Insights into the Adaptation of Geotrichum citri-aurrantii in Highly Acidic Environments

Abstract

Sour rot is a significant postharvest disease affecting citrus fruit, causing sourness and decay in various cultivars, particularly lemons. How the pathogen, Geotrichum citri-aurantii, adapts to the highly acidic environment of citrus fruit remains inadequately understood. In this study, the growth characteristics, morphological and structural changes, gene expression profiles, and adaptive mechanisms of G. citri-aurantii under highly acidic conditions were elucidated. The findings indicated that G. citri-aurantii modified the environmental pH by either alkalizing (pH < 3.00) or acidifying (pH > 3.00) the host tissue. It exhibited strong adaptability at pH 2.2, showing shortened and aggregated hyphae, delayed spore germination, and increased vacuoles. Transcriptomic analysis and qRT-PCR identified the significant regulation of key differentially expressed genes involved in cell wall remodeling, cell membrane component synthesis, carbon metabolism, and signal transduction. These regulatory changes enable the pathogen to prevent an influx of external acids and maintain the energy supply under acid stress conditions. Additionally, the Pal/Rim pH signaling pathway genes exhibit distinct response patterns in citrus cultivars with different acidities. These findings enrich the comprehension of the pathogenic process of G. citri-aurantii and offer a theoretical foundation for preventing and managing citrus sour rot.

Keywords: Geotrichum citri-aurantii; Pal/Rim pH signaling pathway; acid tolerance; citrus fruit.

Figure 1

The lesion diameter (A) and pH change in the decaying area (B) in different citrus varieties inoculated with G. citri-aurantii. The same lowercase letter indicates no significant difference (p > 0.05), and different lowercase letters indicate significant differences (p < 0.05); * represents a significant difference at α = 0.05; ** represents a significant difference at α = 0.01; *** represents a significant difference at α = 0.001.

Figure 2

Change in pH value of culture medium of G. citri-aurantii under different acidic conditions.

Figure 3

G. citri-aurantii growth in medium and its influence on pH values ((A) growth curve in PDB medium; (B) variations in pH values in PDB medium; (C) growth curve in IM medium; (D) variations in pH values in IM medium).

Figure 4

G. citri-aurantii growth in different acidic pH ranges ((A) mycelial growth at 2 d; (B) spore germination rate); The different lowercase letters indicate significant differences (p < 0.05).

Figure 5

Morphology and ultrastructural view of G. citri-aurantii under different pH conditions; (A,B) optical microscopic images of mycelium cultured for 1 d; (C,D) SEM images of spores cultured for 6 h; (E,F) SEM images of mycelium cultured for 1 d; (G,H) TEM images of mycelium cultured for 1 d; CW: cell wall; V: vacuole.

Figure 6

Transcriptome profiles of G. citri-aurantii under different pH conditions. (A) Principal component analysis (PCA) of the transcriptome; (B) volcano plots of differentially expressed genes; (C) GO enrichment; (D) KEGG pathway annotation.

Figure 6

Transcriptome profiles of G. citri-aurantii under different pH conditions. (A) Principal component analysis (PCA) of the transcriptome; (B) volcano plots of differentially expressed genes; (C) GO enrichment; (D) KEGG pathway annotation.

Figure 7

Protein–protein interaction sub-networks for differentially expressed genes. (A) cluster 1; (B) cluster 2; (C) cluster 3; (D) cluster 4, (E) cluster 5 and (F) cluster 6.

Figure 8

The relative expression levels of 20 differentially expressed genes. (A) DSE4; (B) CTS1; (C) ECM33; (D) GAS5; (E) TOS1; (F) ATG22; (G) FBA1; (H) SFC1; (I) ALD4; (J) SDH2; (K) ALDH3A1; (L) FDH1; (M) PDC1; (N) MEP2; (O) UGA4; (P) DIP5; (Q) GatA; (R) GRE2; (S) GAP1; (T) IRA2. The different lowercase letters indicate significant differences (p < 0.05).

Figure 9

Expression of genes related to pH signaling pathways in vitro and in vivo. (A) the log2-fold change in genes related to pH signaling pathways from RNA-seq analysis; (B) expression level of genes in decayed Satsuma mandarin; (C) expression level of genes in decayed Eureka lemon. The different lowercase letters indicate significant differences (p < 0.05).