Investigation of the Effectiveness and Molecular Mechanisms of Thiamin Priming to Control Early Blight Disease in Potato

In several plant species, thiamin foliar application primes plant immunity and can be effective in controlling various diseases. However, the effectiveness of thiamin against potato pathogens has seldom been investigated. Additionally, the transcriptomics and metabolomics of immune priming by thiamin have not previously been investigated. Here, we tested the effect of thiamin application against <i>Alternaria solani</i>, the causal agent of early blight in potato, and identified associated changes in gene expression and metabolite content. Thiamin applied on foliage at an optimal concentration of 10 mM reduced lesion size by ∼33%. However, prevention of lesion growth was temporally limited, as a reduction of lesion size occurred when leaves were inoculated 4 h, but not 24 h, following thiamin treatment. Additionally, the effect of thiamin on lesion size was restricted to the application site and was not systemic. RNA-seq analysis showed that thiamin affected the expression of 308 genes involved in the synthesis of salicylic acid, secondary metabolites, fatty acid, chitin, primary metabolism, and photosynthesis. Genes in these pathways were also amongst the thousands of genes differentially regulated in the response to the pathogen alone, though they were often more differentially expressed and enriched when thiamin and the pathogen were combined. Thiamin also delayed the downregulation of photosynthesis-associated genes in plants inoculated with <i>A</i>. <i>solani</i>. Metabolite analyses revealed that thiamin treatment in the absence of a pathogen decreased the amounts of several organic compounds involved in the citric acid cycle. We hypothesize that thiamin primes plant defenses through perturbation of primary metabolism.