Henrik Vibe Scheller

Comparison of dilute acid and ionic liquid pretreatment of switchgrass: Biomass recalcitrance, delignification and enzymatic saccharification

The efficiency of two biomass pretreatment technologies, dilute acid hydrolysis and dissolution in an ionic liquid, are compared in terms of delignification, saccharification efficiency and saccharide yields with switchgrass serving as a model bioenergy crop. When subject to ionic liquid pretreatment (dissolution and precipitation of cellulose by anti-solvent) switchgrass exhibited reduced cellulose crystallinity, increased surface area, and decreased lignin content compared to dilute acid pretreatment. Pretreated material was characterized by powder X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy and chemistry methods. Ionic liquid pretreatment enabled a significant enhancement in the rate of enzyme hydrolysis of the cellulose component of switchgrass, with a rate increase of 16.7-fold, and a glucan yield of 96.0% obtained in 24h. These results indicate that ionic liquid pretreatment may offer unique advantages when compared to the dilute acid pretreatment process for switchgrass. However, the cost of the ionic liquid process must also be taken into consideration.

Biosynthesis of Pectin

The plant cell wall is composed of polysaccharides and proteins. In addition, some cells have walls impregnated with lignin. In all cases, the polysaccharides constitute the major part of the wall. The wall polysaccharides are often classified into cellulose, hemicelluloses, and pectin, and these

The PSI-H subunit of photosystem I is essential for state transitions in plant photosynthesis.

Photosynthesis in plants involves two photosystems responsible for converting light energy into redox processes. The photosystems, PSI and PSII, operate largely in series, and therefore their excitation must be balanced in order to optimize photosynthetic performance. When plants are exposed to illumination favouring either PSII or PSI they can redistribute excitation towards the light-limited photosystem. Long-term changes in illumination lead to changes in photosystem stoichiometry. In contrast, state transition is a dynamic mechanism that enables plants to respond rapidly to changes in illumination. When PSII is favoured (state 2), the redox conditions in the thylakoids change and result in activation of a protein kinase. The kinase phosphorylates the main light-harvesting complex (LHCII) and the mobile antenna complex is detached from PSII. It has not been clear if attachment of LHCII to PSI in state 2 is important in state transitions. Here we show that in the absence of a specific PSI subunit, PSI-H, LHCII cannot transfer energy to PSI, and state transitions are impaired.

Balance of power: a view of the mechanism of photosynthetic state transitions

Photosynthesis in plants involves photosystem I and photosystem II, both of which use light energy to drive redox processes. Plants can balance the distribution of absorbed light energy between the two photosystems. When photosystem II is favoured, a mobile pool of light harvesting complex II moves from photosystem II to photosystem I. This short-term and reversible redistribution is known as a state transition. It is associated with changes in the phosphorylation of light harvesting complex II but the regulation is complex. Redistribution of energy during state transitions depends on an altered binding equilibrium between the light harvesting complex II-photosystem II and light harvesting complex II-photosystem I complexes.

ARABINAN DEFICIENT 1 Is a Putative Arabinosyltransferase Involved in Biosynthesis of Pectic Arabinan in Arabidopsis

The function of a putative glycosyltransferase (At2g35100) was investigated in Arabidopsis (Arabidopsis thaliana). The protein is predicted to be a type 2 membrane protein with a signal anchor. Two independent mutant lines with T-DNA insertion in the ARABINAN DEFICIENT 1 (ARAD1) gene were analyzed. The gene was shown to be expressed in all tissues but particularly in vascular tissues of leaves and stems. Analysis of cell wall polysaccharides isolated from leaves and stems showed that arabinose content was reduced to about 75% and 46%, respectively, of wild-type levels. Immunohistochemical analysis indicated a specific decrease in arabinan with no change in other pectic domains or in glycoproteins. The cellular structure of the stem was also not altered. Isolated rhamnogalacturonan I from mutant tissues contained only about 30% of the wild-type amount of arabinose, confirming the specific deficiency in arabinan. Linkage analysis showed that the small amount of arabinan present in mutant tissue was structurally similar to that of the wild type. Transformation of mutant plants with the ARAD1 gene driven by the 35S promoter led to full complementation of the phenotype, but none of the transformants had more arabinan than the wild-type level. The data suggest that ARAD1 is an arabinan α-1,5-arabinosyltransferase. To our knowledge, the identification of other l-arabinosyltransferases has not been published.

Role of subunits in eukaryotic Photosystem I

Photosystem I (PSI) of eukaryotes has a number of features that distinguishes it from PSI of cyanobacteria. In plants, the PSI core has three subunits that are not found in cyanobacterial PSI. The remaining 11 subunits of the core are conserved but several of the subunits have a different role in eukaryotic PSI. A distinguishing feature of eukaryotic PSI is the membrane-imbedded peripheral antenna. Light-harvesting complex I is composed of four different subunits and is specific for PSI. Light-harvesting complex II can be associated with both PSI and PSII. Several of the core subunits interact with the peripheral antenna proteins and are important for proper function of the peripheral antenna. The review describes the role of the different subunits in eukaryotic PSI. The emphasis is on features that are different from cyanobacterial PSI.

Arabidopsis CHL27, located in both envelope and thylakoid membranes, is required for the synthesis of protochlorophyllide

CHL27, the Arabidopsis homologue to Chlamydomonas Crd1, a plastid-localized putative diiron protein, is required for the synthesis of protochlorophyllide and therefore is a candidate subunit of the aerobic cyclase in chlorophyll biosynthesis. δ-Aminolevulinic acid-fed antisense Arabidopsis plants with reduced amounts of Crd1/CHL27 accumulate Mg-protoporphyrin IX monomethyl ester, the substrate of the cyclase reaction. Mutant plants have chlorotic leaves with reduced abundance of all chlorophyll proteins. Fractionation of Arabidopsis chloroplast membranes shows that Crd1/CHL27 is equally distributed on a membrane-weight basis in the thylakoid and inner-envelope membranes.

Nearest-neighbor analysis of higher-plant photosystem I holocomplex

Photosystem I (PSI) preparations from barley (Hordeum vulgare) and spinach (Spinacia oleracea) were subjected to chemical cross-linking using the cleavable homobifunctional cross-linkers dithiobis(succinimidylpropionate) and 3,3[prime]-dithiobis(sulfosuccinimidyl-propionate). The overall pattern of cross-linked products was analyzed by the simple but powerful technique of diagonal electrophoresis, in which the disulfide bond in the cross-linker was cleaved between the first and second dimensions of the gel, and immunoblotting. A large number of cross-linked products were identified. Together with preexisting data on the structure of PSI, it was deduced that the subunits PSI-D, PSI-H, PSI-I, and PSI-L occupy one side of the complex, whereas PSI-E, PSI-F, and PSI-J occupy the other. PSI-K and PSI-G appear to be adjacent to Lhca3 and Lhca2, respectively, and not close to the other small subunits. Experiments with isolated light-harvesting complex I preparations indicate that the subunits are organized as dimers, which seem to associate to the PSI-A/PSI-B proteins independent of each other. We suggest which PSI subunit corresponds to each membrane-spanning helix in the cyanobacterial PSI structure, and present a model for higher-plant PSI.

The PSI-K subunit of photosystem I is involved in the interaction between light-harvesting complex I and the photosystem I reaction center core.

PSI-K is a subunit of photosystem I. The function of PSI-K was characterized in Arabidopsis plants transformed with a psaK cDNA in antisense orientation, and several lines without detectable PSI-K protein were identified. Plants without PSI-K have a 19% higher chlorophylla/b ratio and 19% more P700 than wild-type plants. Thus, plants without PSI-K compensate by making more photosystem I. The photosystem I electron transport in vitro is unaffected in the absence of PSI-K. Light response curves for oxygen evolution indicated that the photosynthetic machinery of PSI-K-deficient plants have less capacity to utilize light energy. Plants without PSI-K have less state 1-state 2 transition. Thus, the redistribution of absorbed excitation energy between the two photosystems is reduced. Low temperature fluorescence emission spectra revealed a 2-nm blue shift in the long wavelength emission in plants lacking PSI-K. Furthermore, thylakoids and isolated PSI without PSI-K had 20–30% less Lhca2 and 30–40% less Lhca3, whereas Lhca1 and Lhca4 were unaffected. During electrophoresis under mildly denaturing conditions, all four Lhca subunits were partially dissociated from photosystem I lacking PSI-K. The observed effects demonstrate that PSI-K has a role in organizing the peripheral light-harvesting complexes on the core antenna of photosystem I.

Arabidopsis - a powerful model system for plant cell wall research

Plant cell walls are composites of various carbohydrates, proteins and other compounds. Cell walls provide plants with strength and protection, and also represent the most abundant source of renewable biomass. Despite the importance of plant cell walls, comparatively little is known about the identities of genes and functions of proteins involved in their biosynthesis. The model plant Arabidopsis and the availability of its genome sequence have been invaluable for the identification and functional characterization of genes encoding enzymes involved in plant cell-wall biosynthesis. This review covers recent progress in the identification and characterization of genes encoding proteins involved in the biosynthesis of Arabidopsis cell-wall polysaccharides and arabinogalactan proteins. These studies have improved our understanding of both the mechanisms of cell-wall biosynthesis and the functions of various cell-wall polymers, and have highlighted areas where further research is needed.