The transcriptome of Pinus pinaster under Fusarium circinatum challenge

Abstract

Background: Fusarium 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. Results: A 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. Conclusions: We 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.

Dual RNA-Sequencing Analysis of Resistant (Pinus pinea) and Susceptible (Pinus radiata) Hosts during Fusarium circinatum Challenge

“…In previous studies, a marked trend has been observed in the increase in DEGs as the disease was progressing [ 29 , 38 , 42 , 45 ]. The transcriptomic comparison of the present study at an early stage of disease (4 dpi) showed that the number of DEGs in P. pinea (1822) was vastly higher than in P. radiata (371).…”

“…MeJA-mediated cellular responses include the induction of genes encoding terpene synthases [ 63 ] and enzymes of the JA biosynthesis such as LOX [ 64 ], both widely up-regulated in the two pine species. Although these results could indicate an active biosynthesis of JA mainly in P. pinea , the down-regulation of COI1 together with the high induction of JAZ genes could denote a suppression of JA signaling, as observed in P. pinaster infected with F. circinatum at 5 and 10 dpi [ 42 ]. Our results further support that F. circinatum may target and block JA signaling by COI1 suppression as hypothesized by Hernandez-Escribano et al [ 42 ].…”

“…On the other hand, the results showed a moderate activation of SA signaling in P. pinea together with the high up-regulation of two PR1 genes. Induction of PR1 genes upon F. circinatum infection has also been observed in P. pinaster and in the resistant genotype of P. radiata [ 38 , 42 ]. In addition, the synergistic cooperation of SA and JA has been extensively reported [ 42 , 45 , 68 ].…”

“…Current sequencing platforms generate an unprecedented number of reads, making possible the simultaneous analysis of transcripts both of pathogens and their hosts [ 41 ]. This approach, called dual RNA-Seq, has been employed to reveal the interaction of Pinus pinaster with F. circinatum [ 42 ]. The moderate resistance of P. pinaster was reflected in the early induction of genes associated with a complex phytohormone signaling of jasmonic acid (JA), ethylene (ET), and salicylic acid (SA).…”

“…The moderate resistance of P. pinaster was reflected in the early induction of genes associated with a complex phytohormone signaling of jasmonic acid (JA), ethylene (ET), and salicylic acid (SA). On the other hand, it was suggested a perturbation in the host phytohormone homeostasis by F. circinatum through the expression of fungal genes related to these phytohormones [ 42 ]. Plant hormones are important regulators of metabolism being closely related to the plant defense [ 43 ], resulting in a complex signaling cross-talk of phytohormones such as JA, ET, SA, auxins, cytokinins (CKs), gibberellins (GAs), abscisic acid (ABA), and brassinosteroids (BRs).…”