image: In WT, PDLP7 interacts with BG10 through its GnK2-1 domain to restrict BG10 from hydrolyzing callose. Free callose in the cell wall is deposited at the PD neck to maintain a dynamically balanced steady state of callose (Fig.a). In the pdlp7 mutant, the absence of PLDP7 leads to increased BG10 activity, resulting in decreased callose content at the PD neck and ultimately to PD opening (Fig.b). When virus attack or in the PDLP7 OE lines, the highly expressed PDLP7 interacts with BG10, potentially inhibiting BG10 from hydrolyzing callose. As a result, more callose is deposited into the neck of the cell wall, effectively sealing off the plasmodesmata and providing resistance against the virus ((Fig.c).
Credit: ©Science China Press
This study is led by Prof. Yu-Xian Zhu (Institute for Advanced Studies, Wuhan University; State Key Laboratory of Protein and Plant Gene Research, College of Life Sciences, Peking University)
Plasmodesmata (PD) are specialized channels in the cell wall that facilitate communication between adjacent cells. PD microdomains consist of distinct proteins, sphingolipids, and sterol. One key component, Plasmodesmata-located protein (PDLP), is known for its crucial role in defending against pathogenic bacterial infections. However, the precise impact of PDLP on the structure and function of plasmodesmata remains uncertain.
In Arabidopsis thaliana, the PDLP family consists of eight members. This study identified that PDLP5 and PDLP7 are capable of binding to sphingosine t18:0 via the sphingolipid-binding motif located in the transmembrane region of their carboxyl terminus (C terminus). Further virus inoculation experiments showed that PDLP1, PDLP5, and PDLP7 were notably upregulated following Turnip mosaic virus (TuMV) infection, while PDLP2, PDLP3, and PDLP7 exhibited significant upregulation after Cucumber mosaic virus (CMV) infection. The significant upregulation of PDLP7 suggests its ability to respond to infections by both viruses simultaneously. In comparison to wild-type (WT) plants, pdlp7 mutant exhibits accelerated spread of TuMV and increased accumulation of CMV, pointing towards increased PD permeability. By high-pressure freezing sample preparation in conjunction with focused ion beam-scanning electron microscopy (FIB-SEM) observation, it was discovered that the PD diameter in pdlp7 mutant was enlarged compared to WT. pdlp7 mutant plants showed significantly reduced callose deposition. These plants had increased β-1,3-glucanase activity, but no change in callose synthase activity. Co-immunoprecipitation–mass spectrometry revealed that the top-ranked protein in the Gene Ontology Annotation of PDLP7-related proteins was associated with the “Hydrolase activity”. Furthermore, in yeast two-hybrid experiments, it was found that PDLP7 interacted specifically with glucan endo-1,3-β-glucosidase 10 (BG10). Consistently, higher levels of callose deposition and slower virus transmission were observed in bg10 mutant plants. Furthermore, the interaction between PDLP7 and BG10 depended on the presence of the Gnk2-homologous 1 (GnK2-1) domain at the N terminus of PDLP7 but not the presence of the GnK2-2 domain. Structural simulation and molecular docking analysis indicated that 35-Aspartic acid (D35), 42-Lysine (C42), 44-Glutamine (Q44) and 116-Leucine (L116) in the GnK2-1 domain are necessary for this interaction.
Biochemical and structural simulation analyses have revealed that PDLP7 has the ability to directly bind to callose. Molecular docking results indicate that the GnK2-1 domain exhibits a more stable binding to callose in comparison to the GnK2-2 homology domain. Far-UV circular dichroism experiments showed that conformational change may depend on the relationship between callose and GnK2-1. GnK2-1 (ProPDLP7:GnK2-1×2), but not GnK2-2 (ProPDLP7:GnK2-2×2) complements both the callose content and viral spreading phenotypes in pdlp7 plants.
These data suggest that the GnK2 domain of PDLP7 and BG10 collectively facilitate both the accumulation and degradation of callose, control the opening and closing of plasmodesmata, and consequently restrict the spread of viruses. The findings of this research elucidate the interplay between proteins and plant polysaccharides within plasmodesmata microdomains, offering novel insights for enhancing plant antiviral defenses and agricultural productivity.
See the article:
Arabidopsis PDLP7 modulated plasmodesmata function is related to BG10-dependent glucosidase activity required for callose degradation.
https://doi.org/10.1016/j.scib.2024.04.063
Journal
Science Bulletin