Maize Antifungal Protein AFP1 Elevates Fungal Chitin Levels by Targeting Chitin Deacetylases and Other Glycoproteins

Pathogenic fungi convert chitin to chitosan to evade plant perception and disarm chitin-triggered immune responses. Whether plants have evolved factors to counteract this evasion mechanism remains obscure. Here, we decipher the mechanism underlying the antifungal activity of maize secretory mannose-binding cysteine-rich receptor-like secreted protein (CRRSP), antifungal protein 1 (AFP1). AFP1 binds to multiple sites on the surface of sporidial cells, filaments, and germinated spores of the biotrophic fungus Ustilago maydis. It inhibits cell growth and budding, as well as spore germination. AFP1 promiscuously interacts with most chitin deacetylases (CDAs) by recognizing the conserved NodB domain to interfere with the enzyme activity. Deletion of O-mannosyltransferase 4 decreases protein mannosylation, which correlates with reduced AFP1 binding and antifungal activity, suggesting that AFP1 interacts with mannosylated proteins to exhibit an inhibitory effect. AFP1 also has extended inhibitory activity against Saccharomyces cerevisiae; however, AFP1 did not reduce binding to the double ΔΔcda1,2 mutant, suggesting the targets of AFP1 have expanded to other cell surface glycoproteins, probably facilitated by its mannose-binding property. Increasing chitin levels by modulating the activity of cell surface glycoproteins is a universal feature of AFP1 interacting with a broad spectrum of fungi to inhibit their growth. IMPORTANCE Plants alert immune systems by recognizing the fungal pathogen cell wall component chitin via pattern recognition cell surface receptors. Successful fungal pathogens escape the perception by deacetylating chitin to chitosan, which is also necessary for fungal cell development and virulence. Targeting glycoproteins that are associated with regulating chitin metabolism and maintaining cell wall morphogenesis presents an effective strategy to combat fungal pathogens by simultaneously altering cell wall plasticity, activating chitin-triggered immunity, and impairing fungal viability. Our study provides molecular insights into a plant DUF26 domain-containing secretory protein in warding off a broad range of fungal pathogens by acting on more than one glycoprotein target.

Genome-Wide Identification of DUF26 Domain-Containing Genes in Dongxiang Wild Rice and Analysis of Their Expression Responses under Submergence

The DUF26 domain-containing protein is an extracellular structural protein, which plays an important role in signal transduction. Dongxiang wild rice (Oryza rufipogon Griff.) is the northern-most common wild rice in China. Using domain analysis, 85 DUF26 domain-containing genes were identified in Dongxiang wild rice (DXWR) and further divided into four categories. The DUF26 domain-containing genes were unevenly distributed on chromosomes, and there were 18 pairs of tandem repeats. Gene sequence analysis showed that there were significant differences in the gene structure and motif distribution of the DUF26 domain in different categories. Motifs 3, 8, 9, 13, 14, 16, and 18 were highly conserved in all categories. It was also found that there were eight plasmodesmata localization proteins (PDLPs) with a unique motif 19. Collinearity analysis showed that DXWR had a large number of orthologous genes with wheat, maize, sorghum and zizania, of which 17 DUF26 domain-containing genes were conserved in five gramineous crops. Under the stress of anaerobic germination and seedling submergence treatment, 33 DUF26 domain-containing genes were differentially expressed in varying degrees. Further correlation analysis with the expression of known submergence tolerance genes showed that these DUF26 domain-containing genes may jointly regulate the submergence tolerance process with these known submergence tolerance genes in DXWR.

The Cysteine-Rich Repeat Protein TaCRR1 Participates in Defense against Both Rhizoctonia cerealis and Bipolaris sorokiniana in Wheat

The domain of unknown function 26 (DUF26), harboring a conserved cysteine-rich motif (C-X8-C-X2-C), is unique to land plants. Several cysteine-rich repeat proteins (CRRs), belonging to DUF26-containing proteins, have been implicated in the defense against fungal pathogens in ginkgo, cotton, and maize. However, little is known about the functional roles of CRRs in the important staple crop wheat (Triticum aestivum). In this study, we identified a wheat CRR-encoding gene TaCRR1 through transcriptomic analysis, and dissected the defense role of TaCRR1 against the soil-borne fungi Rhizoctonia cerealis and Bipolaris sorokiniana, causal pathogens of destructive wheat diseases. TaCRR1 transcription was up-regulated in wheat towards B. Sorokiniana or R. cerealis infection. The deduced TaCRR1 protein contained a signal peptide and two DUF26 domains. Heterologously-expressed TaCRR1 protein markedly inhibited the mycelia growth of B. sorokiniana and R. cerealis. Furthermore, the silencing of TaCRR1 both impaired host resistance to B. sorokiniana and R. cerealis and repressed the expression of several pathogenesis-related genes in wheat. These results suggest that the TaCRR1 positively participated in wheat defense against both B. sorokiniana and R. cerealis through its antifungal activity and modulating expression of pathogenesis-related genes. Thus, TaCRR1 is a candidate gene for improving wheat resistance to B. sorokiniana and R. cerealis.

Ectodomain of plasmodesmata-localized protein 5 in Arabidopsis: expression, purification, crystallization and crystallographic analysis

Plasmodesmata-localized protein 5 (PDLP5) is a cysteine-rich receptor-like protein which is localized on the plasmodesmata of Arabidopsis thaliana. Overexpression of PDLP5 can reduce the permeability of the plasmodesmata and further affect the cell-to-cell movement of viruses and macromolecules in plants. The ectodomain of PDLP5 contains two DUF26 domains; however, the function of these domains is still unknown. Here, the ectodomain of PDLP5 from Arabidopsis was cloned and overexpressed using an insect expression system and was then purified and crystallized. X-ray diffraction data were collected to 1.90 Å resolution and were indexed in space group P1, with unit-cell parameters a = 41.9, b = 48.1, c = 62.2 Å, α = 97.3, β = 103.1, γ = 99.7°. Analysis of the crystal content indicated that there are two molecules in the asymmetric unit, with a Matthews coefficient of 2.51 Å3 Da-1 and a solvent content of 50.97%.