Effective mitigation by Pseudomonas Aeruginosa and a specialized organic fertiliser against early ganoderma boninense infection in oil palm

Basal stem rot caused by ganoderma boninense infection shortens the economic life span of oil palm trees resulting in major revenue loss to the oil palm industry in Malaysia. Control measures using organic fertilizer formulation and biocontrol agent could provide a long-term sustainable solution. However, information on the molecular mechanisms to guide effective application of these control strategies is lacking. This study aimed to compare these two control strategies on how they influence early-stage infection mechanisms of the fungal pathogen through ultrastructural, molecular, and physiological analyses. Oil palm seedlings were artificially inoculated with G. boninense and treated with Pseudomonas Aeruginosa (T2) or anti-Ganoderma organic fertilizer (T3) and the roots were harvested at four different time points within three weeks post inoculation.

While G. boninense inoculated but untreated seedlings served as positive control (T1). Severe cell wall degradation and fungal hyphae proliferation were observed in T1 through scanning and transmission electron microscopic analysis with T2 and T3 demonstrating much reduced damage. Superior cell wall protection was evident in T3. This finding was supported by DNA quantification which showed significantly lower pathogen DNA levels in T3. Gene expression profiling of selected defense response genes through RT-qPCR suggested that the endophytic bacteria, Pseudomonas aeruginosa activated ethylene/jasmonate defense signaling pathway earlier while the anti-Ganoderma fertilizer extended the biotrophic defense response. Furthermore, EgGAMYB expression was enhanced under both treatments suggesting strengthening of cell wall structure. Ca2+-signaling shift that promotes earlier, and more effective defense response was observed in T3. Physiological data on chlorophyll index and stomatal conductance supported much enhanced stability in T2 and T3 seedlings in contrast to the rapidly degrading T1. After 25 weeks, T1 showed necrotic roots and leaves, while T2 and T3 resembled healthy controls. Together the results of the study demonstrate the mitigation effects of both disease control strategies and how they differentially modulate biotrophic-to-necrotrophic defense phase transition in oil palm.

Keywords: anti-Ganoderma fertilizer, electron microscopy, Ganoderma boninense, gene expression, oil palm, plant defense system, Pseudomonas aeruginosa