Systematic Analysis and Functional Validation of Citrus Pectin Acetylesterases (CsPAEs) Reveals that CsPAE2 Negatively Regulates Citrus Bacterial Canker Development

Oomycete pathogen pectin acetylesterase targets host lipid transfer protein to reduce salicylic acid signaling

During initial stages of microbial invasion, the extracellular space (apoplast) of plant cells is a vital battleground between plants and pathogens. The oomycete plant pathogens secrete an array of apoplastic carbohydrate active enzymes, which are central molecules for understanding the complex plant-oomycete interactions. Among them, pectin acetylesterase (PAE) plays a critical role in the pathogenesis of plant pathogens including bacteria, fungi, and oomycetes. Here, we demonstrated that Peronophythora litchii (syn. Phytophthora litchii) PlPAE5 suppresses litchi (Litchi chinensis) plant immunity by interacting with litchi lipid transfer protein 1 (LcLTP1). The LcLTP1-binding activity and virulence function of PlPAE5 depend on its PAE domain but not on its PAE activity. The high expression of LcLTP1 enhances plant resistance to oomycete and fungal pathogens, and this disease resistance depends on BRASSINOSTEROID INSENSITIVE 1-associated receptor kinase 1 (BAK1) and Suppressor of BIR1 (SOBIR1) in Nicotiana benthamiana. LcLTP1 activates the plant salicylic acid (SA) signaling pathway, while PlPAE5 subverts the LcLTP1-mediated SA signaling pathway by destabilizing LcLTP1. Conclusively, this study reports a virulence mechanism of oomycete PAE suppressing plant LTP-mediated SA immune signaling and will be instrumental for boosting plant resistance breeding.

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.

Genome-wide analysis of LysM gene family members and their expression in response to Colletotrichum fructicola infection in Octoploid strawberry(Fragaria × ananassa)

The cultivated octoploid strawberry (Fragaria × ananassa) is an economically important fruit that is planted worldwide. The lysin motif (LysM) protein family is composed of the major class of plant pattern recognition receptors, which play important roles in sensing pathogen-associated molecular patterns (PAMPs), and subsequently triggers downstream plant immunity. In the present study, a comprehensive, genome-wide analysis of F. × ananassa LysM (FaLysM) genes was performed to investigate gene structures, phylogenic relationships, chromosome location, collinear relationships, transcription factor binding sites, and protein model analysis. We aimed to identify the LysM genes involved in the defense against plant pathogens. A total of 14 FaLysM genes were identified in the F. × ananassa genome and divided into 2 subgroups (LYP and LYK) on the basis of the phylogenetic analysis. The Ka/Ks ratio for the duplicated pair of most FaLysM genes was less than 1, which indicates that the selection pressure was mostly subject to the purifying selection during evolution. The protein model analysis revealed that FaLysM2-10 contain conserved mode of chitin binding, which suggest the potential role of FaLysM2-10 in pathogen perception and plant immunity. The RNA-Seq results showed the differential regulation of 14 FaLysM genes in response to Colletotrichum fructicola infection, implying the complex interaction between C. fructicola and strawberry. Knockout of candidate effector gene CfLysM2, which was previously proved to be highly expressed during C. fructicola infection, resulted in the up-regulation of six FaLysM genes (FaLysM1, FaLysM2, FaLysM3, FaLysM7, FaLysM8, and FaLysM12), indicating the competitive relations between CfLysM2 and FaLysM genes. Overall, this study provides fundamental information on the roles of LysM proteins in octoploid strawberry and its interaction with C. fructicola, laying useful information for further investigation on the C. fructicola-strawberry interaction and strawberry resistance breeding.

Silencing of a Pectin Acetylesterase (PAE) Gene Highly Expressed in Tobacco Pistils Negatively Affects Pollen Tube Growth

Successful plant reproduction and fruit formation depend on adequate pollen and pistil development, and pollen-pistil interactions. In Nicotiana tabacum, pollen tubes grow through the intercellular spaces of pistil-specialized tissues, stigmatic secretory zone, and stylar transmitting tissue (STT). These intercellular spaces are supposed to be formed by the modulation of cell wall pectin esterification. Previously we have identified a gene preferentially expressed in pistils encoding a putative pectin acetylesterase (PAE), named NtPAE1. Here, we characterized the NtPAE1 gene and performed genome-wide and phylogenetic analyses of PAEs. We identified 30 PAE sequences in the N. tabacum genome, distributed in four clades. The expression of NtPAE1 was assessed by RT-qPCR and in situ hybridization. We confirmed NtPAE1 preferential expression in stigmas/styles and ovaries and demonstrated its high expression in the STT. Structural predictions and comparisons between NtPAE1 and functional enzymes validated its identity as a PAE. Transgenic plants were produced, overexpressing and silencing the NtPAE1 gene. Overexpressed plants displayed smaller flowers while silencing plants exhibited collapsed pollen grains, which hardly germinate. NtPAE1 silencing plants do not produce fruits, due to impaired pollen tube growth in their STTs. Thus, NtPAE1 is an essential enzyme regulating pectin modifications in flowers and, ultimately, in plant reproduction.

Plant Cell Wall Integrity Perturbations and Priming for Defense

A plant cell wall is a highly complex structure consisting of networks of polysaccharides, proteins, and polyphenols that dynamically change during growth and development in various tissues. The cell wall not only acts as a physical barrier but also dynamically responds to disturbances caused by biotic and abiotic stresses. Plants have well-established surveillance mechanisms to detect any cell wall perturbations. Specific immune signaling pathways are triggered to contrast biotic or abiotic forces, including cascades dedicated to reinforcing the cell wall structure. This review summarizes the recent developments in molecular mechanisms underlying maintenance of cell wall integrity in plant-pathogen and parasitic interactions. Subjects such as the effect of altered expression of endogenous plant cell-wall-related genes or apoplastic expression of microbial cell-wall-modifying enzymes on cell wall integrity are covered. Targeted genetic modifications as a tool to study the potential of cell wall elicitors, priming of signaling pathways, and the outcome of disease resistance phenotypes are also discussed. The prime importance of understanding the intricate details and complete picture of plant immunity emerges, ultimately to engineer new strategies to improve crop productivity and sustainability.

Identification of Candidate Genes Associated With Tolerance to Apple Replant Disease by Genome-Wide Transcriptome Analysis

Apple replant disease (ARD) is a worldwide economic risk in apple cultivation for fruit tree nurseries and fruit growers. Several studies on the reaction of apple plants to ARD are documented but less is known about the genetic mechanisms behind this symptomatology. RNA-seq analysis is a powerful tool for revealing candidate genes that are involved in the molecular responses to biotic stresses in plants. The aim of our work was to find differentially expressed genes in response to ARD in Malus. For this, we compared transcriptome data of the rootstock ‘M9’ (susceptible) and the wild apple genotype M. ×robusta 5 (Mr5, tolerant) after cultivation in ARD soil and disinfected ARD soil, respectively. When comparing apple plantlets grown in ARD soil to those grown in disinfected ARD soil, 1,206 differentially expressed genes (DEGs) were identified based on a log2 fold change, (LFC) ≥ 1 for up– and ≤ −1 for downregulation (p < 0.05). Subsequent validation revealed a highly significant positive correlation (r = 0.91; p < 0.0001) between RNA-seq and RT-qPCR results indicating a high reliability of the RNA-seq data. PageMan analysis showed that transcripts of genes involved in gibberellic acid (GA) biosynthesis were significantly enriched in the DEG dataset. Most of these GA biosynthesis genes were associated with functions in cell wall stabilization. Further genes were related to detoxification processes. Genes of both groups were expressed significantly higher in Mr5, suggesting that the lower susceptibility to ARD in Mr5 is not due to a single mechanism. These findings contribute to a better insight into ARD response in susceptible and tolerant apple genotypes. However, future research is needed to identify the defense mechanisms, which are most effective for the plant to overcome ARD.

Cell wall associated immunity in plants

The plant cell wall is the first physical and defensive barrier against pathogens. The plant cell wall usually undergoes dynamic remodeling as an immune response to prevent infection by pathogens. In this review, we summarize advances on relationship between cell wall and immunity in plants. In particular, we outline current progresses regarding the regulation of the cell wall components, including cellulose, hemicellulose, pectin and lignin, on plant disease resistance. We also discuss the impacts of cell wall-derived cellodextrin, oligogalacturonic acid and xyloglucan/xylan oligosaccharides as potent elicitors or signal molecules to trigger plant immune response. We further propose future studies on dissecting the molecular regulation of cell wall on plant immunity, which have potentials in practical application of crop breeding aiming at improvement of plant disease resistance.

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.