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Callose (a β-1,3 glucan) is a unique hemicellulose consisting of repeating units of D-glucose which are joined by β-1,3-glycosidic bond. Callose accumulates at a very high level in special cell walls such as the pollen mother cell wall, microspore cell wall, pollen tube cell wall, and the cell plate of dividing cells and when plant cells are damaged by pathogen attacks or mechanical wounding [Fig 1]. When callose is not needed, it has to be degraded in a timely manner.
Callose has long been thought to be a by-product of the cellulose synthase (CelS) enzyme when calcium ion concentration is high. This perception was put to rest by the discovery of a novel family of callose synthase genes that have no sequence homology with the CelS genes (Verma & Hong, 2001) [Fig 2]. We were the first group to identify this novel family of genes that are responsible for callose synthesis in plants and have coined the term Callose Synthase (CalS) to refer to this family of genes (Hong et al., 2001a; 2001b; Verma & Hong, 2001) [Fig 3], which have also been referred to as Glucan Synthase-like (GSL) (Richmond and Somerville, 2000). We have characterized a series of cals (or cs in short) mutants in Arabidopsis and demonstrated that different CalS genes have different temporal and spatial expression patterns (Dong et al., 2008). CalS1 and CalS2 are responsible for the formation of the cell plate (Hong et al., 2001a); CalS5 is required for pollen development and male sterility (Dong et al., 2005; Xie et al., 2010; 2011); CalS7 plays a key role in the deposition of callose at the sieve plate of sieve tube elements of phloems (Xie et al., 2011; Xie & Hong, 2011); CalS9 and CalS10 are required for microgametogenesis during pollen maturation and stomatal patterning (Xie et al., 2010; Guseman et al., 2010); and CalS11 and CalS12 participate in the formation of the interstitial callose wall of microspore tetrads during microsporogenesis (Xie et al., 2010). Thus, different CalS genes are expressed in different tissues at different stages of plant development and in response to environmental cues.
Fig 1. Occurence of callose in plants.
Callose is an essential constituent of many specialized cell walls, including the cell plate of dividing cells, cell wall of pollen mother cells and pollen tube, and sieve plate of phloems. Callose synthesis is also induced by biotic and abiotic stress conditions. As compared with cellulose (a β-1,4 glucan), which exists in a crystalline-like structure, provides strength to the cell wall, and is difficult to synthesize and difficult to degrade, callose is deposited to cell wall as amorpholous aggrigates, easier to synthesize, more accessible to the aquaous phase inside the cell and in extract, and easier to be degraded. It is an emergence sealant to a wounding site or in a fast-forming cell plate and competitively growing pollen tube.
Fig 2. Callose synthase enzyme complex at the cell plate.
Callose is synthesized by an enzyme complex known as the callose synthase complex, with callose synthase (CalS) as the essential and catalytic subunit. Arabidopsis CalS1 contains 16 predicted transmembrane domains and utilizes UDP-glucose transferase (UGT1) for substrate binding. UGT1 interacts with phragmoplastin (Phr) and Rho-like GTPase (Rop) (Hong et al., 2001a; 2001b). Sucrose synthase (SuSy) and annexin (ANN) may also be part of the CalS complex (Andrawis et al. 1993; Shin and Brown 1999). The CalS enzyme activity is stimulated by calcium ion (Ca2+).
Fig 3. Phylogenetic tree and gene structure of the CalS genes in Arabidopsis.
(A) Phylogenetic tree of Arabidopsis CalS genes and their biological functions in plant development. Protein sequences of Arabidopsis CalS peptide sequences were compiled using the Clustal method. The cotton fiber-related CalS (GhCalS; GenBank acc. # AF085717; Cui et al., 2001) and yeast 1,3-β-glucan synthase (FKS1; GenBank acc. # SCU12893; Douglas et al., 1994) are included for phylogenetic analysis.
(B) Genomic structure of Arabidopsis CalS genes. Exons are represented in solid boxes. The CalS genes constitute a family of long genes, spanning 10-14 kb of genomic DNA, and have complex intron-exon structures, having approximately 40 exons per gene. CalS11 and CalS12 are exceptional, because they are approximately 5 kb in length and have only 2-3 exons per gene, suggesting they may have evolved differently from the rest of the CalS genes. The percentages of amino acid identity with CalS1 is shown before each gene, whereas the percentages in parentheses indicate amino acid identity between adjacent genes (Hong et al., 2001a).
References
Kanaoka M, Shimizu K, Xie B, Urban S, Freeman M, Hong Z, Okada K (2022) KOMPEITO, an atypical Arabidopsis Rhomboid-related gene, is required for callose accumulation and pollen wall development. Int J Mol Sci 23: 5959 (DOI: 10.3390/ijms23115959)
Xie B, Hong Z (2011) Unplugging the callose plug from sieve pores. Plant Signal & Behav, 6:491-493. PMID: 21386663
Xie B, Wang X, Zhu M, Zhang Z, Hong Z (2011) CalS7 encodes a callose synthase responsible for callose deposition in the phloem. Plant J 65:1–14. PMID: 21175885
Xie B, Deng Y, Kanaoka M, Okada K, Hong Z (2011) Expression of Arabidopsis callose synthase 5 (CalS5) in tobacco BY-2 cells: subcellular localization and effect on cell wall permeability. Plant Sci 183: 1-8. PMID: 22195570
Guseman JM, Lee JS, Bogenschutz NL, Peterson KM, Virata RE, Xie B, Kanaoka MM, Hong Z, Torii KU (2010) Dysregulation of cell-to-cell connectivity and stomatal patterning by loss-of-function mutation in Arabidopsis CHORUS (GLUCAN SYNTHASE-LIKE 8). Development 137: 1731-1741
Xie B, Wang X, Hong Z (2010) Precocious pollen germination in Arabidopsis lines with altered callose deposition during microsporogenesis. Planta 231: 809-823
Dong X, Hong Z, Chatterjee J, Kim S, Verma DPS (2008) Expression of callose synthase genes and its connection with Npr1 signaling pathway during pathogen infection. Planta 229: 87-98
Dong X, Hong Z, Sivaramakrishnan M, Mahfouz M, Verma DPS (2005) Callose synthase (CalS5) is required for exine formation during microgametogenesis and pollen viability in Arabidopsis. Plant J. 42: 315-328
Hong Z, Delauney AJ and Verma DPS (2001a) A cell plate-specific callose synthase and its interaction with phragmoplastin and UDP-glucose transferase. Plant Cell 13: 755-768
Hong Z, Zhang Z, Olson J. and Verma DPS (2001b) A novel UDP glucose transferase interacts with callose synthase and phragmoplastin at the forming cell plate. Plant Cell 13, 769-779
Verma DPS, Hong Z (2001) Plant callose synthase complexes. Plant Mol. Biol. 47: 693-701.
Cui X, Shin H, Song C, Laosinchai W, Amano Y, Brown RM Jr (2001) A putative plant homolog of the yeast β-1,3-glucan synthase subunit FKS1 from cotton (Gossypium hirsutum L.) fibers. Planta 213:223-230.
Richmond TA, Somerville CR (2000) The callose synthase superfamily. Plant Physiol 124: 495-498.
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