Starch biosynthesis in cereal endosperms: An updated review over the last decade

Starch is a vital energy source for living organisms and is a key raw material and additive in the food and non-food industries.Starch has received continuous attention in multiple research fields.The endosperm of cereals(e.g.,rice,corn,wheat,and barley)is the most important site for the synthesis of storage starch.Around 2010,several excellent reviews summarized key progress in various fields of starch research,serving as important references for subsequent research.In the past 10 years,many achievements have been made in the study of starch synthesis and regulation in cereals.The present review provides an update on research progress in starch synthesis of cereal endosperms over the past decade,focusing on new enzymes and non-enzymatic proteins involved in starch synthesis,regulatory networks of starch synthesis,and the use of elite alleles of starch synthesis-related genes in cereal breeding programs.We also provide perspectives on future research directions that will further our understanding of cereal starch biosynthesis and regulation to support the rational design of ideal quality grain.

OsPKpa_1 encodes a plastidic pyruvate kinase that affects starch biosynthesis in the rice endosperm

Pyruvate kinase （PK） is a key enzyme in glycolysis and carbon metabolism. Here, we isolated a rice （Oryza sativa） mutant, w59, with a white-core floury endosperm. Map-based cloning of w59 identified a mutation in OsPKpα1, which encodes a plastidic isoform of PK （PKp）. OsPKpα1 localizes to the amyloplast stroma in the developing endosperm, and the mutation of OsPKpα1 in w59 decreases the plastidic PK activity, resulting in dramatic changes to the lipid biosynthesis in seeds. The w59 grains were also characterized by a marked decrease in starch content. Consistent with a decrease in number and size of the w59 amyloplasts, large empty spaces were observed in the central region of the w59 endosperm, at the early grain-filling stage. Moreover, a phylogenetic analysis revealed four potential rice isoforms of OsPKp. We validated the in vitro PK activity of these OsPKps through reconstituting active PKp complexes derived from inactive individual OsPKps, revealing the heteromeric structure of rice PKps, which was further confirmed using a protein- protein interaction analysis. These findings suggest a functional connection between lipid and starch synthesis in rice endosperm amyloplasts.

Starch Metabolism in Plant and Its Applications in Food Industry

Starch, a polymer of sugars in plants, is widely used in various industries due to its properties. It is synthesized through ADP-glucose formation and enzyme-mediated processes. Starch is formed during the day and broken down into sugars at night, which are then transported and converted back to starch in storage tissues. This review explores starch metabolism pathways and its role in the food industry, providing valuable insights on energy storage in plants.

Origin and evolution of the main starch biosynthetic enzymes

Starch,a semi-crystalline energy storage form primarily found in plant plastids plays a crucial role in various food or no-food applications.Despite the starch biosynthetic pathway’s main enzymes have been characterized,their origin and evolution remained a subject of debate.In this study,we conducted the comprehensive phylogenetic and structural analysis of three types of starch biosynthetic enzymes:starch synthase(SS),starch branching enzyme(SBE)and isoamylase-type debranching enzyme(ISA)from 51,151 annotated genomes.Our findings provide valuable insights into the possible scenario for the origin and evolution of the starch biosynthetic pathway.Initially,the ancestor of SBE can be traced back to an unidentified bacterium that existed before the formation of the last eukaryotic common ancestor(LECA)via horizontal gene transfer(HGT).This transfer event likely provided the eukaryote ancestor with the ability to synthesize glycogen.Furthermore,during the emergence of Archaeplastida,one clade of SS was transferred from Deltaproteobacteria by HGT,while ISA and the other clade of SS originated from Chlamydiae through endosymbiosis gene transfer(EGT).Both these transfer events collectively contributed to the establishment of the original starch biosynthetic pathway.Subsequently,after the divergence of Viridiplantae from Rhodophyta,all three enzymes underwent multiple duplications and N-terminus extension domain modifications,resulting in the formation of functionally specialized isoforms and ultimately leading to the complete starch biosynthetic pathway.By shedding light on the evolutionary origins of key enzymes involved in the starch biosynthetic pathway,this study provides important insights into the evolutionary events of plants.

Effects of high CO_2 treatment on green-ripening and peel senescence in banana and plantain fruits

Banana fruit（Musa,AAA group,cv.Brazil） peel fails to fully degreen but the pulp ripens normally at temperatures above24°C.This abnormal ripening,known as green-ripening,does not occur in plantains（Musa,ABB group,cv.Dajiao）.Based on the fact that un-completely yellowing was also observed for bananas in poorly ventilated atmospheres,in the present study,the effect of high CO2 with regular O2（21%） on banana ripening was investigated along with that on plantains at20℃.The results showed that high CO2 conferred different effects on the color changing of bananas and plantains.After6 d ripening in 20%CO2,plantains fully yellowed,while bananas retained high chlorophyll content and stayed green.In contrast to the differentiated color changing patterns,the patterns of the softening,starch degradation and soluble sugar accumulation in the pulp of 20%CO2 treated bananas and plantains displayed similarly as the patterns in the fruits ripening in regular air,indicating that the pulp ripening was not inhibited by 20%CO2,and the abnormal ripening of bananas in 20%CO2 can be considered as green ripening.Similar expression levels of chlorophyll degradation related genes,SGR,NYC and PaO,were detected in the peel of the control and treated fruits,indicating that the repressed degreening in 20%CO2treated bananas was not due to the down-regulation of the chlorophyll degradation related genes.Compared to the effect on plantains,20%CO2 treatment delayed the decline in the chlorophyll florescence（F√F_m values and in the mRNA levels of a gene coding small subunit of Rubisco（SSU）,and postponed the disruption of the ultrastructure of chloroplast in the peel tissue of bananas,indicating that the senescence of the green cells in the exocarp layer was delayed by 20%CO2,to more extent in bananas than in plantains.High CO2 reduced the ethylene production and the expression of the related biosynthesis gene,ACS,but elevated the respiration rates in both cultivars.The up-regulation of the expression of anaerobic respiration pathway genes,ADH and PDC,might be responsible for the subtle effect of high CO2 on the pulp ripening.Taken together,the atmosphere of high CO2 and regular O2,delayed the senescence of the green cells in the exocarp layer of the banana peel,but conferred no obvious inhibition on the pulp ripening,leading to a distinct green-ripening that was different from the phenomenon induced by high temperatures.

Genome-wide transcriptional analysis of maize endosperm in response to ae wx double mutations

Starch biosynthesis is important during endosperm development. Much has been known for the regulation of gene expression involved in starch synthesis, less information is available on the genome-wide expression profiles as a consequence of impaired starch synthesis. In this study, we examined the transcriptional responses through microarray analysis in an ae wx double-mutant with loss-of-function starch branching enzyme IIb （SBEIIb） and granule-bound starch synthase I （GBSSI）. Through Gene Ontology enrichment analysis, we identified differentially expressed genes （DEGs） involved in chromatin organization and lipid transport. The DEGs also include alcohol dehydrogenase genes and pyruvate decarboxylase genes involved in sugar metabolism. In summary, the ae wx double mutations caused pleiotropic effects and transcriptional changes for a number of genes involved in metabolism, cellular response and organization. Therefore, a block in starch synthesis triggers transcriptional responses to favour the flux of excess carbohydrates into glycolysis, pentose phosphate pathway, and cell wall biosynthesis, but not toward the synthesis of alternative storage compounds.