Abstract
Various phenolic compounds have been screened against Ganoderma boninense, the fungal pathogen causing basal stem rot in oil palms. In this study, we focused on the effects of salicylic acid (SA) on the growth of three G. boninense isolates with different levels of aggressiveness. In addition, study on untargeted metabolite profiling was conducted to investigate the metabolomic responses of G. boninense towards salicylic acid. The inhibitory effects of salicylic acid were both concentration- (P < 0.001) and isolate-dependent (P < 0.001). Also, growth-promoting effect was observed in one of the isolates at low concentrations of salicylic acid where it could have been utilized by G. boninense as a source of carbon and energy. Besides, adaptation towards salicylic acid treatment was evident in this study for all isolates, particularly at high concentrations. In other words, inhibitory effect of salicylic acid treatment on the fungal growth declined over time. In terms of metabolomics response to salicylic acid treatment, G. boninense produced several metabolites such as coumarin and azatyrosine, which suggests that salicylic acid modulates the developmental switch in G. boninense towards the defense mode for its survival. Furthermore, the liquid chromatography time-of-flight mass spectrometry (LC-TOF-MS) analysis showed that the growth of G. boninense on potato dextrose agar involved at least four metabolic pathways: amino acid metabolism, lipid pathway, tryptophan pathway and phenylalanine pathway. Overall, there were 17 metabolites that contributed to treatment separation, each with P<0.005. The release of several antimicrobial metabolites such as eudistomin I may enhance G. boninense’s competitiveness against other microorganisms during colonisation. Our findings demonstrated the metabolic versatility of G. boninense towards changes in carbon sources and stress factors. G. boninense was shown to be capable of responding to salicylic acid treatment by switching its developmental stage.
Highlights
Ganoderma boninense Pat., the causal agent of basal and upper stem rot of oil palm (Elaeis guineensis), is a major threat to the oil palm industry in Southeast Asia [1]
In the presence of salicylic acid (SA) regardless of their concentrations, pale reddish zone formed around the fungal mycelia (S2–S4 Figs) of all G. boninense isolates cultured on the SA-amended-potato dextrose agar (PDA) medium
This study provides the first evidence detailing the metabolic responses of G. boninense that enable consequent modulation and switching of its developmental growth stage to combat salicylic acid stress
Summary
Three G. boninense isolates were tested in this study, i.e., G8 (least aggressive), PER71 (moderately aggressive), and G10 (most aggressive), each with different levels of aggressiveness in causing disease in the nursery screening [19]. The cultures were grown on malt extract agar (MEA) (Difco, Becton Dickinson Diagnostics, Sparks, Maryland) at 24 ̊C in the dark for 14 days prior to the commencement of experiment. SA solution was filter-sterilized before adding into potato dextrose agar (PDA) (Difco, Becton Dickinson Diagnostics, Sparks, Maryland). SA was replaced by distilled water with minimal amount of absolute ethanol (0.24% v/v). Ten millimeter of G. boninense mycelial plugs from the culture plates were transferred to SA-amended and positive control petri dishes with a diameter of 88 mm. The cultures were kept under the same growing conditions as culture maintenance on MEA
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