Arabinogalactan Proteins: Focus on the Role in Cellulose Synthesis and Deposition during Plant Cell Wall Biogenesis

Abstract

Arabinogalactan proteins (AGPs) belong to a family of glycoproteins that are widely present in plants. AGPs are mostly composed of a protein backbone decorated with complex carbohydrate side chains and are usually anchored to the plasma membrane or secreted extracellularly. A trickle of compelling biochemical and genetic evidence has demonstrated that AGPs make exciting candidates for a multitude of vital activities related to plant growth and development. However, because of the diversity of AGPs, functional redundancy of AGP family members, and blunt-force research tools, the precise functions of AGPs and their mechanisms of action remain elusive. In this review, we put together the current knowledge about the characteristics, classification, and identification of AGPs and make a summary of the biological functions of AGPs in multiple phases of plant reproduction and developmental processes. In addition, we especially discuss deeply the potential mechanisms for AGP action in different biological processes via their impacts on cellulose synthesis and deposition based on previous studies. Particularly, five hypothetical models that may explain the AGP involvement in cellulose synthesis and deposition during plant cell wall biogenesis are proposed. AGPs open a new avenue for understanding cellulose synthesis and deposition in plants.

The plant cell wall—dynamic, strong, and adaptable—is a natural shapeshifter

“…Genetic analysis has, however, now confirmed roles in lignification for some peroxidases (e.g. Zhang et al 2022a ), and the current model proposes involvement of both enzymes in most lignin biosynthesis, with an exception in the Casparian strip of the root, where peroxidase alone is believed to be necessary ( Lee et al 2013 ).…”

“…The AGPs are the largest family of cell wall proteins and perhaps the most multi-functional, with roles including plant growth and development, cell expansion, division, signaling, embryogenesis, vascular and gametophyte development, and biotic and abiotic stress response ( Seifert and Roberts 2006 ; Silva et al 2020 ; Lin et al 2022a ; Leszczuk et al 2023 ; Ma and Johnson 2023 ; Tan et al 2023a ). Mechanisms for how they may carry out these functions include mediation of information between the cell wall, plasma membrane, and cytoplasm in part due to their anchoring to the plasma membrane and covalent and/or noncovalent interactions with wall carbohydrates and/or proteins.…”

“…Root hair tip growth is a somewhat different process that shows oscillations in which the growth phase is promoted by lowered apoplastic pH and elevated ROS gradients as well as the creation of a tip-focused [Ca 2+ cyt ] gradient stimulating actin reorganization and polar exocytosis of wall polymers to the tip ( Zhang et al 2022a , 2022b ). One of FER's roles involves its association with LLG1 in root hair tips, activation of a specific associated ROP that further activates NADPH oxidase leading to apoplastic ROS production ( Duan et al 2010 ; Li et al 2015 ; Zhang et al 2022a , 2022b ) ( Fig. 15C ; right).…”

Logistics of defense: The contribution of endomembranes to plant innate immunity

“…To date, only the AGP arabinoxylan pectin arabinogalactan protein 1 (APAP1) has been ascribed a role in immunity, as supported by the evidence that an APAP1 knock-out mutant, apap1-4 , is hypersusceptible to Pst DC3000 ( Kim et al, 2023 ). While the mechanisms of AGP deposition in the CW are not yet known, AGPs have been implicated in cellulose synthesis and are likely trafficked through the conventional secretory pathway ( Lin et al, 2022 ). In addition to callose and AGP, lignin is deposited upon immune activation ( Lee et al, 2019 ).…”

Identification of fasciclin-like arabinogalactan protein family genes associated with fruit ripening in tomato

“…The family is typically divided into three multigene families: the arabinogalactan-proteins (AGPs), proline-rich proteins (PRPs) and extensins (EXTs) [9]. Among them, AGP is one of the family of glycoproteins in plant cell wall, which is abundant in plant cell wall and plasma membra [10]. As a highly glycosylated glycoprotein rich in Hyp, it plays several roles in plant growth and development, including tissue differentiation, reproduction, cell expansion and secondary cell wall deposition [11,12].…”