vestitol and sativan (Bonde et al. 1973 ), but
additional isoflavonoid compounds and some
derived glycosides are also thought to be pro-
duced. Natural variation in isoflavonoid compo-
sition seems to exist within theL. japonicus
species, for instance, we have identified isosativan
as a prominent compound in the MG74 accession.
InL. japonicus, vestitol is the isoflavonoid that has
been investigated the most extensively in terms of
its biosynthesis and its physiological role in the
defense against microbial pathogens. Inoculation
of lotus roots withCalonectoria ilicola, a fungus
pathogenic to soybean but not to lotus, induced the
expression of key genes in vestitol biosynthesis
and led to vestitol exudation from the roots
(Masunaka et al. 2011 ). In contrast, inoculation
withTrichoderma koningi, a plant growth pro-
moting fungus able to colonize the roots of
L. japonicus, only led to brief transient induction
of gene expression. Inoculation with the symbiotic
bacteriumMesorhizobium lotidid not upregulate
isoflavonoid biosynthetic gene expression or
vestitol levels in root exudates. Vestitol also has a
defense role in plant–plant interactions as its
biosynthesis was highly upregulated in lotus roots
challenged with the incompatible parasitic plant
Striga hermonthica(Ueda and Sugimoto 2010 ).
Flavonoids and isoflavonoids not only func-
tion as antimicrobial defense compounds but also
as plant-derived signaling molecules that induce
the expression of nodulation genes in symbiotic
Rhizobiumspecies and are as such an important
early factor in the successful establishment of
symbiosis (reviewed in Hassan and Mathesius
2012 ). The specificity for which plant compound
acts as an inducer of nod gene expression
depends on the bacterial strain. This was dem-
onstrated by Kosslak et al. ( 1987 ) who monitored
nodgene expression by measuring theβ-galac-
tosidase activity resulting from anodABC-lacZ
translational fusion construct. Using the response
of Bradyrhizobium japonicum, the original
source of thenodABCoperon, twonodgene-
inducing compounds present in soybean root
extracts were identified as the isoflavones
daidzein and genistein (Kosslak et al. 1987 ).
Following the transfer of thenodABC-lacZcon-
struct into Rhizobium trifolii, it showed no
induction by soybean root extracts or individual
isoflavones, but the construct could be induced
byflavones able to inducedR. trifolii nodgenes,
such as 4′,7-dihydroxyflavone, apigenin, and
luteolin. Surprisingly, for a model system to
study symbiosis, the identity of theflavonoid or
non-flavonoid compounds produced byLotus
species that induce nod factor production in its
symbiotic partnerMesorhizobium lotihave not
been identified, but the phytoalexins vestitol and
sativan seem not to be involved (Cooper 2007 ;
Rispail et al. 2010 ).Fig. 14.2 Early steps in
the isoflavonoid
biosynthesis pathway
branching off from general
flavonoid metabolism.
Enzymatic steps are
described in the text
14 Plant-Specialized Metabolism and Its Genomic Organization... 153