Growth and physiological and biochemical responses of enset (Ensete ventricosum (Welw.) Cheesman) clones infected with bacterial wilt: Induction of resistance using medicinal plant leaf extract
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Date
2016-06
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Addis Ababa Universty
Abstract
Enset (Ensete ventricosum (Welw.) Cheesman) is a perennial staple food crop widely cultivated
in the south and southwestern Ethiopia. It supports the lives of approximately 20 million people.
However, its cultivation is impeded by enset bacterial wilt (EBW). Therefore, the aim of the
study was to evaluate the antibacterial activity of medicinal plant extracts and antibiotics against
Xanthomonas campestris pv. musacearum (Xcm); to study induction of resistance in enset by
leaf extract and evaluate its contribution in controlling enset bacterial wilt disease and to
investigate growth and metabolic responses of enset clones under different treatments.The
antibacterial activities of methanol leaf extracts of Achyranthes aspera, Agarista salicifolia,
Datura stramonium, Melia azedarach, Pycnostachys abyssinica and Vernonia amygdalina and
some antibiotics; amoxicillin, cephalexin, chloramphenicol, streptomycin sulphate and
tetracycline were evaluated in vitro. Qualitative analysis of leaf extracts was carried out using
standard methods. Total phenolics content was estimated with Folin Ciocalteau method. The
antibacterial activity of methanol leaf extracts was evaluated by disc diffusion method. Minimum
inhibitory concentration was determined using agar dilution method. The potential role of A.
salicifolia leaf extract to trigger induced resistance, metabolic changes in resistant and
susceptible enset clones and the subsequent suppression of EBW was evaluated. Growth, photosynthetic and biochemical responses of the two enset clones, Viz., resistant (Genticha) and
susceptible (Midasho) were evaluated 120, 270 and 360 days after treatment application. The
phytochemical screening results revealed the existence of alkaloids, phenols, terpenoids,
saponnins, tannins and glycosides in extract of A. salicifolia in abundant amount. Similar result
was observed in P.abyssinica extract. However, glycosides were absent. Moreover, A. salicifolia
leaf extract had the highest total phenolics content. It showed a 100, 85.7, 100, 61.2 and 12.3 %
increment from A. aspera, A. salicifolia, D. stramonium, M. azedarach, P. abyssinica and V.
amygdalina. The widest inhibition zone diameter was recorded in A. salicifolia extract followed
by P. abyssinica. A statistically significant difference in inhibition zone diameter was seen
between species and among serial concentrations within a species extract. The two species
extracts also showed the lowest minimum inhibitory (MIC) and minimum bactericidal
concentrations (MBC). Similarly, antibiotics showed antibacterial activity against Xcm.
However, the inhibition zone diameters of antibiotics were significantly higher fivefold times
than leaf extracts. The highest anti-Xcm activity was shown by streptomycin sulphate (50.3 mm
at 50 mg/mL) followed by amoxicillin (49.7 mm at 50 mg/mL). However, the lowest MIC (0.02
mg/mL) and MBC (0.049 mg/mL) values of antibiotics were recorded by tetracycline. In
addition, extract treated and Xcm inoculated plants of both clones had an increased activity of
phenylalanine ammonia lyase (PAL), peroxidase, polyphenol oxidase and greater amount of total
phenolics content (TPC) and total soluble sugars (TSS). Induced plants of the resistant clone
showed an 8, 4.6, 3, 13 and 4.6 fold increments in PAL activity compared to controls at 24, 48,
72, 96 and 120 h post treatment, respectively. Infected control plants on the other hand
demonstrated a 7, 3.6, 2.3, 10.6 and 2.4 fold increments compared to controls. Similarly, induced
plants of the susceptible clone exhibited 1.5, 1.7, 1.4, 4.2 and 4.4 fold increments in PAL activity compared to controls at 24, 48, 72, 96 and 120 h post treatment, respectively. While infected
control plants showed a 0, 1.3, 1.4, 3 and 2 fold increments in PAL activity compared to controls
at similar sampling time. The activity of tested enzymes and amounts of TPC and TSS were
significantly increased in plants of the resistant clone treated with leaf extract and Xcm as
compared to plants of the susceptible clone. The results of the study suggested that the difference
between the two clones was partially caused by variations in biochemical responses. Extract
induced resistance brought a 33.33 % and 3.1% disease incidence reduction in plants of the
susceptible and resistant clones, respectively, compared to infected controls. Plants of the two
enset clones also showed variations in growth and physiological responses among themselves
and between treatments at different measuring time periods. Nine months after treatment
application, induced plants of the resistant clone had significantly lower pseudostem (22.2%),
total leaf area (6.8%) and leaf area index (32%) than control plants. Similarly, infected control
plants of the susceptible clone showed lower pseudostem girth (29.4%), total leaf area (37.3%)
and leaf area index (37.3%) as compared to controls. Generally, the results suggest that extracts
of A. salicifolia and P. abyssinica have high potency and the potential for further formulation
development as biocides with broad spectrum activity. Tetracycline with the lowest MIC and
MBC values is also a potential alternative in controlling EBW. Moreover, the results also suggest
the feasibility of induced resistance to control EBW. However, further research should be
conducted to formulate the extracts, identify the active compounds responsible for the
antibacterial activity of extracts, and evaluate the performance of other application methods
under field conditions. Keywords: Agarista salicifolia, Antibiotics, Assimilation rate, Inhibition zone, Ensete
ventricosum, Induced resistance, methanol extracts, phenylalanine ammonia lyase, Total
phenolic compounds. Xanthomonas campestris pv. musacearum
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Keywords
Agarista salicifolia, Antibiotics, Assimilation rate, Inhibition zone, Ensete ventricosum,, Induced resistance, Methanol extracts, Phenylalanine ammonia lyase, Total phenolic compounds. Xanthomonas campestris pv. musacearum