or the poor performance of those free of cost, might explain why smaller operations did not take full advantage of this technique. The lack of adapted inducers is especially felt in tropical countries, where inducer breeding efforts are more recent. Therefore, defining optimal breeding approaches for inducer development could benefit many breeding programs which are in the process of adopting the DH technique. In this manuscript, we review traits important to maize maternal haploid inducers, explain their genetic basis, listing known genes and quantitative trait loci (QTL), and discuss different breeding approaches for inducer development. The performance of haploid inducers has an important impact on the cost of DH line production.

A New SNP in Rice Gene Encoding Pyruvate Phosphate Dikinase (PPDK) Associated with Floury Endosperm

Rice varieties with suitable flour-making qualities are required to promote the rice processed-food industry and to boost rice consumption. A rice mutation, Namil(SA)-flo1, produces grains with floury endosperm. Overall, grains with low grain hardness, low starch damage, and fine particle size are more suitable for use in flour processing grains with waxy, dull endosperm with normal grain hardness and a high amylose content. In this study, fine mapping found a C to T single nucleotide polymorphism (SNP) in exon 2 of the gene encoding cytosolic pyruvate phosphate dikinase (cyOsPPDK). The SNP resulted in a change of serine to phenylalanine acid at amino acid position 101. The gene was named FLOURY ENDOSPERM 4-5 (FLO4-5). Co-segregation analysis with the developed cleaved amplified polymorphic sequence (CAPS) markers revealed co-segregation between the floury phenotype and the flo4-5. This CAPS marker could be applied directly for marker-assisted selection. Real-time RT-PCR experiments revealed that PPDK was expressed at considerably higher levels in the flo4-5 mutant than in the wild type during the grain filling stage. Plastid ADP-glucose pyrophosphorylase small subunit (AGPS2a and AGPS2b) and soluble starch synthase (SSIIb and SSIIc) also exhibited enhanced expression in the flo4-5 mutant.

Starch Trek: The Search for Yield

Starch is a plant storage polyglucan that accumulates in plastids. It is composed of two polymers, amylose and amylopectin, with different structures and plays several roles in helping to determine plant yield. In leaves, it acts as a buffer for night time carbon starvation. Genetically altered plants that cannot synthesize or degrade starch efficiently often grow poorly. There have been a number of successful approaches to manipulate leaf starch metabolism that has resulted in increased growth and yield. Its degradation is also a source of sugars that can help alleviate abiotic stress. In edible parts of plants, starch often makes up the majority of the dry weight constituting much of the calorific value of food and feed. Increasing starch in these organs can increase this as well as increasing yield. Enzymes involved in starch metabolism are well known, and there has been much research analyzing their functions in starch synthesis and degradation, as well as genetic and posttranslational regulatory mechanisms affecting them. In this mini review, we examine work on this topic and discuss future directions that could be used to manipulate this metabolite for improved yield.

Fundamental differences in starch synthesis in the maize leaf, embryo, ovary and endosperm

Enzymological and starch analyses of various ADP-glucose pyrophosphorylase (AGPase) null mutants point to fundamental differences in the pathways for starch synthesis in the maize leaf, embryo, ovary and endosperm. Leaf starch is synthesized via the AGPase encoded by the small and large subunits shown previously to be expressed at abundant levels in the leaf, whereas more than one AGPase isoform functions in the embryo and in the ovary. Embryo starch content is also dependent on genes functioning in the leaf and in the endosperm. AGPase encoded by shrunken-2 and brittle-2 synthesizes ~75% of endosperm starch. The gene, agpsemzm, previously shown to encode the small subunit expressed in the embryo, and agpllzm, the leaf large subunit gene, are here shown to encode the endosperm, plastid-localized AGPase. Loss of this enzyme does not reduce endosperm starch. Rather, the data suggest that AGPase-independent starch synthesis accounts for ~25% of endosperm starch. Three maize genes encode the small subunit of the AGPase. Data here show that the triple mutant lacking all three small subunits is lethal in early seed development but can be viable in both male and female gametes. Seed and plant viability is restored by any one of the three small subunit genes, including one previously thought to function only in the cytosol of the endosperm. Data herein also show the functionality of a fourth gene encoding the large subunit of this enzyme. Although adenosine diphosphate glucose pyrophosphorylase is shown here to be essential for maize viability, strong evidence for starch synthesis in the endosperm that is independent of this enzyme is also presented. Starch synthesis is distinct in the maize embryo, ovary, leaf and endosperm, and is coordinated among the various tissues.

Combinatorial interaction of two adjacent cis-active promoter regions mediates the synergistic induction of Bt2 gene by sucrose and ABA in maize endosperm

The accumulation of starch in cereal endosperm is a key process that determines crop yield and quality. Research has reported that sucrose and abscisic acid (ABA) synergistically regulate the synthesis of crop starch. However, little is known about the molecular mechanisms behind this synergistic effect. In this study, the effect of sucrose and ABA on starch synthesis in maize endosperm was investigated. The starch content, the ADP-Glc pyrophosphorylase (AGPase) concentration, and the expression of AGPase-encoding genes were found to be enhanced slightly by sucrose or ABA alone, but were elevated significantly by the co-treatment of sucrose and ABA. Truncation analysis of the Bt2 promoter via transient expression in maize endosperm showed that the promoter region (-370/-186) is involved in sucrose response, and that an adjacent region (-186/-43) responds to ABA. The synergistic induction of sucrose and ABA on Bt2 promoter activity requires interaction with both of these regions. Interestingly, removal of the sucrose-responsive region (-370 to -186) abolishes ABA responsiveness in the Bt2 promoter, even in the presence of ABA-responsive region (-186 to -43). This study provides novel insights into the regulatory mechanisms that underlie the synergistic regulation of starch synthesis and grain filling from sucrose and ABA in cereal endosperm.

Ovary abortion is prevalent in diverse maize inbred lines and is under genetic control

Crop improvement programs focus on characteristics that are important for plant productivity. Typically genes underlying these traits are identified and stacked to create improved cultivars. Hence, identification of valuable traits for plant productivity is critical for plant improvement. Here we describe an important characteristic for maize productivity. Despite the fact mature maize ears are typically covered with kernels, we find that only a fraction of ovaries give rise to mature kernels. Non-developed ovaries degenerate while neighboring fertilized ovaries produce kernels that fill the ear. Abortion occurs throughout the ear, not just at the tip. We show that the fraction of aborted ovaries/kernels is genetically controlled and varies widely among maize lines, and low abortion genotypes are rare. Reducing or eliminating ovary abortion could substantially increase yield, making this characteristic a new target for selection in maize improvement programs.