PHD-Department of Chemistry

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Browsing PHD-Department of Chemistry by Subject "Allelochemicals"

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    Identification and Mechanisms of Allelochemicals Regulating Root-Knot Nematode Parasitism
    (Kenyatta University, 2023-10) Kihika, Ruth Muthoni; Margaret M. Ng'ang'a; David P. Tchouassi; Baldwyn Torto
    Global crop production is hampered by numerous pests and parasites including plant parasitic nematodes (PPNs). Root-knot nematodes (RKNs; Meloidogyne spp.) are among the most pervasive economically important PPNs accounting for crop losses of over USD 100 billion globally, thus posing a severe risk to food security. Despite the current measures deployed in the management of RKNs, their damage still persists, necessitating more effective approaches. Recent studies identified constitutive plant compounds that influence the host seeking behavior of RKNs. The current study sought to elucidate the chemical dialogue between host plants and RKNs during parasitism and investigate the chemical mechanisms of non-host plants in reducing nematode infestation. The influence of the infective stage juveniles (J2) of RKNs in inducing tomato (Solanum lycopersicum) root and leaf volatiles, and chemotactic effects on conspecifics was investigated. The hypothesis that the non-host Asteraceae plant vegetable black-jack (Bidens pilosa) suppresses infection of the PPN Meloidogyne incognita, in two susceptible Solanaceae host plants; tomato (Solanum lycopersicum) and black nightshade (S. nigrum) was also tested. In behavioral olfactometer assays, J2 avoided roots of 2-day infected plants but preferred 7-day infected tomato compared to healthy plants. Coupled gas chromatography-mass spectrometry (GC/MS) was used to identify root and leaf volatiles from healthy and RKN-infected tomato at two and seven-days post infection. Chemical analysis showed a two- to seven-fold increase in the amounts of monoterpenes emitted from tomato roots infected with M. javanica relative to healthy roots. Additionally, infected plants released ~4 to 225-fold quantitatively more leaf volatiles, and were compositionally richer than the healthy plants. Analysis of similarities (ANOSIM) and non-metric multidimensional scaling (NMDS) of the leaf volatile organic compounds (VOCs) showed that five terpenes, 2-δ-carene (25), β-phellandrene (29), δ- caryophyllene (13), elemene (58), and α- humulene (66) contributed to the dissimilarity trends between healthy and infected tomato irrespective of the nematode species. In further bioassays, the monoterpenes β-pinene (24), 2-δ-carene (25), α-phellandrene (26), and β-phellandrene (29) differentially attracted (51-87%) J2 relative to control. Concurrent reduction and increase in the levels of methyl salicylate (3) and (Z)-methyl dihydrojasmonate (45), respectively, in the root volatiles reduced J2 responses. Screenhouse pot experiments combined with laboratory in vitro hatching and mortality assays and chemical analysis to test the effect of non-host plant in growth and development of RKNs in susceptible host plants were used. In intercrop and drip pot experiments, blackjack significantly reduced the number of galls and egg masses in RKN- susceptible host plants by 3-9-fold compared to controls (susceptible plants). LC-QQQ-MS analysis of the most bioactive fraction from the root exudates of blackjack identified several classes of compounds, including aromatic acids, a dicarboxylic acid, vitamins, amino acids, and a flavonoid. In in vitro assays, the vitamins, ascorbic acid (86) and nicotinic acid (88) and the aromatic acids, p-coumaric acid (92) and 2-hydroxybenzoic acid (13) caused the highest inhibition in egg hatching, whereas ascorbic acid (86) (vitamin) and 2-hydroxybenzoic acid (13) (aromatic acid) elicited strong nematicidal activity against M. incognita, with LC50/48 h values of 12 and 300 ng µl-1, respectively. These results demonstrate that RKN infection induces chemical changes both locally and systemically in the host plant. Additionally, the host plant can alter its root volatile composition to inhibit PPN attack. The observed plant-produced inhibition of J2 warrants further investigation as a potential management tool for growers. The findings also provide insights into how certain non-host plants can be used as companion crops to disrupt PPN infestation.