The plant cell wall (PCW) is a dynamic structure that plays a pivotal role in various essential processes throughout the plant life cycle. The PCW is principally composed of cellulose, and also contains non-cellulosic polysaccharides, lignin, and proteins. Cellulose, comprising unbranched β-(1,4)-linked glucan chains, is regarded as the principal structural polymer within the cell walls of the majority of plants, representing 40.6–51.2 % of their dry weight.

Pectin (30–35 % of dry weight) is a complex family of polysaccharides enriched in galacturonic acid (GalA). The primary polysaccharide domains of pectin molecules are homogalacturonan (HG) and branched rhamnogalacturonan-I (RG-I). The RG-I backbone is composed of repeating units of GalA and Rha residues: [→4)-α-D-GalpA-(1→2)-α-L-Rhap-(1→4)-α-D-GalpA-(1→]. The GalA residues in the RG-I backbone may be acetylated by O-2 and/or O-3, but are typically unmethylated [1]. RG-I is distinguished by its high degree of structural variability and complexity. It contains neutral side chains of varying length, composition, and configuration. The aforementioned side chains are typically enriched in 1,5-α-L-arabinan and/or 1,4-β-D-galactan and/or arabinogalactan (AG) type I, and potentially AG type II [2,3].

Pectin has been utilised with great success in the food, cosmetics, pharmaceutical, and biomedical industries. Furthermore, pectin has demonstrated potential for application in environmental remediation and sustainable packaging, including wastewater treatment, heavy metal ion removal, and the production of eco-friendly films and coatings. The health benefits of pectin are currently being investigated in a variety of areas, including tissue repair, drug delivery systems, and anti-cancer applications. The substance exhibits mucoadhesion, controlled drug release, and anticancer properties. The structural modifications unique to pectin render it a versatile biopolymer. It can be used as a gelling agent, stabilizer, emulsifier, and sugar substitute in low-calorie food products. Moreover, pectin is being studied as a prebiotic in functional foods and for creating edible thin films and coatings [4].

Arabinogalactan proteins (AGPs) are a class of highly glycosylated glycoproteins, distinguished by their complex structure comprising a hydroxyproline-rich (Hyp) protein backbone with a molecular mass of approximately 60–300 kDa and carbohydrate side chains, which typically represent over 90 % of the molecular mass. These side chains possess an arabinogalactan type II (AG-II) polysaccharide structure, linked to the protein backbone by O-glycosidic bonds through Hyp residues. The polysaccharide structure of AG-II consists of a β-1,3-galactose backbone decorated with β-1,6-galactose side chains, which can be further modified by α-arabinose (Ara), β-(methyl)glucuronic acid (GlcA), α-rhamnose (Rha), and α-fucose residues [[5], [6], [7], [8], [9]]. AG-IIs exhibit a distinct structural organisation when compared with type I and III arabinogalactans, which are distinguished by a β-1,4-linked and β-1,6-linked galactan backbone, respectively [10]. AGPs have been demonstrated to be integral to a multitude of plant growth and development processes, including vegetative growth, reproductive development, tissue regeneration, stress response, and other vital activities [8,11].

The data available on the interactions between different biomacromolecules, such as arabinogalactan proteins (AGPs) and pectins, in plant cell wall formation and organisation is still limited. The organisation and assembly of cell wall polymers into functional structures has proven to be more complex than previously believed. The presence of covalent bonds between polysaccharides and proteins in plants has been the subject of conflicting results, which hinders the establishment of unambiguous conclusions regarding the existence of polysaccharide-protein complexes.

Nevertheless, research has substantiated that pectins and AGPs are the most frequently identified cell wall biopolymers that are linked. For instance, Tan et al. [10,12] demonstrated that classical AGPs from arabidopsis suspension culture cells were covalently linked to arabinoxylan and RG-I, thereby enabling the formation of a continuous network between cell wall polysaccharides and structural proteins. In this instance, the investigation revealed that RG-I was found to be linked to a Rha residue belonging to the carbohydrate domain of AGPs: RG-I → Rha-(1 → 4)-GlcA [10]. The authors herein describe the identification and structural characterization of AGP from Arabidopsis thaliana, presented in two glycoforms, YS1 and YS2, both of which contain pectin and arabinoxylan glycans. YS2 was examined for cross-reactivity with 47 antibodies raised against RG-I, xylan, and AG. Of the four selected anti-xylan antibodies, only CCRC-M149 demonstrated strong reactivity with YS2. The authors thus concluded that YS is an AGP proteoglycan with two arabinoxylan-containing sites, one attached to the pectin motif and the other not [12].

AtAGP31 has been demonstrated to engage in cross-linking interactions with pectins via PAC (Proline-rich Arabinogalactan protein and Conserved Cysteines) domain [14]. Villa-Rivera [15] showed that the role of AGPs in plant defence responses to pathogens involves the secretion and association of AGPs at infection sites and the formation of cross-links with pectin. The identification of connections between individual biomacromolecules will contribute to a more complete and objective elucidation of PCW function and structure, thus expanding our understanding of its molecular architecture.

This study should contribute to our understanding of how diverse components of the cell wall interact with each other, thereby participating in the creation and maintenance of cell wall structures.

Pine (Pinus sylvestris) is an evergreen coniferous tree that grows in a variety of forest types, including mixed and coniferous forests, as well as small-leaved, broad-leaved, coniferous, riparian, and tundra forests. Coniferous pine greenery is of particular importance for the study of polysaccharide complexes. This is due to the fact that it is the predominant component of sawmill waste, and it is available in significant quantities throughout the year. The medicinal properties of pine wood greens have been employed in the treatment of numerous diseases, including colds, atherosclerosis, cardiovascular diseases, skin diseases, lung diseases, urinary tract diseases, kidney diseases, musculoskeletal system diseases, rheumatism, sciatica, joint pain, avitaminosis, and neuroses. Nevertheless, studies on the polysaccharide structure of pine greens are, at the present time, somewhat limited in number, despite the demand and economic value of this raw material [16]. The significance of such studies lies in their capacity to ascertain the possible properties and potential applications of polysaccharides derived from pine greens. It is vital that research in this area is increased in order to facilitate a more comprehensive understanding of the chemical composition of tree green polysaccharides, as well as the macromolecular structure of its individual components. This will optimise and improve the use of pine greens in various sectors, which in turn will contribute to solving the problem of utilisation of this large tonnage waste from the timber industry.

In a previous study [17], a polysaccharide containing mainly highly branched type II arabinogalactan and pectin-containing polysaccharides was isolated from the coniferous foliage of P. sylvéstris by hot water extraction. The objective of the present study was to further investigate the structural features of the biopolymers isolated from P. sylvéstris conifer needles by water extraction and to identify the linkages between them. The biopolymers were subjected to a process of enzymatic hydrolysis using exo- and endo-1,4-α-D-polygalacturonase for the purpose of analysis. The analysis encompassed the enzymatic cleavage of the initial polymer, in addition to polymers that were obtained subsequent to preliminary fractionation by ion exchange chromatography and decomposition by partial acid hydrolysis. The principal cleavage products, selected according to monosaccharide composition and yield, were subjected to analysis by 1D/2D NMR.