Abutilon theophrasti’s Defense Against the AllelochemicalBenzoxazolin-2(3H)-One: Support by Actinomucor elegans

Abutilon theophrasti Medik., previously found tobe rather insensitive to benzoxazinoid containing rye mulchand the allelochemical benzoxazolin-2(3H)-one (BOA), canbe associated with the zygomycete Actinomucor elegans,whereby the fungus colonizes the root relatively superficiallyand mainly in the maturation zone. The fungus mitigatesnecrosis of the cotyledons when seedlings are incubated with2 mMBOA, in contrast to those that lack the fungus. In liquidcultures of the fungus, tryptophan was identified. The accumulationof tryptophan is increased in presence of BOA. Thisamino acid seems to be important in protecting Abutilonagainst BOA and its derivatives since it suppressed the accumulationof BOA derived, highly toxic 2-aminophen-oxazin-3-one (APO) in the medium and on the root surface duringBOA incubations of Abutilon seedlings. Although A. elegansis insensitive to BOA and APO, the fungus is not able toprotect the plant against harmful effects of APO, when seedlingsare treated with the compound. Abutilon can detoxifyBOA via BOA-6-OH glucosylation probably by a cell wallassociated glucosyltransferase, but only low amounts of theproduct accumulate. Low tryptophan concentrations can contributeto a degradation of the toxic intermediate BOA-6-OHby Fenton reactions, whereby the amino acid is oxidized. Oneof the oxidation products was identified as 4(1H)-quinolinone,which is the core substructure of the quorum sensing molecule2-heptyl-3-hydroxy-4-quinolone. The mutualistic associationof Abutilon theophrasti with Actinomucor elegans is consideredas opportunistic and facultative. Such plant-fungus associationsdepend rather likely on environmental conditions,such as the mode of fertilization