The role of flavonoids in the interaction between the plant, Medicago truncatula and the nematode, Meloidogyne javanica
Date
2019
Authors
Chin, Sabrina
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Abstract
Flavonoids are plant secondary metabolites which are common in all vascular plants. They play numerous functions in plants, ranging from UV protection, pigments, signalling molecules, defence to auxin transport inhibition for plant development. Flavonoids are potentially manipulated by the root-knot nematodes, Meloidogyne spp. to hijack multiple plant functions. This is especially of interest in the study of Meloidogyne parasitism because these nematodes have an extraordinarily broad host range (>2000 plant species), thus indicating their mastery of basal and essential plant machineries. Flavonoids may mediate the plant-nematode interactions by acting as defence compounds, signalling molecules in the rhizosphere and as auxin transport modulators in nematode feeding site (gall) development. To test these hypotheses, flavonoids in wild-type Medicago truncatula cv. 2HA were systematically profiled using tandem mass spectrometry across seven time points which represented different stages of nematode parasitism. The results revealed that flavonoid induction began at 24 hours, with upstream flavonoids pathways such as the chalcones and flavanones and specific flavonols eg. kaempferol glycosides and the flavone, 7,4’-DHF were dramatically increased. In contrast, a different pattern of flavonoid accumulation occurred at 4 weeks, where in addition to the chalcones and flavanones, isoflavonoids and pterocarpans accumulated at high levels, suggesting their role in nematode defence. Subsequently, involvement of isoflavonoids and pterocarpans in defence were tested by observing nematode infection in 7 different genotypes of M. truncatula with flavonoid variations. The genotypes included naturally occurring accessions, F83005 and DZA045, bred cultivars, 2HA and A17 and transgenics with silenced isoflavone synthase (IFS) genes and over-expressing IFS gene. Plants that accumulated high levels of the pterocarpan, medicarpin either early in infection, eg. IFS over-expression line, or extremely high levels of medicarpin eg. F83005 exhibited reduced nematode egg numbers. In addition, plants over-expressing IFS also showed better infection outcome, with minimal biomass loss due to infection, less galls and smaller galls. These results were complemented with in vitro chemotaxis and motility assays, which revealed that the medicarpin and afromosin were effective nematode repellent and motility inhibitor respectively. Lastly, the roles of isoflavonoids in auxin transport inhibition in the gall were elucidated using M. truncatula IFS transgenics and wild-type 2HA. Isoflavonoids were found to be unlikely involved in auxin transport inhibition in the gall as the over-production of isoflavonoids resulted in delayed auxin transport inhibition, whereas the under-production of isoflavonoids resulted in strong and early auxin transport inhibition. Conclusively, medicarpin and afromosin played specific roles in protecting the plant against the nematodes, and isoflavonoid pathway modification showed that isoflavonoids were unlikely auxin transport inhibitors in gall development.
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flavonoids, gall development, nematodes, Meloidogyne javanica, Medicago truncatula, plant defense, plant defence, auxin transport, galls
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