Insights into pectin O-acetylation in the plant cell wall: structure, synthesis, and modification

O-Acetyl esterification is an important structural and functional feature of pectins present in the cell walls of all land plants. The amount and positions of pectin acetyl substituents varies across plant tissues and stages of development. Plant growth and response to biotic and abiotic stress are known to be significantly influenced by pectin O-acetylation. Gel formation is a key characteristic of pectins, and many studies have shown that gel formation is dependent upon the degree of acetylation. Previous studies have indicated that members of the TRICHOME BIREFRINGENCE-LIKE (TBL) family may play a role in the O-acetylation of pectin, however, biochemical evidence for acceptor specific pectin acetyltransferase activity remains to be confirmed and the exact mechanism(s) for catalysis must be determined. Pectin acetylesterases (PAEs) affect pectin acetylation as they hydrolyze acetylester bonds and have a role in the amount and distribution of O-acetylation. Several mutant studies suggest the critical role of pectin O-acetylation; however, additional research is required to fully understand this. This review aims to discuss the importance, role, and putative mechanism of pectin O-acetylation.

Bioinformatics analysis of PAE family in Populus trichocarpa and responsiveness to carbon and nitrogen treatment

Plant Pectin acetylesterase (PAE) belongs to family CE13 of carbohydrate esterases in the CAZy database. The ability of PAE to regulate the degree of acetylation of pectin, an important polysaccharide in the cell wall, affects the structure of plant cell wall. In this study, ten PtPAE genes were identified and characterized in Populus trichocarpa genome using bioinformatics methods, and the physiochemical properties such as molecular weight, isoelectric points, and hydrophilicity, as well as the secondary and tertiary structure of the protein were predicted. According to phylogenetic analysis, ten PtPAEs can be divided into three evolutionary clades, each of which had similar gene structure and motifs. Tissue-specific expression profiles indicated that the PtPAEs had different expression patterns. Real-time quantitative PCR (RT-qPCR) analysis showed that transcription level of PtPAEs was regulated by different CO₂ and nitrogen concentrations. These results provide important information for the study of the phylogenetic relationship and function of PtPAEs in Populus trichocarpa.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02918-1.

Plant pectin acetylesterase structure and function: new insights from bioinformatic analysis

Background

Pectins are plant cell wall polysaccharides that can be acetylated on C2 and/or C3 of galacturonic acid residues. The degree of acetylation of pectin can be modulated by pectin acetylesterase (EC 3.1.1.6, PAE). The function and structure of plant PAEs remain poorly understood and the role of the fine-tuning of pectin acetylation on cell wall properties has not yet been elucidated.

Results

In the present study, a bioinformatic approach was used on 72 plant PAEs from 16 species among 611 plant PAEs available in plant genomic databases. An overview of plant PAE proteins, particularly Arabidopsis thaliana PAEs, based on phylogeny analysis, protein motif identification and modeled 3D structure is presented. A phylogenetic tree analysis using protein sequences clustered the plant PAEs into five clades. AtPAEs clustered in four clades in the plant kingdom PAE tree while they formed three clades when a phylogenetic tree was performed only on Arabidopsis proteins, due to isoform AtPAE9. Primitive plants that display a smaller number of PAEs clustered into two clades, while in higher plants, the presence of multiple members of PAE genes indicated a diversification of AtPAEs. 3D homology modeling of AtPAE8 from clade 2 with a human Notum protein showed an α/β hydrolase structure with the hallmark Ser-His-Asp of the active site. A 3D model of AtPAE4 from clade 1 and AtPAE10 from clade 3 showed a similar shape suggesting that the diversification of AtPAEs is unlikely to arise from the shape of the protein. Primary structure prediction analysis of AtPAEs showed a specific motif characteristic of each clade and identified one major group of AtPAEs with a signal peptide and one group without a signal peptide. A multiple sequence alignment of the putative plant PAEs revealed consensus sequences with important putative catalytic residues: Ser, Asp, His and a pectin binding site. Data mining of gene expression profiles of AtPAE revealed that genes from clade 2 including AtPAE7, AtPAE8 and AtPAE11, which are duplicated genes, are highly expressed during plant growth and development while AtPAEs without a signal peptide, including AtPAE2 and AtPAE4, are more regulated in response to plant environmental conditions.

Conclusion

Bioinformatic analysis of plant, and particularly Arabidopsis, AtPAEs provides novel insights, including new motifs that could play a role in pectin binding and catalytic sites. The diversification of AtPAEs is likely to be related to neofunctionalization of some AtPAE genes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3833-0) contains supplementary material, which is available to authorized users.

Transcripts of pectin-degrading enzymes and isolation of complete cDNA sequence of a pectate lyase gene induced by coffee white stem borer (Xylotrechus quadripes) in the bark tissue of Coffea canephora (robusta coffee)

Of the two commercially cultivated coffee (Coffea) species, C. arabica (arabica) is highly susceptible and C. canephora (robusta) is highly resistant to the insect pest Xylotrechus quadripes (Coleoptera: Cerambycidae), commonly known as coffee white stem borer (CWSB). We constructed a forward-subtracted cDNA library by Suppression Subtractive Hybridization (SSH) from robusta bark tissue for profiling genes induced by CWSB infestation. Among the 265 unigenes of the SSH EST library, 7 unigenes (5 contigs and 2 singletons) matching different pectin-degrading enzymes were discovered. These ESTs matched one pectate lyase, three polygalacturonases, and one pectin acetylesterase gene. Quantitative real-time PCR (qRT-PCR) revealed that CWSB infestation strongly induces the pectate lyase gene at 72 h. Complete cDNA sequence of the pectate lyase gene was obtained through 3' and 5' RACE reactions. It was a 1595 bp long sequence that included full CDS and both UTRs. Against C. canephora genome sequences in Coffee Genome Hub database ( http://coffee-genome.org/ ), it had 22 matches to different pectate lyase genes mapped on 9 of the 11 pseudochromosomes, the top match being Cc07_g00190 Pectate lyase. In NCBI database, it matched pectate lyase sequences of several plants. Apart from C. canephora, the closest pectate lyase matches were from Sesamum indicum and Nicotiana tabacum. The pectinolytic enzymes discovered here are thought to play a role in the production of oligogalacturonides (OGs) which act as Damage-Associated Molecular Pattern (DAMP) signals eliciting innate immunity in plants. The pectate lyase gene, induced by CWSB infestation, along with other endogenous pectinolytic enzymes and CWSB-specific elicitors, may be involved in triggering basal defense responses to protect the CWSB-damaged tissue against pathogens, as well as to contain CWSB in robusta.

Structural alteration of cell wall pectins accompanies pea development in response to cold

Pea (Pisum sativum) cell wall metabolism in response to chilling was investigated in a frost-sensitive genotype 'Terese' and a frost-tolerant genotype 'Champagne'. Cell walls isolated from stipules of cold acclimated and non-acclimated plants showed that cold temperatures induce changes in polymers containing xylose, arabinose, galactose and galacturonic acid residues. In the tolerant cultivar Champagne, acclimation is accompanied by increases in homogalacturonan, xylogalacturonan and highly branched Rhamnogalacturonan I with branched and unbranched (1→5)-α-arabinans and (1→4)-β-galactans. In contrast, the sensitive cultivar Terese accumulates substantial amounts of (1→4)-β-xylans and glucuronoxylan, but not the pectins. Greater JIM7 labeling was observed in Champagne compared to Terese, indicating that cold acclimation also induces an increase in the degree of methylesterification of pectins. Significant decrease in polygalacturonase activities in both genotypes were observed at the end of cold acclimation. These data indicate a role for esterified pectins in cold tolerance. The possible functions for pectins and their associated arabinans and galactans in cold acclimation are discussed.