Bacterial Endophytes From Oil Palm (Elaeis Guineensis) and Their Antagonistic Activity Against Ganoderma Boninense

The oil palm (Elaeis guineensis) is the most important oil crop in Malaysia, producing an average yield of approximately four tonnes of palm oil per hectare per year. In Malaysia, the crop is blessed by being largely disease free, suffering from but one major disease, Basal Stem Rot (BSR), caused by Ganoderma boninense. With no known cure at present, it is the major cause of disease in oil palm and therefore of great economic importance to the Malaysian oil palm industry. Endophytic bacteria have been found able to induce systemic resistance in plants and shown biological traits like antibiotic activity analysis. The use of endophytic microorganisms should be preferable to other biological control agents as they are internal colonizers, and therefore more able to compete in the vascular systems, depriving Ganoderma from both nutrients and space for its proliferation. The potential of symptomless endophyte colonization for protection against disease has provided great impetus for research. Understanding their endophytic life stages may help in the design of management strategies for controlling BSR. The objectives of this study were, therefore, to isolate, characterize and identify the symptomless bacterial endophytes in oil palm and to determine their potential as biocontrol agents against Ganoderma in vitro and in vivo. The bacterial endophytes were detected and isolated from the roots of oil palm on different soils, palm age and BSR infection status. Observations of histological root sections under a light microscope showed the presence of bacterial endophytes, stained red with safranin. The bacterial cells were found both intra- and intercellular in the root tissues and distributed uniformly in the epidermis, sclerenchyma and cortical cells. They were also detected in the vascular tissues around the phloem and xylem vessels in the roots of symptomless palms. However, in the roots of palms with BSR, Gram-negative bacteria were also found randomly in the epidermis, sclerenchyma and cortex, but not in the phloem and xylem vessels. Of 1,323 microbial endophytes isolated from 3,600 oil palm roots, 65.23% were bacteria, 32.73% fungi and 2.04% actinomycetes. 93.04% of the bacteria were Gram-negative and only 6.95% gram-positive. Contrast comparisons between the frequency of bacterial and fungal isolates for different age palms showed a significant difference with more bacterial and fungal endophytes in mature (≥11 years) followed by middle-aged (6-10 years) and young (1-5 years) palms from all the sampling areas. In comparing the soils, the frequency of endophytic bacteria recovered from symptomless palms was not significantly different between coastal and peat soils. However, all the inland soils had a significantly lower frequency of endophytic bacteria with high abundance of the genera Pseudomonas and Burkholderia. The fungal abundance was not significantly different between all the areas sampled. From the symptomatic palms, abundant bacteria were found in inland but lower in coastal and peat areas. The isolated bacteria were screened in vitro for their antagonistic properties towards G. boninense. Three bacterial endophytes (B3, P3 and S19) were established to have potential as biocontrol agents based on their percentage inhibition of radial growth (PIRG) of 70.80%, 75.95% and 55.63%, respectively. They were also effective host colonizers and could be re-isolated from the roots, 24 hours after inoculation. B3, P3 and S19 were identified by the Biolog® system as Burkholderia cepacia, Pseudomonas aeruginosa and Serratia marcescens respectively. Based on their colonization, establishment and localization in oil palm roots, B. cepacia (B3) and P. aeruginosa (P3) were selected for further in vivo screening for their efficacy in controlling BSR in oil palm seedlings in the glasshouse. When tested on oil palm seedlings inoculated with G. boninense, both of them, alone and in mixture suppressed G. boninense compared to the control. Not only did they delay the onset of BSR symptoms but also promoted the growth of the seedlings. Pre-inoculating the seedlings with P. aeruginosa had their BSR incidence reduced by 76.27%, B. cepacia reduced the disease incidence by 42.2% and the mixture of P. aeruginosa and B. cepacia by 54.24%. Based on disease reduction, the oil palm seedlings with P. aeruginosa were most effective in suppressing BSR as compared to other treatments.

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