CAGEs are Golgi-localized GT31 enzymes involved in cellulose biosynthesis in Arabidopsis

Cytosolic UDP-L-arabinose synthesis by bifunctional UDP-glucose 4-epimerases in Arabidopsis

L-Arabinose (L-Ara) is a plant-specific sugar found in cell wall polysaccharides, proteoglycans, glycoproteins, and small glycoconjugates, which play physiologically important roles in cell proliferation and other essential cellular processes. L-Ara is synthesized as UDP-L-arabinose (UDP-L-Ara) from UDP-xylose (UDP-Xyl) by UDP-Xyl 4-epimerases (UXEs), a type of de novo synthesis of L-Ara unique to plants. In Arabidopsis, the Golgi-localized UXE AtMUR4 is the main contributor to UDP-L-Ara synthesis. However, cytosolic bifunctional UDP-glucose 4-epimerases (UGEs) with UXE activity, AtUGE1, and AtUGE3 also catalyze this reaction. For the present study, we first examined the physiological importance of bifunctional UGEs in Arabidopsis. The uge1 and uge3 mutants enhanced the dwarf phenotype of mur4 and further reduced the L-Ara content in cell walls, suggesting that bifunctional UGEs contribute to UDP-L-Ara synthesis. Through the introduction of point mutations exchanging corresponding amino acid residues between AtUGE1 with high UXE activity and AtUGE2 with low UXE activity, two mutations that increase relative UXE activity of AtUGE2 were identified. The crystal structures of AtUGE2 in complex forms with NAD+ and NAD+/UDP revealed that the UDP-binding domain of AtUGE2 has a more closed conformation and smaller sugar-binding site than bacterial and mammalian UGEs, suggesting that plant UGEs have the appropriate size and shape for binding UDP-Xyl and UDP-L-Ara to exhibit UXE activity. The presented results suggest that the capacity for cytosolic synthesis of UDP-L-Ara was acquired by the small sugar-binding site and several mutations of UGEs, enabling diversified utilization of L-Ara in seed plants.

Applications of the Yariv reagent in polysaccharide analysis and plant physiology from theory to practice

Arabinogalactan (AG), a biologically active substance found abundantly in plants, is of significant interest in plant physiology due to its unique physicochemical properties. Yariv reagent, widely utilized in AG-II related applications, forms insoluble precipitates when bound to AG-II. This paper provides a comprehensive overview of the synthesis methods, physicochemical properties, and various dissociation methods of the Yariv reagent to enhance its utility in AG-II studies. Furthermore, the review explores the binding mechanisms and applications of the Yariv reagent, highlighting the advancements in studying the Yariv-AG complex in plant physiology. The aim of this review is to inspire new research ideas and foster novel applications of the Yariv reagent from synthesis to implementation.

Expression Quantitative Trait Locus of Wood Formation-Related Genes in Salix suchowensis

Shrub willows are widely planted for landscaping, soil remediation, and biomass production, due to their rapid growth rates. Identification of regulatory genes in wood formation would provide clues for genetic engineering of willows for improved growth traits on marginal lands. Here, we conducted an expression quantitative trait locus (eQTL) analysis, using a full sibling F1 population of Salix suchowensis, to explore the genetic mechanisms underlying wood formation. Based on variants identified from simplified genome sequencing and gene expression data from RNA sequencing, 16,487 eQTL blocks controlling 5505 genes were identified, including 2148 cis-eQTLs and 16,480 trans-eQTLs. eQTL hotspots were identified, based on eQTL frequency in genomic windows, revealing one hotspot controlling genes involved in wood formation regulation. Regulatory networks were further constructed, resulting in the identification of key regulatory genes, including three transcription factors (JAZ1, HAT22, MYB36) and CLV1, BAM1, CYCB2;4, CDKB2;1, associated with the proliferation and differentiation activity of cambium cells. The enrichment of genes in plant hormone pathways indicates their critical roles in the regulation of wood formation. Our analyses provide a significant groundwork for a comprehensive understanding of the regulatory network of wood formation in S. suchowensis.

Callose synthesis at the center point of plant development-An evolutionary insight

Polar callose deposition into the extracellular matrix is tightly controlled in time and space. Its presence in the cell wall modifies the properties of the surrounding area, which is fundamental for the correct execution of numerous processes such as cell division, male gametophyte development, intercellular transport, or responses to biotic and abiotic stresses. Previous studies have been invaluable in characterizing specific callose synthases (CalSs) during individual cellular processes. However, the complex view of the relationships between a particular CalS and a specific process is still lacking. Here we review the recent proceedings on the role of callose and individual CalSs in cell wall remodelling from an evolutionary perspective and with a particular focus on cytokinesis. We provide a robust phylogenetic analysis of CalS across the plant kingdom, which implies a 3-subfamily distribution of CalS. We also discuss the possible linkage between the evolution of CalSs and their function in specific cell types and processes.

Fern cell walls and the evolution of arabinogalactan proteins in streptophytes

Significant changes have occurred in plant cell wall composition during evolution and diversification of tracheophytes. As the sister lineage to seed plants, knowledge on the cell wall of ferns is key to track evolutionary changes across tracheophytes and to understand seed plant-specific evolutionary innovations. Fern cell wall composition is not fully understood, including limited knowledge of glycoproteins such as the fern arabinogalactan proteins (AGPs). Here, we characterize the AGPs from the leptosporangiate fern genera Azolla, Salvinia, and Ceratopteris. The carbohydrate moiety of seed plant AGPs consists of a galactan backbone including mainly 1,3- and 1,3,6-linked pyranosidic galactose, which is conserved across the investigated fern AGPs. Yet, unlike AGPs of angiosperms, those of ferns contained the unusual sugar 3-O-methylrhamnose. Besides terminal furanosidic arabinose, Ara (Araf), the main linkage type of Araf in the ferns was 1,2-linked Araf, whereas in seed plants 1,5-linked Araf is often dominating. Antibodies directed against carbohydrate epitopes of AGPs supported the structural differences between AGPs of ferns and seed plants. Comparison of AGP linkage types across the streptophyte lineage showed that angiosperms have rather conserved monosaccharide linkage types; by contrast bryophytes, ferns, and gymnosperms showed more variability. Phylogenetic analyses of glycosyltransferases involved in AGP biosynthesis and bioinformatic search for AGP protein backbones revealed a versatile genetic toolkit for AGP complexity in ferns. Our data reveal important differences across AGP diversity of which the functional significance is unknown. This diversity sheds light on the evolution of the hallmark feature of tracheophytes: their elaborate cell walls.

Arabinogalactan protein polysaccharide chains are required for normal biogenesis of plasmodesmata

Arabinogalactan proteins (AGPs) are a plant-specific family of extracellular proteoglycans characterized by large and complex galactose-rich polysaccharide chains. Functional elucidation of AGPs, however, has been hindered by the high degree of redundancy of AGP genes. To uncover as yet unexplored roles of AGPs in Arabidopsis, a mutant of Hyp O-galactosyltransferase (HPGT), a critical enzyme that catalyzes the common initial step of Hyp-linked arabinogalactan chain biosynthesis, was used. Here we show, using the hpgt1,2,3 triple mutant, that a reduction in functional AGPs leads to a stomatal patterning defect in which two or more stomata are clustered together. This defect is attributed to increased and dysregulated symplastic transport following changes in plasmodesmata structure, such that highly permeable complex branched plasmodesmata with cavities in branching parts increased in the mutant. We also found that the hpgt1,2,3 mutation causes a reduction of cellulose in the cell wall and accumulation of pectin, which controls cell wall porosity. Our results highlight the importance of AGPs in the correct biogenesis of plasmodesmata, possibly acting through the regulation of cell wall properties surrounding the plasmodesmata.

In vivo structural modification of type II arabinogalactans with fungal endo-β-1, 6-galactanase in Arabidopsis

Arabinogalactan-proteins (AGPs) are mysterious extracellular glycoproteins in plants. Although AGPs are highly conserved, their molecular functions remain obscure. The physiological importance of AGPs has been extensively demonstrated with β-Yariv reagent, which specifically binds to AGPs and upon introduction into cells, causes various deleterious effects including growth inhibition and programmed cell death. However, structural features of AGPs that determine their functions have not been identified with β-Yariv reagent. It is known that AGPs are decorated with large type II arabinogalactans (AGs), which are necessary for their functions. Type II AGs consist of a β-1,3-galactan main chain and β-1,6-galactan side chains with auxiliary sugar residues such as L-arabinose and 4-O-methyl-glucuronic acid. While most side chains are short, long side chains such as β-1,6-galactohexaose (β-1,6-Gal₆) also exist in type II AGs. To gain insight into the structures important for AGP functions, in vivo structural modification of β-1,6-galactan side chains was performed in Arabidopsis. We generated transgenic Arabidopsis plants expressing a fungal endo-β-1,6-galactanase, Tv6GAL, that degrades long side chains specifically under the control of dexamethasone (Dex). Two of 6 transgenic lines obtained showed more than 40 times activity of endo-β-1,6-galactanase when treated with Dex. Structural analysis indicated that long side chains such as β-1,6-Gal₅ and β-1,6-Gal₆ were significantly reduced compared to wild-type plants. Tv6GAL induction caused retarded growth of seedlings, which had a reduced amount of cellulose in cell walls. These results suggest that long β-1,6-galactan side chains are necessary for normal cellulose synthesis and/or deposition as their defect affects cell growth in plants.

A Golgi-localized glycosyltransferase, OsGT14;1, is required for growth of both roots and shoots in rice

Glycosyltransferases (GTs) form a large family in plants and are important enzymes for the synthesis of various polysaccharides, but only a few members have been functionally characterized. Here, through mutant screening with gene mapping, we found that an Oryza sativa (rice) mutant with a short-root phenotype was caused by a frame-shift mutation of a gene (OsGT14;1) belonging to the glycosyltransferase gene family 14. Further analysis indicated that the mutant also had a brittle culm and produced lower grain yield compared with wild-type rice, but the roots showed similar root structure and function in terms of the uptake of mineral nutrients. OsGT14;1 was broadly expressed in all organs throughout the entire growth period, with a relatively high expression in the roots, stems, node I and husk. Furthermore, OsGT14;1 was expressed in all tissues of these organs. Subcellular observation revealed that OsGT14;1 encoded a Golgi-localized protein. Mutation of OsGT14;1 resulted in decreased cellulose content and increased hemicellulose, but did not alter pectin in the cell wall of roots and shoots. The knockout of OsGT14;1 did not affect the tolerance to toxic mineral elements, including Al, As, Cd and salt stress, but did increase the sensitivity to low pH. Taken together, OsGT14;1 located at the Golgi is required for growth of both roots and shoots in rice through affecting cellulose synthesis.