Cellular localization and biochemical characterization of a chimeric fluorescent protein fusion of Arabidopsis cellulose synthase-like A2 inserted into Golgi membrane

Ride to cell wall: Arabidopsis XTH11, XTH29 and XTH33 exhibit different secretion pathways and responses to heat and drought stress

The xyloglucan endotransglucosylase/hydrolases (XTHs) are enzymes involved in cell wall assembly and growth regulation, cleaving and re-joining hemicellulose chains in the xyloglucan-cellulose network. Here, in a homologous system, we compare the secretion patterns of XTH11, XTH33 and XTH29, three members of the Arabidopsis thaliana XTH family, selected for the presence (XTH11 and XTH33) or absence (XTH29) of a signal peptide, and the presence of a transmembrane domain (XTH33). We show that XTH11 and XTH33 reached, respectively, the cell wall and plasma membrane through a conventional protein secretion (CPS) pathway, whereas XTH29 moves towards the apoplast following an unconventional protein secretion (UPS) mediated by exocyst-positive organelles (EXPOs). All XTHs share a common C-terminal functional domain (XET-C) that, for XTH29 and a restricted number of other XTHs (27, 28 and 30), continues with an extraterminal region (ETR) of 45 amino acids. We suggest that this region is necessary for the correct cell wall targeting of XTH29, as the ETR-truncated protein never reaches its final destination and is not recruited by EXPOs. Furthermore, quantitative real-time polymerase chain reaction analyses performed on 4-week-old Arabidopsis seedlings exposed to drought and heat stress suggest a different involvement of the three XTHs in cell wall remodeling under abiotic stress, evidencing stress-, organ- and time-dependent variations in the expression levels. Significantly, XTH29, codifying the only XTH that follows a UPS pathway, is highly upregulated with respect to XTH11 and XTH33, which code for CPS-secreted proteins.

Actin and Microtubules Differently Contribute to Vacuolar Targeting Specificity during the Export from the ER

Plants rely on both actin and microtubule cytoskeletons to fine-tune sorting and spatial targeting of membranes during cell growth and stress adaptation. Considerable advances have been made in recent years in the comprehension of the relationship between the trans-Golgi network/early endosome (TGN/EE) and cytoskeletons, but studies have mainly focused on the transport to and from the plasma membrane. We address here the relationship of the cytoskeleton with different endoplasmic reticulum (ER) export mechanisms toward vacuoles. These emergent features of the plant endomembrane traffic are explored with an in vivo approach, providing clues on the traffic regulation at different levels beyond known proteins’ functions and interactions. We show how traffic of vacuolar markers, characterized by different vacuolar sorting determinants, diverges at the export from the ER, clearly involving different components of the cytoskeleton.

Root responses of contrasting rice genotypes to low temperature stress

Rice (Oryza sativa L.) ssp. indica is the most cultivated species in the South of Brazil. However, these plants face low temperature stress from September to November, which is the period of early sowing, affecting plant development during the initial stages of growth, and reducing rice productivity. This study aimed to characterize the root response to low temperature stress during the early vegetative stage of two rice genotypes contrasting in their cold tolerance (CT, cold-tolerant; and CS, cold-sensitive). Root dry weight and length, as well as the number of root hairs, were higher in CT than CS when exposed to cold treatment. Histochemical analyses indicated that roots of CS genotype present higher levels of lipid peroxidation and H₂O₂ accumulation, along with lower levels of plasma membrane integrity than CT under low temperature stress. RNAseq analyses revealed that the contrasting genotypes present completely different molecular responses to cold stress. The number of over-represented functional categories was lower in CT than CS under cold condition, suggesting that CS genotype is more impacted by low temperature stress than CT. Several genes might contribute to rice cold tolerance, including the ones related with cell wall remodeling, cytoskeleton and growth, signaling, antioxidant system, lipid metabolism, and stress response. On the other hand, high expression of the genes SRC2 (defense), root architecture associated 1 (growth), ACC oxidase, ethylene-responsive transcription factor, and cytokinin-O-glucosyltransferase 2 (hormone-related) seems to be related with cold sensibility. Since these two genotypes have a similar genetic background (sister lines), the differentially expressed genes found here can be considered candidate genes for cold tolerance and could be used in future biotechnological approaches aiming to increase rice tolerance to low temperature.

CesA6 and PGIP2 Endocytosis Involves Different Subpopulations of TGN-Related Endosomes

Endocytosis is an essential process for the internalization of plasma membrane proteins, lipids and extracellular molecules into the cells. The mechanisms underlying endocytosis in plant cells involve several endosomal organelles whose origins and specific role needs still to be clarified. In this study we compare the internalization events of a GFP-tagged polygalacturonase-inhibiting protein of Phaseolus vulgaris (PGIP2-GFP) to that of a GFP-tagged subunit of cellulose synthase complex of Arabidopsis thaliana (secGFP-CesA6). Through the use of endocytic traffic chemical inhibitors (tyrphostin A23, salicylic acid, wortmannin, concanamycin A, Sortin 2, Endosidin 5 and BFA) it was evidenced that the two protein fusions were endocytosed through distinct endosomes with different mechanisms. PGIP2-GFP endocytosis is specifically sensitive to tyrphostin A23, salicylic acid and Sortin 2; furthermore, SYP51, a tSNARE with interfering effect on late steps of vacuolar traffic, affects its arrival in the central vacuole. SecGFP-CesA6, specifically sensitive to Endosidin 5, likely reaches the plasma membrane passing through the trans Golgi network (TGN), since the BFA treatment leads to the formation of BFA bodies, compatible with the aggregation of TGNs. BFA treatments determine the accumulation and tethering of the intracellular compartments labeled by both proteins, but PGIP2-GFP aggregated compartments overlap with those labeled by the endocytic dye FM4-64 while secGFP-CesA6 fills different compartments. Furthermore, secGFP-CesA6 co-localization with RFP-NIP1.1, marker of the direct ER-to-Vacuole traffic, in small compartments separated from ER suggests that secGFP-CesA6 is sorted through TGNs in which the direct contribution from the ER plays an important role. All together the data indicate the existence of a heterogeneous population of Golgi-independent TGNs.

Drought and Heat Differentially Affect XTH Expression and XET Activity and Action in 3-Day-Old Seedlings of Durum Wheat Cultivars with Different Stress Susceptibility

Heat and drought stress have emerged as major constraints for durum wheat production. In the Mediterranean area, their negative effect on crop productivity is expected to be exacerbated by the occurring climate change. Xyloglucan endotransglucosylase/hydrolases (XTHs) are chief enzymes in cell wall remodeling, whose relevance in cell expansion and morphogenesis suggests a central role in stress responses. In this work the potential role of XTHs in abiotic stress tolerance was investigated in durum wheat. The separate effects of dehydration and heat exposure on XTH expression and its endotransglucosylase (XET) in vitro activity and in vivo action have been monitored, up to 24 h, in the apical and sub-apical root regions and shoots excised from 3-day-old seedlings of durum wheat cultivars differing in stress susceptibility/tolerance. Dehydration and heat stress differentially influence the XTH expression profiles and the activity and action of XET in the wheat seedlings, depending on the degree of susceptibility/tolerance of the cultivars, the organ, the topological region of the root and, within the root, on the gradient of cell differentiation. The root apical region was the zone mainly affected by both treatments in all assayed cultivars, while no change in XET activity was observed at shoot level, irrespective of susceptibility/tolerance, confirming the pivotal role of the root in stress perception, signaling, and response. Conflicting effects were observed depending on stress type: dehydration evoked an overall increase, at least in the apical region of the root, of XET activity and action, while a significant inhibition was caused by heat treatment in most cultivars. The data suggest that differential changes in XET action in defined portions of the root of young durum wheat seedlings may have a role as a response to drought and heat stress, thus contributing to seedling survival and crop establishment. A thorough understanding of the mechanisms underlying these variations could represent the theoretical basis for implementing breeding strategies to develop new highly productive hybrids adapted to future climate scenarios.

Identification of genes involved in biosynthesis of mannan polysaccharides in Dendrobium officinale by RNA-seq analysis

Dendrobium officinale is a traditional Chinese medicinal plant. The stems of D. officinale contain mannan polysaccharides, which are promising bioactive polysaccharides for use as drugs. However, the genes involved in the biosynthesis of mannan polysaccharides in D. officinale have not yet been identified. In this study, four digital gene expression profiling analyses were performed on developing stems of greenhouse-grown D. officinale to identify such genes. Based on the accumulation of mannose and on gene expression levels, eight CELLULOSE SYNTHASE-LIKE A genes (CSLA), which are highly likely to be related to the biosynthesis of bioactive mannan polysaccharides, were identified from the differentially expressed genes database. In order to further analyze these DoCSLA genes, a full-length cDNA of each was obtained by RACE. The eight genes, belonging to the CSLA family of the CesA superfamily, contain conserved domains of the CesA superfamily. Most of the genes, which were highly expressed in the stems of D. officinale, were related to abiotic stress. Our results suggest that the CSLA family genes from D. officinale are involved in the biosynthesis of bioactive mannan polysaccharides.

Identification of genes involved in biosynthesis of mannan polysaccharides in Dendrobium officinale by RNA-seq analysis

Dendrobium officinale is a traditional Chinese medicinal plant. The stems of D. officinale contain mannan polysaccharides, which are promising bioactive polysaccharides for use as drugs. However, the genes involved in the biosynthesis of mannan polysaccharides in D. officinale have not yet been identified. In this study, four digital gene expression profiling analyses were performed on developing stems of greenhouse-grown D. officinale to identify such genes. Based on the accumulation of mannose and on gene expression levels, eight CELLULOSE SYNTHASE-LIKE A genes (CSLA), which are highly likely to be related to the biosynthesis of bioactive mannan polysaccharides, were identified from the differentially expressed genes database. In order to further analyze these DoCSLA genes, a full-length cDNA of each was obtained by RACE. The eight genes, belonging to the CSLA family of the CesA superfamily, contain conserved domains of the CesA superfamily. Most of the genes, which were highly expressed in the stems of D. officinale, were related to abiotic stress. Our results suggest that the CSLA family genes from D. officinale are involved in the biosynthesis of bioactive mannan polysaccharides.

Molecular dissection of Phaseolus vulgaris polygalacturonase-inhibiting protein 2 reveals the presence of hold/release domains affecting protein trafficking toward the cell wall

The plant endomembrane system is massively involved in the synthesis, transport and secretion of cell wall polysaccharides and proteins; however, the molecular mechanisms underlying trafficking toward the apoplast are largely unknown. Besides constitutive, the existence of a regulated secretory pathway has been proposed. A polygalacturonase inhibitor protein (PGIP2), known to move as soluble cargo and reach the cell wall through a mechanism distinguishable from default, was dissected in its main functional domains (A, B, C, D), and C sub-fragments (C1-10), to identify signals essential for its regulated targeting. The secretion patterns of the fluorescent chimeras obtained by fusing different PGIP2 domains to the green fluorescent protein (GFP) were analyzed. PGIP2 N-terminal and leucine-rich repeat domains (B and C, respectively) seem to operate as holding/releasing signals, respectively, during PGIP2 transit through the Golgi. The B domain slows down PGIP2 secretion by transiently interacting with Golgi membranes. Its depletion leads, in fact, to the secretion via default (Sp2-susceptible) of the ACD-GFP chimera faster than PGIP2. Depending on its length (at least the first 5 leucine-rich repeats are required), the C domain modulates B interaction with Golgi membranes allowing the release of chimeras and their extracellular secretion through a Sp2 independent pathway. The addition of the vacuolar sorting determinant Chi to PGIP2 diverts the path of the protein from cell wall to vacuole, suggesting that C domain is a releasing rather than a cell wall sorting signal.

Subcellular compartmentalization in protoplasts from Artemisia annua cell cultures: engineering attempts using a modified SNARE protein

Plants are ideal bioreactors for the production of macromolecules but transport mechanisms are not fully understood and cannot be easily manipulated. Several attempts to overproduce recombinant proteins or secondary metabolites failed. Because of an independent regulation of the storage compartment, the product may be rapidly degraded or cause self-intoxication. The case of the anti-malarial compound artemisinin produced by Artemisia annua plants is emblematic. The accumulation of artemisinin naturally occurs in the apoplast of glandular trichomes probably involving autophagy and unconventional secretion thus its production by undifferentiated tissues such as cell suspension cultures can be challenging. Here we characterize the subcellular compartmentalization of several known fluorescent markers in protoplasts derived from Artemisia suspension cultures and explore the possibility to modify compartmentalization using a modified SNARE protein as molecular tool to be used in future biotechnological applications. We focused on the observation of the vacuolar organization in vivo and the truncated form of AtSYP51, 51H3, was used to induce a compartment generated by the contribution of membrane from endocytosis and from endoplasmic reticulum to vacuole trafficking. The artificial compartment crossing exocytosis and endocytosis may trap artemisinin stabilizing it until extraction; indeed, it is able to increase total enzymatic activity of a vacuolar marker (RGUSChi), probably increasing its stability. Exploring the 51H3-induced compartment we gained new insights on the function of the SNARE SYP51, recently shown to be an interfering-SNARE, and new hints to engineer eukaryote endomembranes for future biotechnological applications.