Plant and algal structure: from cell walls to biomechanical function

Effect of acetochlor on the symbiotic relationship between microalgae and bacteria

In this study, two strains of symbiotic bacteria (SOB-1 and SOB-2) were isolated from Scenedesmus obliquus, and various algal-bacterial mutualistic systems were established under acetochlor (ACT) stress conditions. Following exposure to varying ACT concentrations from 2.0 to 25.0 μg/L, the capacity for co-cultured bacteria to degrade ACT was enhanced in 7 days by up to 226.9% (SOB-1) and 193.0% (SOB-2), compared with axenic algae, although bacteria exposed to higher ACT concentrations exacerbated algal metabolic stress, oxidative states, apoptosis and cellular lysis. ACT reduced carbohydrates in the phycosphere by up to 31.5%; compensatory nutrient plunder and structural damage by bacteria were the potential exploitation pathways determined based on the inhibition of bacterial infection using a glucanase inhibitor. The ACT-induced reduction in algal antimicrobial substances, including fatty acids and phenolics (by up to 58.1% and 56.6%, respectively), also facilitated bacterial exploitation of algae. ACT-dependent interspecific interaction coefficients between algae and bacteria generated from long-term symbiosis cultures implied that bacteria moved from mutualism (0 and 2.0 μg/L ACT) to exploitation (7.9 and 25.0 μg/L ACT). The population dynamic model under incremental ACT-concentration scenarios inferred that theoretical systematic extinction may occur in algal-bacterial systems earlier than in axenic algae. These outcomes provide interspecific insights into the distortion of algal-bacterial reciprocity due to the ecotoxicological effects of ACT.

Genome evolution and transcriptome plasticity is associated with adaptation to monocot and dicot plants in Colletotrichum fungi

BACKGROUND

Colletotrichum fungi infect a wide diversity of monocot and dicot hosts, causing diseases on almost all economically important plants worldwide. Colletotrichum is also a suitable model for studying gene family evolution on a fine scale to uncover events in the genome associated with biological changes.

RESULTS

Here we present the genome sequences of 30 Colletotrichum species covering the diversity within the genus. Evolutionary analyses revealed that the Colletotrichum ancestor diverged in the late Cretaceous in parallel with the diversification of flowering plants. We provide evidence of independent host jumps from dicots to monocots during the evolution of Colletotrichum, coinciding with a progressive shrinking of the plant cell wall degradative arsenal and expansions in lineage-specific gene families. Comparative transcriptomics of 4 species adapted to different hosts revealed similarity in gene content but high diversity in the modulation of their transcription profiles on different plant substrates. Combining genomics and transcriptomics, we identified a set of core genes such as specific transcription factors, putatively involved in plant cell wall degradation.

CONCLUSIONS

These results indicate that the ancestral Colletotrichum were associated with dicot plants and certain branches progressively adapted to different monocot hosts, reshaping the gene content and its regulation.

Lotus-Flower- and Lotus-Seedpod-Derived Polysaccharide: Structural Characterization and Biological Activity

Lotus flower polysaccharide (LFP) and lotus seedpod polysaccharide (LSP) were separated by water extract-alcohol precipitation, and their structures and biological activities were investigated. The results of monosaccharide composition showed that LFP and LSP were composed of nine monosaccharides, fucose, rhamnose, arabinose, glucose, galactose, mannose, fructose, galacturonic acid, and glucuronic acid, with the molar percentages of 0.18: 0.43: 2.26: 45.22: 32.14: 4.28: 8.20: 6.28: 1.01 and 2.70: 1.02: 8.15: 45.63: 20.63: 1.44: 2.59: 16.45. LSP and LFP exhibited molecular weights of 9.37 × 104 Da and 1.24 × 106 Da, respectively. SEM showed that LFP and LSP have similar structures; XRD analysis showed that both polysaccharides had crystalline structure and amorphous structure. The results of ABTS+, DPPH, hydroxyl radical scavenging experiment, and a reducing power experiment showed that LFP and LSP had good antioxidant capacity. Cell viability findings showed that polysaccharide concentrations of lotus flower and lotus seedpod could enhance cellular proliferation ranging from 25 to 400 μg/mL without cytotoxicity. By inducing the production of crucial proteins in the TLR4/NF-κB pathway, LFP and LSP were able to induce autophagy in RAW264.7, according to the results of the RT-PCR and Western blotting assays.

Transcriptomic Characterization of Miscanthus sacchariflorus × M. lutarioriparius and Its Implications for Energy Crop Development in the Semiarid Mine Area

Miscanthus interspecific hybrids have been proved to have better adaptability in marginal lands than their parents. Miscanthus sacchariflorus and Miscanthus lutarioriparius were used as the parents to develop hybrids. We performed the transcriptome for 110 F1 hybrids of Miscanthus sacchariflorus × Miscanthus lutarioriparius and their parents that had been established on the Loess Plateau mine area, to estimate the population’s genetic expression variation, and illuminate the adaptive mechanism of the F1 population. The result speculated that the F1 population has mainly inherited the stress response metabolic pathway of its female parent (M. sacchariflorus), which may be responsible for its higher environmental adaptability and biomass yield compared with male parents. Based on PopART, we assembled a leaf reference transcriptome for M. sacchariflorus (LRTMS) and obtained 8116 high-quality transcripts. When we analyze the differential expression of genes between F1 population and parent, 39 and 56 differentially expressed genes were screened out in the female parent and male parent, respectively. The enrichment analysis showed that pathways of carbohydrate metabolism, lipid metabolism, biosynthesis of secondary metabolites and circadian rhythm-plant played a key role in resisting the harsh environment. The carbohydrate metabolism and lipid metabolism were also significantly enriched, and the synthesis of these substances facilitated the yield. The results provided an insight into breeding Miscanthus hybrids more suited to the harsh environment of the Loess Plateau.

Physcomitrium (Physcomitrella) patens endo-glucanase 16 is involved in the cell wall development of young tissue

Plants maintain large repertoires of carbohydrate-active enzymes (CAZymes)-comprising between 3% and 10% of their genomes-to synthesize, modify, and degrade the polysaccharide components of the cell wall. We recently identified a unique group of plant endo-glucanases from Glycoside Hydrolase Family 16, viz. EG16 orthologs, which constitute a sister clade to the well-known XYLOGLUCAN ENDO-TRANSGLYCOSYLASE/HYDROLASE (XTH) gene products. Biochemical analysis of EG16 orthologs from poplar (Populus trichocarpa), grapevine (Vitis vinifera), and spreading earthmoss (Physcomitrium patens) has demonstrated that these endo-glucanases are distinctly active on cell wall matrix glycans, mixed-linkage β(1,3);β(1,4)-glucan and xyloglucan (XyG), and that enzyme structure and specificity is highly conserved across diverse plant lineages. However, the physiological role of EG16 orthologs in any species is presently unknown. To shed light on EG16 function in vivo, here we performed reverse genetics and protein localization analyses of the single EG16 ortholog in the model moss P. patens, where this gene is highly expressed in young, expanding tissues, particularly in protonema. Surprisingly, deletion of the PpEG16 gene by homologous recombination led to an increase in growth, as well as accelerated senescence. Notably, the PpEG16 protein was shown to co-localize with XyG in the cell wall of protonema tissue, specifically at cell tips, despite lacking a secretion signal peptide. Although the precise biological role of EG16 orthologs remains elusive, our results implicate these highly conserved glycoside hydrolases in cell wall polysaccharide remodeling and recycling. We anticipate that these foundational results will inform future studies on EG16 function across plant lineages.

Microalgal nanocellulose - opportunities for a circular bioeconomy

Over 3 billion years, photosynthetic algae have evolved complex uses for cellulose, the most abundant polymer worldwide. A major cell-wall component of lignocellulosic plants, seaweeds, microalgae, and bacteria, cellulose can be processed to nanocellulose, a promising nanomaterial with novel properties. The structural diversity of macro- and microalgal nanocelluloses opens opportunities to couple low-impact biomass production with novel, green-chemistry processing to yield valuable, sustainable nanomaterials for a multitude of applications ranging from novel wound dressings to organic solar cells. We review the origins of algal cellulose and the applications and uses of nanocellulose, and highlight the potential for microalgae as a nanocellulose source. Given the limited state of current knowledge, we identify research challenges and strategies to help to realise this potential.

Microwave-Assisted Defibrillation of Microalgae

The first production of defibrillated celluloses from microalgal biomass using acid-free, TEMPO-free and bleach-free hydrothermal microwave processing is reported. Two routes were explored: i. direct microwave process of native microalgae ("standard"), and ii. scCO2 pre-treatment followed by microwave processing. ScCO2 was investigated as it is commonly used to extract lipids and generates considerable quantities of spent algal biomass. Defibrillation was evidenced in both cases to afford cellulosic strands, which progressively decreased in their width and length as the microwave processing temperature increased from 160 °C to 220 °C. Lower temperatures revealed aspect ratios similar to microfibrillated cellulose whilst at the highest temperature (220 °C), a mixture of microfibrillated cellulose and nanocrystals were evidenced. XRD studies showed similar patterns to cellulose I but also some unresolved peaks. The crystallinity index (CrI), determined by XRD, increased with increasing microwave processing temperature. The water holding capacity (WHC) of all materials was approximately 4.5 g H2O/g sample. The materials were able to form partially stable hydrogels, but only with those processed above 200 °C and at a concentration of 3 wt% in water. This unique work provides a new set of materials with potential applications in the packaging, food, pharmaceutical and cosmetic industries.

Assessing the Interfacial Dynamic Modulus of Biological Composites

Biological composites (biocomposites) possess ultra-thin, irregular-shaped, energy dissipating interfacial regions that grant them crucial mechanical capabilities. Identifying the dynamic (viscoelastic) modulus of these interfacial regions is considered to be the key toward understanding the underlying structure-function relationships in various load-bearing biological materials including mollusk shells, arthropod cuticles, and plant parts. However, due to the submicron dimensions and the confined locations of these interfacial regions within the biocomposite, assessing their mechanical characteristics directly with experiments is nearly impossible. Here, we employ composite-mechanics modeling, analytical formulations, and numerical simulations to establish a theoretical framework that links the interfacial dynamic modulus of a biocomposite to the extrinsic characteristics of a larger-scale biocomposite segment. Accordingly, we introduce a methodology that enables back-calculating (via simple linear scaling) of the interfacial dynamic modulus of biocomposites from their far-field dynamic mechanical analysis. We demonstrate its usage on zigzag-shaped interfaces that are abundant in biocomposites. Our theoretical framework and methodological approach are applicable to the vast range of biocomposites in natural materials; its essence can be directly employed or generally adapted into analogous composite systems, such as architected nanocomposites, biomedical composites, and bioinspired materials.

Advances in Understanding of Desiccation Tolerance of Lichens and Lichen-Forming Algae

Lichens are symbiotic associations (holobionts) established between fungi (mycobionts) and certain groups of cyanobacteria or unicellular green algae (photobionts). This symbiotic association has been essential in the colonization of terrestrial dry habitats. Lichens possess key mechanisms involved in desiccation tolerance (DT) that are constitutively present such as high amounts of polyols, LEA proteins, HSPs, a powerful antioxidant system, thylakoidal oligogalactolipids, etc. This strategy allows them to be always ready to survive drastic changes in their water content. However, several studies indicate that at least some protective mechanisms require a minimal time to be induced, such as the induction of the antioxidant system, the activation of non-photochemical quenching including the de-epoxidation of violaxanthin to zeaxanthin, lipid membrane remodeling, changes in the proportions of polyols, ultrastructural changes, marked polysaccharide remodeling of the cell wall, etc. Although DT in lichens is achieved mainly through constitutive mechanisms, the induction of protection mechanisms might allow them to face desiccation stress in a better condition. The proportion and relevance of constitutive and inducible DT mechanisms seem to be related to the ecology at which lichens are adapted to.

Solanales Stem Biomechanical Properties Are Primarily Determined by Morphology Rather Than Internal Structural Anatomy and Cell Wall Composition

Self-supporting plants and climbers exhibit differences in their structural and biomechanical properties. We hypothesized that such fundamental differences originate at the level of the material properties. In this study, we compared three non-woody members of the Solanales exhibiting different growth habits: (1) a self-supporting plant (potato, Solanum tuberosum), (2) a trailing plant (sweet potato, Ipomoea batatas), and (3) a twining climber (morning glory, Ipomoea tricolor). The mechanical properties investigated by materials analyses were combined with structural, biochemical, and immunohistochemical analyses. Generally, the plants exhibited large morphological differences, but possessed relatively similar anatomy and cell wall composition. The cell walls were primarily composed of hemicelluloses (~60%), with α-cellulose and pectins constituting ~25% and 5%-8%, respectively. Immunohistochemistry of specific cell wall components suggested only minor variation in the occurrence and localization between the species, although some differences in hemicellulose distribution were observed. According to tensile and flexural tests, potato stems were the stiffest by a significant amount and the morning glory stems were the most compliant and showed differences in two- and three-orders of magnitude; the differences between their effective Young's (Elastic) modulus values (geometry-independent parameter), on the other hand, were substantially lower (at the same order of magnitude) and sometimes not even significantly different. Therefore, although variability exists in the internal anatomy and cell wall composition between the different species, the largest differences were seen in the morphology, which appears to be the primary determinant of biomechanical function. Although this does not exclude the possibility of different mechanisms in other plant groups, there is apparently less constraint to modifying stem morphology than anatomy and cell wall composition within the Solanales.

Ultrastructural and biochemical analyses reveal cell wall remodelling in lichen-forming microalgae submitted to cyclic desiccation-rehydration

BACKGROUND AND AIMS

One of the most distinctive features of desiccation-tolerant plants is their high cell wall (CW) flexibility. Most lichen microalgae can tolerate drastic dehydration-rehydration (D/R) conditions; however, their mechanisms of D/R tolerance are scarcely understood. We tested the hypothesis that D/R-tolerant microalgae would have flexible CWs due to species-specific CW ultrastructure and biochemical composition, which could be remodelled by exposure to cyclic D/R.

METHODS

Two lichen microalgae, Trebouxia sp. TR9 (TR9, adapted to rapid D/R cycles) and Coccomyxa simplex (Csol, adapted to seasonal dry periods) were exposed to no or four cycles of desiccation [25-30 % RH (TR9) or 55-60 % RH (Csol)] and 16 h of rehydration (100 % RH). Low-temperature SEM, environmental SEM and freeze-substitution TEM were employed to visualize structural alterations induced by D/R. In addition, CWs were extracted and sequentially fractionated with hot water and KOH, and the gel permeation profile of polysaccharides was analysed in each fraction. The glycosyl composition and linkage of the main polysaccharides of each CW fraction were analysed by GC-MS.

KEY RESULTS

All ultrastructural analyses consistently showed that desiccation caused progressive cell shrinkage and deformation in both microalgae, which could be rapidly reversed when water availability increased. Notably, the plasma membrane of TR9 and Csol remained in close contact with the deformed CW. Exposure to D/R strongly altered the size distribution of TR9 hot-water-soluble polysaccharides, composed mainly of a β-3-linked rhamnogalactofuranan and Csol KOH-soluble β-glucans.

CONCLUSIONS

Cyclic D/R induces biochemical remodelling of the CW that could increase CW flexibility, allowing regulated shrinkage and expansion of D/R-tolerant microalgae.

Nano-indentation reveals a potential role for gradients of cell wall stiffness in directional movement of the resurrection plant Selaginella lepidophylla

As a physical response to water loss during drought, inner Selaginella lepidophylla stems curl into a spiral shape to prevent photoirradiation damage to their photosynthetic surfaces. Curling is reversible and involves hierarchical deformation, making S. lepidophylla an attractive model with which to study water-responsive actuation. Investigation at the organ and tissue level has led to the understanding that the direction and extent of stem curling can be partially attributed to stiffness gradients between adaxial and abaxial stem sides at the nanoscale. Here, we examine cell wall elasticity to understand how it contributes to the overall stem curling. We compare the measured elastic moduli along the stem length and between adaxial and abaxial stem sides using atomic force microscopy nano-indentation testing. We show that changes in cortex secondary cell wall development lead to cell wall stiffness gradients from stem tip to base, and also between adaxial and abaxial stem sides. Changes in cortical cell wall morphology and secondary cell wall composition are suggested to contribute to the observed stiffness gradients.

Lipid extraction from wet Nannochloropsis biomass via enzyme-assisted three phase partitioning

A green and efficient enzyme assisted three phase partitioning (EA-TPP) process was firstly developed to extract microalgal lipids using wet Nannochloropsis sp. biomass. In the pretreatment of microalgal biomass by four hydrolytic enzymes, TPP obtained a higher TFAs lipid extraction efficiency by cellulase compared with the resting enzymes. After optimization by EA-TPP of the wet disrupted Nannochloropsis biomass (3 g), the maximum TFAs extraction yield (90.40%) was attained at 20% ammonium sulphate, 6-7 pH, 1:2 slurry/tert-butanol ratio and 70 °C for 2 h incubation time and two extraction cycles. Moreover, results also revealed that the lipidic species compositions of Nannochloropsis sp. biomass were greatly related with the EA-TPP parameters. In the laboratory scale for wet disrupted microalgae biomass, EA-TPP process achieved 88.70% TFAs extraction yield under the optimized conditions. In all, EA-TPP process could be a promising approach to extract microalgae lipids for food application using wet microalgae biomass.

Gazing at Cell Wall Expansion under a Golden Light

In plants, cell growth is constrained by a stiff cell wall, at least this is the way textbooks usually present it. Accordingly, many studies have focused on the elasticity and plasticity of the cell wall as prerequisites for expansion during growth. With their specific evolutionary history, cell wall composition, and environment, brown algae present a unique configuration offering a new perspective on the involvement of the cell wall, viewed as an inert material yet with intrinsic mechanical properties, in growth. In light of recent findings, we explore here how much of the functional relationship between cell wall chemistry and intrinsic mechanics on the one hand, and growth on the other hand, has been uncovered in brown algae.