Protective andCurative Activity of the Synergism of Knifefish (Chitala ornata) Skin and Banaba (Lagerstroemia speciosa) Bark Extracts against Rice Bacterial Blight (Xanthomonas oryzae pv. oryzae)

Abstract

INTRODUCTION

The most serious disease in rice is bacterial blight, caused by Xanthomonas oryzae pv. oryzae (Xoo). Affected fields lose 80% of grains, but 100% loss is common. As treatments are toxic, studies suggest antimicrobial peptides (AMP) as pesticides. Some fish possess broad-spectrum AMP but none have been found in Chitala ornata Knifefish, yet. Quorum Sensing (QS) is a weakness of AMPs, though. Thus, Banaba (Lagerstroemia speciosa) was chosen as it inhibits AHL-based QS, a QS similar to that of Xoo.

METHODS

Knifefish skin was homogenized, centrifuged and purified (Agapito 2014). Banaba bark was macerated and purified (Nautiyal 2012). Antibacterial synergism was tested using ratios 1:0, 1:1, 0:1 of knife fish and banaba extracts via well-diffusion (Cui 2016). Only the significantly higher 1:1 ratio was used further. Concentration-dependent antibacterial activity was tested using 25%, 50%, 75% concentrations of the mixture via well-diffusion, and foliar application pre-(protective activity) or post-inoculation (curative activity) (Shi et al. 2011). MIC was determined via broth dilution (Rahman et al. 2014). Swarming Inhibition test was done using 25%MIC, 50%MIC, 75%MIC based on Cho(2013).

RESULTS

Synergistic antibacterial activity was found -higher inhibition zones (IZ) in a 1:1 ratio (mean=40mm), with 163% and 78% increase for the activities of knife fish skin and banaba bark extracts, individually (p<0.003). The activity of the 1:1 ratio is concentration-dependent (p<0.003), with 57mm mean IZ at 75% concentration, while 3.33mm for Ampicillin. The MIC of the 1:1 ratio is 52Âμg/mL. Swarming was also inhibited, with lower colony radii at higher concentrations (p<0.003). The mean radius in 75%MIC was 10mm, while 58.67mm in water. In the protective and curative assays, shorter lesions were observed at higher concentrations, on the 7th and 18th day (p<0.003). The lesions in the protective assay were shorter than in the curative assay.

DISCUSSIONS

The observed antibacterial activity is likely due to AMP, in knife fish, and phytochemicals, for banaba (Nautiyal 2012). Their synergistic and concentration-dependent antibacterial activity is probably by complementing modes of action (Rakers 2013). As the extracts inhibit QS-based swarming of Xoo, resistance development is unlikely. The results of the protective and curative assay imply that the treatment can be a pesticide (curative agent) or immunostimulant (a protective agent), though further studies on toxicity and mechanisms are needed.