Search

Recent work has identified several proteins involved in starch granule initiation, the first step of starch synthesis. However, the degree of conservation in the granule initiation process remains poorly understood, especially among grass species differing in patterns of carbohydrate turnover in leaves, and granule morphology in the endosperm. We therefore compared mutant phenotypes of Hordeum vulgare (barley), Triticum turgidum (durum wheat), and Brachypodium distachyon defective in PROTEIN TARGETING TO STARCH 2 (PTST2), a key granule initiation protein. We report striking differences across species and organs. Loss of PTST2 from leaves resulted in fewer, larger starch granules per chloroplast and normal starch content in wheat, fewer granules per chloroplast and lower starch content in barley, and almost complete loss of starch in Brachypodium. The loss of starch in Brachypodium leaves was accompanied by high levels of ADP-glucose and detrimental effects on growth and physiology. Additionally, we found that loss of PTST2 increased granule initiation in Brachypodium amyloplasts, resulting in abnormal compound granule formation throughout the seed. These findings suggest that the importance of PTST2 varies greatly with the genetic and developmental background and inform the extent to which the gene can be targeted to improve starch in crops.

A reduction in cytosolic invertase in Arabidopsis roots changes the sucrose to hexose ratio, resulting in profound alterations of metabolism, growth, development, and patterns of gene expression., Plant roots depend on sucrose imported from leaves as the substrate for metabolism and growth. Sucrose and hexoses derived from it are also signalling molecules that modulate growth and development, but the importance for signalling of endogenous changes in sugar levels is poorly understood. We report that reduced activity of cytosolic invertase, which converts sucrose to hexoses, leads to pronounced metabolic, growth, and developmental defects in roots of Arabidopsis (Arabidopsis thaliana) seedlings. In addition to altered sugar and downstream metabolite levels, roots of cinv1 cinv2 mutants have reduced elongation rates, cell and meristem size, abnormal meristematic cell division patterns, and altered expression of thousands of genes of diverse functions. Provision of exogenous glucose to mutant roots repairs relatively few of the defects. The extensive transcriptional differences between mutant and wild-type roots have hallmarks of both high sucrose and low hexose signalling. We conclude that the mutant phenotype reflects both low carbon availability for metabolism and growth and complex sugar signals derived from elevated sucrose and depressed hexose levels in the cytosol of mutant roots. Such reciprocal changes in endogenous sucrose and hexose levels potentially provide rich information about sugar status that translates into flexible adjustments of growth and development.

Starch, the major storage carbohydrate in plants, is synthesized in plastids as semi-crystalline, insoluble granules. Many organs and cell types accumulate starch at some point during their development and maturation. The biosynthesis of the starch polymers, amylopectin and amylose, is relatively well understood and mostly conserved between organs and species. However, we are only beginning to understand the mechanism by which starch granules are initiated, and the factors that control the number of granules per plastid and the size/shape of granules. Here, we review recent progress in understanding starch granule initiation and morphogenesis. In Arabidopsis, granule initiation requires several newly discovered proteins with specific locations within the chloroplast, and also on the availability of maltooligosaccharides which act as primers for initiation. We also describe progress in understanding granule biogenesis in the endosperm of cereal grains-within which there is large interspecies variation in granule initiation patterns and morphology. Investigating whether this diversity results from differences between species in the functions of known proteins, and/or from the presence of novel, unidentified proteins, is a promising area of future research. Expanding our knowledge in these areas will lead to new strategies for improving the quality of cereal crops by modifying starch granule size and shape in vivo.

Since starch is by far the major component of the mature wheat grain, it has been assumed that variation in the capacity for starch synthesis during grain filling can influence final grain weight. We investigated this assumption by studying a total of 54 wheat genotypes including elite varieties and landraces that were grown in two successive years in fields in the east of England. The weight, water content, sugars, starch, and maximum catalytic activities of two enzymes of starch biosynthesis, ADP-glucose pyrophosphorylase and soluble starch synthase, were measured during grain filling. The relationships between these variables and the weights and starch contents of mature grains were analysed. Final grain weight showed few or no significant correlations with enzyme activities, sugar levels, or starch content during grain filling, or with starch content at maturity. We conclude that neither sugar availability nor enzymatic capacity for starch synthesis during grain filling significantly influenced final grain weight in our field conditions. We suggest that final grain weight may be largely determined by developmental processes prior to grain filling. Starch accumulation then fills the grain to a physical limit set by developmental processes. This conclusion is in accord with those from previous studies in which source or sink strength has been artificially manipulated.

Changes in the capacities of enzymes in various metabolic pathways have been measured during infection of cotyledons of Cucurbita pepo L. with cucumber mosaic virus (CMV). Starch accumulation and low sucrose content, which are characteristic of the early stages of infection, are reversed in the later stages of infection. The decline in starch correlated with a reduced capacity for starch synthesis (ADP-glucose pyrophosphorylase) and a rise in the capacity for starch degradation (total starch hydrolase, starch phosphorylase). 14CO2 feeding experiments, conducted at saturating C0 2 concentration, show that the newlyassimilated carbon was lost at a lower rate from infected cotyledons and less was incorporated into structural carbohydrates, phosphorylated intermediates plus organic acids, more into soluble sugars, amino acids and proteins. At a later stage of infection there were dramatic increases in respiratory capacity and a substantial alteration of carbohydrate metabolism. The infection had a large stimulatory effect on the capacity for oxidative pentose-phosphate pathway (glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase), glycolysis (ATP- and pyrophosphate-dependent phosphofructokinases), tricarboxylic acid cycle (isocitrate dehydrogenase, fumarate hydratase), anaplerotic reactions (NADdependent malic enzyme, phosphoenolpyruvate carboxylase) and oxidative electron transport (cytochrome c oxidase). While there were no overall changes in photosynthetic rate (measured in saturating CO-J, infection either reduced (Rubisco and glycerate kinase) or did not affect (chloroplastic fructose bisphosphatase and hydroxypyruvate kinase) the capacities of the photosynthetic carbon reduction pathway or the photosynthetic carbon oxidation pathway.

It has frequently been observed that starch accumulates in plant tissues after virus infection. In pea plants infected with red clover mottle comovirus, strain «O», virus replicates in the inoculated leaves and at the shoot apex where it induces lethal top necrosis. Concomitant with the onset of top necrosis, starch accumulates in the intervening leaves which remain substantially virus-free. This pattern of starch accumulation can be mimicked by removal of the apex in uninfected plants. We conclude that in this plant-virus interaction starch accumulation is an indirect consequence of virus infection associated with the removal of the physiological sink for photosynthate