Comparative Transcriptome Analysis Reveals the Effect of the DHN Melanin Biosynthesis Pathway on the Appressorium Turgor Pressure of the Poplar Anthracnose-Causing Fungus Colletotrichum gloeosporioides

Abstract

Anthracnose of poplar caused by Colletotrichum gloeosporioides is a leaf disease that seriously affects poplar growth. The pathogen invades the host in the form of adherent cells, which generate turgor pressure through the metabolism of intracellular substances prior to penetrating the epidermis of poplar leaves. In this study, the expansion-related pressure of the mature appressorium of the wild-type C. gloeosporioides was approximately 13.02 ± 1.54 MPa at 12 h, whereas it was 7.34 ± 1.23 MPa and 9.34 ± 2.22 MPa in the melanin synthesis-related gene knockout mutants ΔCgCmr1 and ΔCgPks1, respectively. The CgCmr1 and CgPks1 genes were highly expressed at 12 h in the wild-type control, implying that the DHN melanin biosynthesis pathway may play an important role in the mature appressorium stage. The transcriptome sequencing analysis indicated that the upregulated melanin biosynthesis genes in C. gloeosporioides, such as CgScd1, CgAyg1, CgThr1, CgThr2, and CgLac1, are involved in specific KEGG pathways (i.e., fatty acid biosynthesis, fatty acid metabolism, and biotin metabolism). Therefore, we speculate that the melanin synthesis-related genes and fatty acid metabolism pathway genes contribute to the regulation of the turgor pressure in the mature C. gloeosporioides appressorium, ultimately leading to the formation of infection pegs that enter plant tissues. These observations may reflect the co-evolution of C. gloeosporioides and its host.

Keywords: Colletotrichum gloeosporioides; DHN melanin biosynthesis; appressorium; turgor generation.

Figure 1

Morphological characteristics of the Colletotrichum gloeosporioides appressorium on a hydrophobic surface at different time points (scale = 10 μm).

Figure 2

Appressorium turgor pressure for the wild-type (WT) control (A) and ΔCgCmr1 (B) and ΔCgPks1 (C) mutants at different time points.

Figure 3

Relative expression of CgCmr1 (A) and CgPks1 (B) in the wild-type control at different time points. * Significant differences among time points as determined by the t-test (p < 0.05).

Figure 4

Number of DEGs in the CgWT12h sample. Red and green indicate upregulated and downregulated expression, respectively.

Figure 5

GO term annotation of the DEGs in the CgWT12h transcriptome. The DEGs in the CgWT12h sample were assigned GO terms from the following three main categories: cellular component (A), molecular function (B), and biological process (C).

Figure 6

Results of the qRT-PCR analysis performed to validate key candidate DEGs in the wild-type (WT) control and ΔCgCmr1 and ΔCgPks1 mutants at 0 and 12 h. The qRT-PCR data represent the mean of three replicates. Bars indicate the standard error. Different letters indicate significant differences among treatments, as determined by Duncan’s test (p < 0.05).