The Fusarium crown rot pathogen Fusarium pseudograminearum triggers a suite of transcriptional and metabolic changes in bread wheat (Triticum aestivum L.)

Abstract

Background and aims: Fusarium crown rot caused by the fungal pathogen Fusarium pseudograminearum is a disease of wheat and barley, bearing significant economic cost. Efforts to develop effective resistance to this disease have been hampered by the quantitative nature of resistance and a lack of understanding of the factors associated with resistance and susceptibility. Here, we aimed to dissect transcriptional responses triggered in wheat by F. pseudograminearum infection.

Methods: We used an RNA-seq approach to analyse host responses during a compatible interaction and identified >2700 wheat genes differentially regulated after inoculation with F. pseudograminearum . The production of a few key metabolites and plant hormones in the host during the interaction was also analysed.

Key results: Analysis of gene ontology enrichment showed that a disproportionate number of genes involved in primary and secondary metabolism, signalling and transport were differentially expressed in infected seedlings. A number of genes encoding pathogen-responsive uridine-diphosphate glycosyltransferases (UGTs) potentially involved in detoxification of the Fusarium mycotoxin deoxynivalenol (DON) were differentially expressed. Using a F. pseudograminearum DON-non-producing mutant, DON was shown to play an important role in virulence during Fusarium crown rot. An over-representation of genes involved in the phenylalanine, tryptophan and tyrosine biosynthesis pathways was observed. This was confirmed through metabolite analyses that demonstrated tryptamine and serotonin levels are induced after F. pseudograminearum inoculation.

Conclusions: Overall, the observed host response in bread wheat to F. pseudograminearum during early infection exhibited enrichment of processes related to pathogen perception, defence signalling, transport and metabolism and deployment of chemical and enzymatic defences. Additional functional analyses of candidate genes should reveal their roles in disease resistance or susceptibility. Better understanding of host responses contributing to resistance and/or susceptibility will aid the development of future disease improvement strategies against this important plant pathogen.

Keywords: Fusarium; RNA-seq; Tri5; Wheat; biotic stress; deoxynivalenol; jasmonate; salicylic acid; secologanin; serotonin.

Fig. 1.

Molecular inference for induction of phenylalanine, tryptamine and tyrosine pathways that lead to the production of defence-associated hormones and metabolites in wheat following F. pseudograminearum infection in wheat. The phenylalanine, tyrosine and tryptophan biosynthesis pathway (retrieved from KEGG) denoting Fusarium responsive genes encoding enzymes functioning within the pathway. Filled arrows denote up-regulated genes and open arrows denote down-regulated genes. Underlined compound names indicate metabolites which were detected in this study using LC-MS.

Fig. 2.

Uridine di-phospate glycosyltransferase (UGT) genes induced during pathogen infection and putatively involved in deoxynivalenol detoxification. Phylogenetic tree of F. pseudograminearum responsive wheat UGT genes alongside known deoxynivalenol detoxifying UGT encoding genes from barley (HvUGT13248) and Brachypodium (Bradi5g03300). The phylogram shows a redacted version of the phylogram with all Brachypodium UDP-glycosyltransferases given in File S4 displaying the clade with deoxynivalenol detoxifying UDP-glycosyltransferases. The phylogeny was produced using the unweighted pair group method (UPGMA) with the Kimura two-parameter model applied. One hundred replicates were performed with bootstrap values displayed at branch nodes. The scale bar represents 0·500 substitutions per nucleotide position

Fig. 3.

Reduction of virulence in Tri5 F. pseudograminearum knockout mutants. (A) Representative infection assays with the parental (CS3096) isolate and two independent mutants. (B) Shoot length is used as a measure of isolate virulence. Plants inoculated with the CS3096 parental strain are significantly shorter than the plants inoculated with the TRI5 mutants. Student’s t-testing was applied to determine statistically significant differences between mean values.

Fig. 4.

Quantification of salicylic acid and jasmonate in above leaf sheath tissue across an F. pseudograminerum infection time-course using LC-MS. Bar graphs denote mean quantification values (ng metabolite per ng tissue) across four biological replicate samples for mock- versus F. pseudograminearum-inoculated samples at 3 and 7 dpi. Error bars represent the standard error between biological replicates. Student’s t-testing was applied to determine statistically significant differences (*P < 0·05) between mean values.

Fig. 5.

Quantification of tryptamine and serotonin in wheat seedlings across an F. pseudograminerum infection time-course using LC-MS. Bar graphs denote mean quantification values (µg metabolite per g tissue) across four biological replicate samples for mock- versus F. pseudograminearum-inoculated samples at 3 and 7 dpi. Error bars represent standard error between biological replicates. Student’s t-testing was applied to determine statistically significant differences (*P < 0·05) between mean values.