Plastid thylakoid architecture optimizes photosynthesis in diatoms

Photosynthesis is a unique process that allows independent colonization of the land by plants and of the oceans by phytoplankton. Although the photosynthesis process is well understood in plants, we are still unlocking the mechanisms evolved by phytoplankton to achieve extremely efficient photosynthesis. Here, we combine biochemical, structural and in vivo physiological studies to unravel the structure of the plastid in diatoms, prominent marine eukaryotes. Biochemical and immunolocalization analyses reveal segregation of photosynthetic complexes in the loosely stacked thylakoid membranes typical of diatoms. Separation of photosystems within subdomains minimizes their physical contacts, as required for improved light utilization. Chloroplast 3D reconstruction and in vivo spectroscopy show that these subdomains are interconnected, ensuring fast equilibration of electron carriers for efficient optimum photosynthesis. Thus, diatoms and plants have converged towards a similar functional distribution of the photosystems although via different thylakoid architectures, which likely evolved independently in the land and the ocean.ISSN:2041-172

Cecropia peltata accumulates starch or soluble glycogen by differentially regulating starch biosynthetic genes

The branched glucans glycogen and starch are the most widespread storage carbohydrates in living organisms. The production of semicrystalline starch granules in plants is more complex than that of small, soluble glycogen particles in microbes and animals. However, the factors determining whether glycogen or starch is formed are not fully understood. The tropical tree Cecropia peltata is a rare example of an organism able to make either polymer type. Electron micrographs and quantitative measurements show that glycogen accumulates to very high levels in specialized myrmecophytic structures (Müllerian bodies), whereas starch accumulates in leaves. Compared with polymers comprising leaf starch, glycogen is more highly branched and has shorter branches--factors that prevent crystallization and explain its solubility. RNA sequencing and quantitative shotgun proteomics reveal that isoforms of all three classes of glucan biosynthetic enzyme (starch/glycogen synthases, branching enzymes, and debranching enzymes) are differentially expressed in Müllerian bodies and leaves, providing a system-wide view of the quantitative programming of storage carbohydrate metabolism. This work will prompt targeted analysis in model organisms and cross-species comparisons. Finally, as starch is the major carbohydrate used for food and industrial applications worldwide, these data provide a basis for manipulating starch biosynthesis in crops to synthesize tailor-made polyglucans

The Simultaneous Abolition of Three Starch Hydrolases Blocks Transient Starch Breakdown in Arabidopsis

In this study, we investigated which enzymes are involved in debranching amylopectin during transient starch degradation. Previous studies identified two debranching enzymes, isoamylase 3 (ISA3) and limit dextrinase (LDA), involved in this process. However, plants lacking both enzymes still degrade substantial amounts of starch. Thus, other enzymes/mechanisms must contribute to starch breakdown. We show that the chloroplastic α-amylase 3 (AMY3) also participates in starch degradation and provide evidence that all three enzymes can act directly at the starch granule surface. The isa3 mutant has a starch excess phenotype, reflecting impaired starch breakdown. In contrast, removal of AMY3, LDA, or both enzymes together has no impact on starch degradation. However, removal of AMY3 or LDA in addition to ISA3 enhances the starch excess phenotype. In plants lacking all three enzymes, starch breakdown is effectively blocked, and starch accumulates to the highest levels observed so far. This provides indirect evidence that the heteromultimeric debranching enzyme ISA1-ISA2 is not involved in starch breakdown. However, we illustrate that ISA1-ISA2 can hydrolyze small soluble branched glucans that accumulate when ISA3 and LDA are missing, albeit at a slow rate. Starch accumulation in the mutants correlates inversely with plant growth.ISSN:0021-9258ISSN:1083-351

Diurnal Leaf Starch Content: An Orphan Trait in Forage Legumes

Forage legumes have a relatively high biomass yield and crude protein content, but their grazed and harvested biomass lacks the high-energy carbohydrates required to meet the productivity potential of modern livestock breeds. Because of their low carbohydrate content, forage legume diets are typically supplemented with starch rich cereal grains or maize (Zea mays), leading to the disruption of local nutrient cycles. Although plant leaves were first reported to accumulate starch in a diurnal pattern over a century ago, leaf starch content has yet to be exploited as an agronomic trait in forage crops. Forage legumes such as red clover (Trifolium pratense) have the genetic potential to accumulate up to one third of their leaf dry mass as starch, but this starch is typically degraded at night to support nighttime growth and respiration. Even when diurnal accumulation is considered with regard to the time the crop is harvested, only limited gains are realized due to environmental effects and post-harvest losses from respiration. Here we present original data for starch metabolism in red clover and place it in the broader context of other forage legumes such as, white clover (T. repens), and alfalfa (Medicago sativa). We review the application of recent advances in molecular breeding, plant biology, and crop phenotyping, to forage legumes to improve and exploit a potentially valuable trait for sustainable ruminant livestock production

Southeast Asian waxy maize (Zea mays L.), a resource for amylopectin starch quality types?

Amylose-free (waxy) maize has been a vegetable (cooked ears) and staple food in Southeast Asia for centuries, resulting in hundreds of landraces (LRs) across the region. The recessive waxy allele induces soft grains with preferred cooking and flavour properties. We hypothesized that eating preferences resulted in the additional selection for different starch properties, reflected in altered starch granule morphology or amylopectin structure. A total of 41 LRs were available as starting material that had been used by different ethnic groups in Vietnam and Thailand. Unluckily, some LR were not pure waxy, but we successfully regained the original pure waxy status for most. Twenty LR were chosen for analysis of starch traits according to their purity. Four different waxy mutations were identified, including two unknown alleles. This is a strong proof for parallel independent selection of waxy maize in the region. Starch granule morphology and size were similar among all LRs. Gelatinization properties differed only between waxy and wild-type LR, and all waxy LR were comparable to a commercial waxy hybrid. The fine structure of waxy amylopectin had fewer short chains compared with that in wild-type. So far, the differences observed in starch properties are likely associated exclusively with the waxy trait. Despite the strong selection for amylose-free starch, there was no evidence for additional region wide selection for other special starch properties in our collection. In conclusion, all analyses did not encourage further targeted research on allelic variation of other starch metabolism genes for future use in the food and feed industry.ISSN:1479-2621ISSN:1479-263

Starch Granule Biosynthesis in Arabidopsis Is Abolished by Removal of All Debranching Enzymes but Restored by the Subsequent Removal of an Endoamylase[W][OA]

Several studies have suggested that debranching enzymes (DBEs) are involved in the biosynthesis of amylopectin, the major constituent of starch granules. Our systematic analysis of all DBE mutants of Arabidopsis thaliana demonstrates that when any DBE activity remains, starch granules are still synthesized, albeit with altered amylopectin structure. Quadruple mutants lacking all four DBE proteins (Isoamylase1 [ISA1], ISA2, and ISA3, and Limit-Dextrinase) are devoid of starch granules and instead accumulate highly branched glucans, distinct from amylopectin and from previously described phytoglycogen. A fraction of these glucans are present as discrete, insoluble, nanometer-scale particles, but the structure and properties of this material are radically altered compared with wild-type amylopectin. Superficially, these data support the hypothesis that debranching is required for amylopectin synthesis. However, our analyses show that soluble glucans in the quadruple DBE mutant are degraded by α- and β-amylases during periods of net accumulation, giving rise to maltose and branched malto-oligosaccharides. The additional loss of the chloroplastic α-amylase AMY3 partially reverts the phenotype of the quadruple DBE mutant, restoring starch granule biosynthesis. We propose that DBEs function in normal amylopectin synthesis by promoting amylopectin crystallization but conclude that they are not mandatory for starch granule synthesis

Plasmodesmal connectivity in C₄ Gynandropsis gynandra is induced by light and dependent on photosynthesis

In leaves of C₄ plants the reactions of photosynthesis become restricted between two compartments. Typically, this allows accumulation of C₄ acids in mesophyll cells and subsequent decarboxylation in the bundle sheath. In C₄ grasses proliferation of plasmodesmata between these cell types is thought to increase cell-to-cell connectivity to allow efficient metabolite movement. However, it is not known if C₄ dicotyledons also show this enhanced plasmodesmal connectivity and so whether this is a general requirement for C₄ photosynthesis is not clear. How mesophyll and bundle sheath cells in C₄ leaves become highly connected is also not known. We investigated these questions using 3D- and 2D- electron microscopy on the C₄ dicotyledon Gynandropsis gynandra as well as phylogenetically close C₃ relatives. The mesophyll-bundle sheath interface of C₄ G. gynandra showed higher plasmodesmal frequency compared with closely related C₃ species. Formation of these plasmodesmata was induced by light. Pharmacological agents that perturbed chloroplast development or photosynthesis reduced the number of plasmodesmata, but this inhibitory effect could be reversed by the provision of exogenous sucrose. We conclude that enhanced formation of plasmodesmata between mesophyll and bundle sheath cells is wired to the induction of photosynthesis in C₄ G. gynandra

Plasmodesmal connectivity in C4 Gynandropsis gynandra is induced by light and dependent on photosynthesis.

Publication status: PublishedIn leaves of C4 plants, the reactions of photosynthesis become restricted between two compartments. Typically, this allows accumulation of C4 acids in mesophyll (M) cells and subsequent decarboxylation in the bundle sheath (BS). In C4 grasses, proliferation of plasmodesmata between these cell types is thought to increase cell-to-cell connectivity to allow efficient metabolite movement. However, it is not known whether C4 dicotyledons also show this enhanced plasmodesmal connectivity and so whether this is a general requirement for C4 photosynthesis is not clear. How M and BS cells in C4 leaves become highly connected is also not known. We investigated these questions using 3D- and 2D-electron microscopy on the C4 dicotyledon Gynandropsis gynandra as well as phylogenetically close C3 relatives. The M-BS interface of C4 G. gynandra showed higher plasmodesmal frequency compared with closely related C3 species. Formation of these plasmodesmata was induced by light. Pharmacological agents that perturbed photosynthesis reduced the number of plasmodesmata, but this inhibitory effect could be reversed by the provision of exogenous sucrose. We conclude that enhanced formation of plasmodesmata between M and BS cells is wired to the induction of photosynthesis in C4 G. gynandra