Differential timing of defense-related responses induced by cerato-platanin and cerato-populin, two non-catalytic fungal elicitors.

The cerato-platanin (CP) family consists of fungal-secreted proteins involved in various stages of the host-fungus interaction and acting as phytotoxins and elicitors of defense responses. The founder member of this family is CP, a non-catalytic protein with a six-stranded double-ψß-barrel fold. Cerato-populin (Pop1) is an ortholog showing low sequence identity with CP. CP is secreted by Ceratocystis platani, the causal agent of the canker stain of plane. Pop1 is secreted by Ceratocystis populicola, a pathogen of poplar. CP and Pop1 have been suggested to act as PAMPs (pathogen-associated molecular patterns) because they induce phytoalexin synthesis, transcription of defense-related genes, restriction of conidia growth and cell death in various plants. Here, we treated plane leaves with CP or Pop1, and monitored defense responses to define the role of these elicitors in the plant interactions. Both CP and Pop1 were able to induce mitogen-activated protein kinases (MAPKs) phosphorylation, production of reactive oxygen species and nitric oxide, and overexpression of defense related genes. The characteristic DNA fragmentation and the cytological features indicate that CP and Pop1 induce cell death by a mechanism of programmed cell death. Therefore, CP and Pop1 can be considered as two novel, non-catalytic fungal PAMPs able to enhance primary defense. Of particular interest is the observation that CP showed faster activity compared to Pop1. The different timing in defense activation could potentially be due to the structural differences between CP and Pop1 (i.e. different hydrophobic index and different helix content) therefore constituting a starting point in unraveling their structure-function relationships.

A single amino acid mutation affects elicitor and expansins-like activities of cerato-platanin, a non-catalytic fungal protein.

Cerato-platanin (CP) is a non-catalytic, cysteine-rich protein, the first member of the cerato-platanin family. It is a single-domain protein with a double Ψ/ß barrel domain resembling the D1 domain of plant and bacterial expansins. Similarly to expansins, CP shows a cell wall-loosening activity on cellulose and can be defined as an expanisin-like protein, in spite of the missing D2 domain, normally present in plant expansins. The weakening activity shown on cellulose may facilitate the CP-host interaction, corroborating the role of CP in eliciting plant defence response. Indeed, CP is an elicitor of primary defences acting as a Pathogen-Associated Molecular Patterns (PAMP). So far, structure-function relationship study has been mainly performed on the bacterial BsEXLX1 expansin, probably due to difficulties in expressing plant expansins in heterologous systems. Here, we report a subcloning and purification method of CP in the engineered E. coli SHuffle cells, which proved to be suitable to obtain the properly folded and biologically active protein. The method also enabled the production of the mutant D77A, rationally designed to be inactive. The wild-type and the mutated CP were characterized for cellulose weakening activity and for PAMP activity (i.e. induction of Reactive Oxygen Species synthesis and phytoalexins production). Our analysis reveals that the carboxyl group of D77 is crucial for expansin-like and PAMP activities, thus permitting to establish a correlation between the ability to weaken cellulose and the capacity to induce defence responses in plants. Our results enable the structural and functional characterization of a mono-domain eukaryotic expansin and identify the essential role of a specific aspartic residue in cellulose weakening.

¹H, ¹5N, and ¹³C resonance assignment of cerato-populin, a fungal PAMP from Ceratocystis populicola.

Plant pathogenic fungi secrete several non-catalytic proteins involved in various aspects of the pathogenesis process. Amongst these, cerato-populin (Pop1) produced by Ceratocystis populicola; a protein orthologous of cerato-platanin (CP), the core member of the CP family. These two proteins interact with host and non-host plants. In plane leaves they induce synthesis of phytoalexins, disruption of intercellular and intracellular leaf tissue, cell plasmolysis, programmed cell death, over-expression of defence-related genes, H2O2 and NO production, activation of MAPK cascade and plant resistance. All these features point to CP and Pop1 as defence inducers, though Pop1 shows a reduced efficiency. Pop1/CP similarity is 73%. CD spectroscopy highlights some secondary structure differences between Pop1 and CP. Indeed, the region between the first two cysteines (C20-C57), that in CP includes the ß2-strand and it is involved in GlcNAc (N-acetyl-D-glucosamine) interaction, in Pop1 is predicted to be fully disordered.

Cerato-populin and cerato-platanin, two non-catalytic proteins from phytopathogenic fungi, interact with hydrophobic inanimate surfaces and leaves.

Based on sequence homology, several fungal Cys-rich secreted proteins have been grouped in the cerato-platanin (CP) family, which comprises at least 40 proteins involved mainly in eliciting defense-related responses. The core member of this family is cerato-platanin, a moderately hydrophobic protein with a double ψ-ß barrel fold. CP and the recently identified orthologous cerato-populin (Pop1) are involved in host-fungus interaction, and can be considered non-catalytic fungal PAMPs. CP is more active in inducing defense when in an aggregated conformation than in its native form, but little is known about other CP-orthologous proteins. Here, we cloned, expressed, and purified recombinant Pop1, which was used to characterize the protein aggregates. Our results suggest that the unfolded, self-assembled Pop1 is more active in inducing defense, and that the unfolding process can be induced by interaction with hydrophobic inanimate surfaces such as Teflon, treated mica, and gold sheets. In vivo, we found that both CP and Pop1 interact with the hydrophobic cuticle of leaves. Therefore, we propose that the interaction of these proteins with host cuticle waxes could induce unfolding and consequently trigger their PAMP-like activity.

The expression of the cerato-platanin gene is related to hyphal growth and chlamydospores formation in Ceratocystis platani.

Cerato-platanin (CP) is a protein produced by Ceratocystis platani, the causal agent of canker stain disease of plane trees. CP is the first member of the 'cerato-platanin family', and its role as a pathogen-associated molecular pattern (PAMP), inducing defence responses both in host and nonhost plants, is established. However, the primary role of CP and its homologues in the fungal life remains unknown. In the present work, we investigated the regulation of the cp gene during the in vitro growth of C. platani in different conditions and under the effect of potential stress factors. Fungal growth and conidiogenesis were also analysed. Results showed that cp is a single-copy gene whose expression level is strictly associated with hyphal growth and with chlamydospores formation. The analysis of a 1368 bp 5'-flanking region revealed putative motifs that could be involved in the regulation of gene expression in response to stress and developmental cues. Taking into account the localization of CP in the fungal cell wall and the recently published 3D structure of the protein, our results support a role for CP in growth and developmental processes of C. platani.