Intact salicylic acid signalling is required for potato defence against the necrotrophic fungus Alternaria solani

Abstract

Key Message: Using disease bioassays and transcriptomic analysis we show that intact SA-signalling is required for potato defences against the necrotrophic fungal pathogen Alternaria solani.

Abstract: Early blight, caused by the necrotrophic fungus Alternaria solani, is an increasing problem in potato cultivation. Studies of the molecular components defining defence responses to A. solani in potato are limited. Here, we investigate plant defence signalling with a focus on salicylic acid (SA) and jasmonic acid (JA) pathways in response to A. solani. Our bioassays revealed that SA is necessary to restrict pathogen growth and early blight symptom development in both potato foliage and tubers. This result is in contrast to the documented minimal role of SA in resistance of Arabidopsis thaliana against necrotrophic pathogens. We also present transcriptomic analysis with 36 arrays of A. solani inoculated SA-deficient, JA-insensitive, and wild type plant lines. A greater number of genes are differentially expressed in the SA-deficient mutant plant line compared to the wild type and JA- insensitive line. In wild type plants, genes encoding metal ion transporters, such as copper, iron and zinc transporters were upregulated and transferase-encoding genes, for example UDP-glucoronosyltransferase and Serine-glyoxylate transferase, were downregulated. The SA-deficient plants show upregulation of genes enriched in GO terms related to oxidoreductase activity, respiratory chain and other mitochondrial-related processes. Pathogenesis-related genes, such as genes encoding chitinases and PR1, are upregulated in both the SA-deficient and wild type plants, but not in the JA-insensitive mutants. The combination of our bioassays and the transcriptomic analysis indicate that intact SA signalling, and not JA signalling, is required for potato defences against the necrotrophic pathogen A. solani.

Electronic supplementary material: The online version of this article (10.1007/s11103-020-01019-6) contains supplementary material, which is available to authorized users.

Keywords: Alternaria solani; Coi1; Early blight; JA; NahG; Potato; SA; Solanum tuberosum.

Fig. 1

Salicylic acid-deficient potato plants (NahGD2 and NahGA) show larger leaf lesions and higher fungal biomass after Alternaria solani infection than wild type plants. Representative images (A) and average lesion size (mm) (B) at 10 days post-inoculation (dpi) for one wild type (N = 57 leaflets) and four mutant lines, two jasmonic acid insensitive (coi1H1 (N = 54 leaflets) and coi1 × 5 (N = 35 leaflets)) and two salicylic acid-deficient (NahGD2 (N = 63 leaflets) and NahGA (N = 59 leaflets)) lines. Both SA deficient lines showed significantly larger lesions compared to wild type. The fungal biomass (C) in μg/100 mg fresh weight harvested 5dpi determined by qPCR N = 3. Both SA deficient lines show significantly higher fungal biomass compared to wild type. Error bars represent standard error of the mean. Asterisks represent significant differences in comparison to wild type as tested by one-way ANOVA followed by Fisher’s Pairwise comparison’s test (p < 0.05)

Fig. 2

Salicylic acid-deficient potato tubers (NahGD2 and NahGA) show larger lesions 15 days after Alternaria solani infection than wild type tubers. Schematic figure of A. solani tuber lesion and the formula used to calculate the lesion volume (A) and average lesion size volume (mm³) (B) at 15 days post-inoculation (dpi) for one wild type and four mutant lines, two jasmonic acid insensitive (coi1H1 and coi1 × 5) and two salicylic acid-deficient (NahGD2 and NahGA) lines (N = 16). Both SA deficient lines showed significantly larger lesions compared to wildtype. Error bars represent standard error of the mean. Asterisks represent significant differences in lesion volume in comparison to wildtype as tested one-way ANOVA followed by Fisher’s Pairwise comparison’s test (p < 0.05)

Fig. 3

Salicylic acid deficient and jasmonic acid insensitive lines produce less H₂O₂ in response to A. solani infection than wild type. DAB (3,3–diaminobenzidine) stained area fraction of A. solani infected area. Representative images (A) and average area fraction in % of brownish DAB staining (B) at 72 hpi, wild type, JA insensitive (coi1H1), and SA deficient (NahGD2) lines. Error bars represent standard error of the mean (N = 20). Asterisks represent significant differences in comparison to wild type as tested by one-way ANOVA followed by Fisher’s Pairwise comparison’s test (p < 0.05)

Fig. 4

Venn diagrams depicting the overlap of the total the number of significantly upregulated and downregulated genes (adj. p value < 0.05) at 24, 72 and 120 hpi in the wild type, jasmonic acid insensitive (coi1H1) and salicylic acid-deficient (NahGD2) lines

Fig. 5

Bubble plot showing significantly enriched Gene Ontology (GO) terms for the differentially expressed genes (DEGs) in A. solani infected SA defiencient plant line (NahGD2) versus control at 120 hpi. Size of the bubbles is proportional to the number of DEGs (adj. p value < 0.05) assigned to the GO term. The y-axis represents the negative logarithm of the adjusted p value [false discovery rate (FDR)] for the GO terms, and the x-axis displays the z-score as calculated using the GOplot R-Package (Walter et al. 2015). The threshold for displaying the bubble labels was set to a − log(FDR) of 2.5. Bubbles for GO terms belonging to Biological Process are green, Molecular Function are blue, and Cellular Component are red. Gene name (DMG), gene description and the Log2 Fold change in the infected compared to the control are displayed.