Abstract
BackgroundFusarium circinatum, the causal agent of pitch canker disease, poses a serious threat to several Pinus species affecting plantations and nurseries. Although Pinus pinaster has shown moderate resistance to F. circinatum, the molecular mechanisms of defense in this host are still unknown. Phytohormones produced by the plant and by the pathogen are known to play a crucial role in determining the outcome of plant-pathogen interactions. Therefore, the aim of this study was to determine the role of phytohormones in F. circinatum virulence, that compromise host resistance.ResultsA high quality P. pinaster de novo transcriptome assembly was generated, represented by 24,375 sequences from which 17,593 were full length genes, and utilized to determine the expression profiles of both organisms during the infection process at 3, 5 and 10 days post-inoculation using a dual RNA-sequencing approach. The moderate resistance shown by Pinus pinaster at the early time points may be explained by the expression profiles pertaining to early recognition of the pathogen, the induction of pathogenesis-related proteins and the activation of complex phytohormone signaling pathways that involves crosstalk between salicylic acid, jasmonic acid, ethylene and possibly auxins. Moreover, the expression of F. circinatum genes related to hormone biosynthesis suggests manipulation of the host phytohormone balance to its own benefit.ConclusionsWe hypothesize three key steps of host manipulation: perturbing ethylene homeostasis by fungal expression of genes related to ethylene biosynthesis, blocking jasmonic acid signaling by coronatine insensitive 1 (COI1) suppression, and preventing salicylic acid biosynthesis from the chorismate pathway by the synthesis of isochorismatase family hydrolase (ICSH) genes. These results warrant further testing in F. circinatum mutants to confirm the mechanism behind perturbing host phytohormone homeostasis.
Highlights
Fusarium circinatum, the causal agent of pitch canker disease, poses a serious threat to several Pinus species affecting plantations and nurseries
Expression of F. circinatum genes related to hormone biosynthesis or signaling, besides Gibberellic acid (GA), has not been studied
Pathogen colonization and symptom development Fusarium circinatum was observed growing in the resin drop of P. pinaster seedlings during the first three days
Summary
The causal agent of pitch canker disease, poses a serious threat to several Pinus species affecting plantations and nurseries. The aim of this study was to determine the role of phytohormones in F. circinatum virulence, that compromise host resistance. ET and JA act synergistically in response to the infection of necrotrophic fungi, while SA is most commonly expressed in response to biotrophic or hemibiotrophic fungi [41]. This is only a simplistic classification and crosstalk between phytohormones are much more. Indol-3-acetic acid (IAA), the most common form of auxins, can be synthesized from tryptophan by the indol-3-acetamide (IAM) pathway, and IAM-related genes have been reported in four Fusarium pathogenic fungi: Fusarium verticillioides, Fusarium oxysporum, F. fujikuroi and Fusarium proliferatum [103]. Expression of F. circinatum genes related to hormone biosynthesis or signaling, besides GA, has not been studied
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