Sugar beet defense responses are significantly modulated during a susceptible interaction with Cercospora beticola.

Sugar beet (Beta vulgaris L.) is the sole source of sucrose production in temperate regions worldwide and hence, is a very important economic crop. Among the many constraints to sugar beet cultivation, Cercospora leaf spot (CLS), caused by the fungal pathogen Cercospora beticola, is the most destructive foliar disease globally. To better understand the sugar beet – C. beticola interaction, it is pertinent to elucidate the sugar beet defense mechanisms that are modulated during infection. Herein, we present a time-course RNAseq analysis of a compatible (susceptible) interaction between sugar beet and C. beticola spanning the asymptomatic (3, 7 days post inoculation (DPI)) and symptomatic (11, 17, 25 DPI) phases of the infection process. We found that the defense response was the most altered functional category of differentially expressed genes with various classes of cell surface receptors and intracellular receptors mediating pathogen recognition being differentially expressed. We also observed differential expression of genes associated with reactive oxygen species generation, Ca²⁺ homeostasis, mitogen-activated protein kinase activation, callose deposition, stomatal movement, and defense associated transcription factors. A major proportion of the defense response associated genes that were upregulated at 3 DPI were repressed at 7 DPI but were strongly upregulated again during the symptomatic phase (11-25 DPI). Our results have provided preliminary indication that C. beticola could target and proficiently modulate these mechanisms to successfully evade, suppress or induce sugar beet defense ostensibly for its advantage. Taken together, our results have provided a transcriptomic snapshot of the sugar beet defense responses that were modulated during C. beticola infection.