Regulators of Starch Biosynthesis in Cereal Crops

Starch is the main food source for human beings and livestock all over the world, and it is also the raw material for production of industrial alcohol and biofuel. A considerable part of the world’s annual starch production comes from crops and their seeds. With the increasing demand for starch from food and non-food industries and the growing loss of arable land due to urbanization, understanding starch biosynthesis and its regulators is essential to produce the desirable traits as well as more and better polymers via biotechnological approaches in cereal crops. Because of the complexity and flexibility of carbon allocation in the formation of endosperm starch, cereal crops require a broad range of enzymes and one matching network of regulators to control the providential functioning of these starch biosynthetic enzymes. Here, we comprehensively summarize the current knowledge about regulatory factors of starch biosynthesis in cereal crops, with an emphasis on the transcription factors that directly regulate starch biosynthesis. This review will provide new insights for the manipulation of bioengineering and starch biosynthesis to improve starch yields or qualities in our diets and in industry.

Endosperm starch grains of Andropogon, Arthraxon hispidus, and Hyparrhenia rufa (Andropogoneae, Panicoideae, Poaceae)

Background and Aims: Grasses have five different types of endosperm starch grain morphology. Even though there is high diversity within the family, the morphology of the starch grains is generally represented by one or two species. Some genera, such as Andropogon (Andropogoneae), were reported to have at least three types of starch grains. However, most of the reviewed species were transferred to other genera. Therefore, the question whether the genus has one or more types of starch grain morphology remains unanswered. Methods: Between four and eight mature caryopses were removed from specimens deposited in the herbarium IEB for most species, as well as from plants monitored in the field until they had mature caryopses. The caryopses were attached on a plate with a drop of white adhesive Resistol® or resin and then sectioned with a razor blade. Sections were stained with a drop of diluted Lugol´s solution, microscopically observed and photographed at several magnifications. Key results: All Andropogon species observed have only one type of starch grain morphology, the Andropogon-type. In all species the simple starch grains are much more abundant than the compound ones, except in A. tenuispatheus where the ratio is inverted. The other two reviewed species, Arthraxon hispidus and Hyparrhenia rufa have Andropogon-type and Panicum-type starch grains, respectively. Conclusions: It is confirmed that, so far, all Andropogon species observed have one type (Andropogon-type) of endosperm starch grain morphology. There is variation in the size, size distribution and shape of the starch grains among the species. Andropogon gayanus is the only reviewed species with large starch grains reaching 28 µm, whereas those in the other species measure up to 15 µm in diameter.

Sweet Corn Research around the World 2015–2020

Modern sweet corn is distinguished from other vegetable corns by the presence of one or more recessive alleles within the maize endosperm starch synthesis pathway. This results in reduced starch content and increased sugar concentration when consumed fresh. Fresh sweet corn originated in the USA and has since been introduced in countries around the World with increasing popularity as a favored vegetable choice. Several reviews have been published recently on endosperm genetics, breeding, and physiology that focus on the basic biology and uses in the US. However, new questions concerning sustainability, environmental care, and climate change, along with the introduction of sweet corn in other countries have produced a variety of new uses and research activities. This review is a summary of the sweet corn research published during the five years preceding 2021.

STARCH SYNTHASE 4 is required for normal starch granule initiation in amyloplasts of wheat endosperm

SUMMARYStarch granule initiation is poorly understood at the molecular level. The glucosyltransferase, STARCH SYNTHASE 4 (SS4), plays a central role in granule initiation in Arabidopsis leaves, but its function in cereal endosperms is unknown. We investigated the role of SS4 in wheat, which has a distinct spatiotemporal pattern of granule initiation during grain development.We generated TILLING mutants in tetraploid wheat (Triticum turgidum) that are defective in both SS4 homoeologs. The morphology of endosperm starch was examined in developing and mature grains.SS4 deficiency led to severe alterations in endosperm starch granule morphology. During early grain development, while the wild type initiated single ‘A-type’ granules per amyloplast, most amyloplasts in the mutant formed compound granules due to multiple initiations. This phenotype was similar to mutants deficient in B-GRANULE CONTENT 1 (BGC1). SS4 deficiency also reduced starch content in leaves and pollen grains.We propose that SS4 and BGC1 are required for the proper control of granule initiation during early grain development that leads to a single A-type granule per amyloplast. The absence of either protein results in a variable number of initiations per amyloplast and compound granule formation.

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 to improve heat tolerance in maize seed development

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 peptide coding sequence to the Pgd1 and Pgd2 open reading frames. 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 on 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.Significance StatementHeat stress reduces yield in maize by affecting the number of kernels that develop and the accumulation of seed storage molecules during grain fill. Climate change is expected to increase frequency and duration of high temperature stress, which will lower grain yields. Here we show that one enzyme in central carbon metabolism is sensitive to high temperatures. By providing a heat-resistant form of the enzyme in the correct subcellular compartment, a larger number of kernels develop per plant during heat stress in the field. This genetic improvement could be included as part of integrated approaches to mitigate yield losses due to climate change.

LYS3 encodes a prolamin-box-binding transcription factor that controls embryo growth in barley and wheat

ABSTRACTMutations at the LYS3 locus in barley have multiple effects on grain development, including an increase in embryo size and a decrease in endosperm starch content. The gene underlying LYS3 was identified by genetic mapping and mutations in this gene were identified in all four barley lys3 alleles. LYS3 encodes a transcription factor called Prolamin Binding Factor (PBF). Its role in controlling embryo size was confirmed using wheat TILLING mutants. To understand how PBF controls embryo development, we studied its spatial and temporal patterns of expression in developing grains. The PBF gene is expressed in both the endosperm and the embryos, but the timing of expression in these organs differs. PBF expression in wild-type embryos precedes the onset of embryo enlargement in lys3 mutants, suggesting that PBF suppresses embryo growth. We predicted the down-stream target genes of PBF in wheat and found them to be involved in a wide range of biological processes, including organ development and starch metabolism. Our work suggests that PBF may influence embryo size and endosperm starch synthesis via separate gene control networks.HIGHLIGHTSLYS3 encodes a transcription factor called Prolamin Binding Factor (PBF) that is expressed in grains only.Wheat and barley LYS3/PBF mutants have enlarged embryos suggesting that this gene suppresses embryo growth.The down-stream target genes of PBF in wheat are predicted to be involved in a wide range of biological processes including organ development and starch metabolism.

Maize sugary enhancer1 (se1) is a gene affecting endosperm starch metabolism

sugary enhancer1 (se1) is a naturally occurring mutant allele involved in starch metabolism in maize endosperm. It is a recessive modifier of sugary1 (su1) and commercially important in modern sweet corn breeding, but its molecular identity and mode of action remain unknown. Here, we developed a pair of near-isogenic lines, W822Gse (su1-ref/su1-ref se1/se1) and W822GSe (su1-ref/su1-ref Se1/Se1), that Mendelize the se1 phenotype in an su1-ref background. W822Gse kernels have lower starch and higher water soluble polysaccharide and sugars than W822GSe kernels. Using high-resolution genetic mapping, we found that wild-type Se1 is a gene Zm00001d007657 on chromosome 2 and a deletion of this gene causes the se1 phenotype. Comparative metabolic profiling of seed tissue between these 2 isolines revealed the remarkable difference in carbohydrate metabolism, with sucrose and maltose highly accumulated in the mutant. Se1 is predominantly expressed in the endosperm, with low expression in leaf and root tissues. Differential expression analysis identified genes enriched in both starch biosynthesis and degradation processes, indicating a pleiotropic regulatory effect of se1. Repressed expression of Se1 and Su1 in RNA interference-mediated transgenic maize validates that deletion of the gene identified as Se1 is a true causal gene responsible for the se1 phenotype. The findings contribute to our understanding of starch metabolism in cereal crops.