Transcriptome Analysis and Reactive Oxygen Species Detection Suggest Contrasting Molecular Mechanisms in Populus canadensis' Response to Different Formae Speciales of Marssonina brunnea

Abstract

Revealing plant-pathogen interactions is important for resistance breeding, but it remains a complex process that presents many challenges. Marssonina leaf spot of poplars (MLSP) is the main disease in poplars; in China, its pathogens consist of two formae speciales, namely, Marssonina brunnea f. sp. Monogermtubi (MO) and M. brunnea f. sp. Multigermtubi (MU). However, the mechanism of the molecular interaction between poplars and the two formae speciales, especially for an incompatible system, remains unclear. In this study, we conducted transcriptome sequencing and reactive oxygen species (ROS) staining based on the interactions between Populus canadensis and the two formae speciales. The results show that the gene expression patterns of P. canadensis induced by MO and MU were significantly different, especially for the genes associated with biotic stress. Furthermore, MO and MU also triggered distinct ROS reactions of P. canadensis, and ROS (mainly H₂O₂) burst was only observed around the cells penetrated by MU. In conclusion, this study suggested that P. canadensis experienced different resistance reactions in response to the two formae speciales of M. brunnea, providing valuable insights for further understanding the host-pathogen interactions of MLSP.

Keywords: differentially expressed genes (DEGs); plant–pathogen interaction; poplar; resistance.

Figure 1

Poplar leaves infected by M. brunnea, the cultural colonies, and the penetration process of MLSP. (A) At 7 dpi, no visible spots occur at the point inoculated with the spore suspension of MO, while obvious black spots occur at the point inoculated with MU on the leaf of P. canadensis. (B) The fungal colony of MO is magenta on PDA and the fungal colony of MU is greenish on PDA. (C) The left picture shows spores of MU on the surface of the poplar leaf at 8 hpi; the right picture shows the IVs formed in epidermis cells under the spores shown in the left picture.

Figure 2

A common comparative analysis of poplar DEGs between MO_treat and MU_treat. (A) The number of up− and down−regulated DEGs shown with a histogram. (B) Venn diagram showing the number of overlapping genes between the two treatments. (C) Volcano plot showing the distribution and significance of DEGs.

Figure 3

The enriched GO and KEGG terms analyzed by the DEGs of MO_treat vs. control and MU_treat vs. control. In this figure, only the GO terms belonging to biological processes are listed with a selection condition of p value < 0.01. The KEGG terms shown in this figure were also selected with a p value < 0.01. The values shown in color represent the gene number enriched in each term.

Figure 4

MapMan analysis illustrating the DEGs of MO_treat vs. MU_treat involved in plant–pathogen interaction pathways. The scale showing the value of log2(fold change). The gene details are listed in Supplementary Table S4.

Figure 5

Cytochemical localization of H₂O₂ and O₂⁻ production in poplar–M. brunnea interactions. (A) An App produced by a Co of MO, stained with NBT. (B) An App produced by a Co of MO, stained with DAB. (C) An App produced by a Co of MU, stained with NBT. (D) An App produced by a Co of MU, stained with DAB. (E) IVs formed in the host epidemic cells, stained with DAB. (F) An App germinated by a Co of MU on the surface of the poplar upper epidermis and IV formed in the epidemic cells under the Co, stained with DAB. (G) A penetration pore (PP) produced by MU on the host surface and IV formed under PP, stained with DAB. (H) IV of MU formed in the cells surrounding stomatal guard cells, stained with DAB. Bar = 5 μm.