some of the evolutionary dynamics resulting from
the interaction between a plant species and its
biotic environment. Gene duplication is highly
relevant to the evolution of metabolic diversity as
it is often followed by functional divergence, or
alternatively the formation of pseudogenes. As a
result, clusters of functional and non-functional
versions of a specific biosynthetic gene are com-
monly observed in plant genomes, particular in
plant-specialized metabolism. However, more
remarkable is the organization of several of these
metabolic pathways in gene clusters consisting of
non-homologous genes encoding different bio-
synthetic enzymes of the same pathway. Here, we
describe three such biosynthetic gene clusters
present in the genome ofL. japonicusand discuss
the evolutionary mechanism responsible for their
formation.
14.2 Proanthocyanidins
Proanthocyanidins (PAs), also known as con-
densed tannins, are polyphenolic compounds
synthesized by a branch of the phenylpropanoid
pathway that also produces anthocyanins and
flavonols. They have a polymeric structure con-
sisting offlavan-3-ol units linked in various ways.
PAs occur in a wide range of plants where their
primary role is defense against herbivores by being
toxic to insects and by decreasing protein digestion
in vertebrates (Barbehenn and Constabel 2011 ).
There has been significant interest in manipulation
of PA levels in forage legume crop species as their
ability to precipitate protein reduces pasture bloat
and improves animal productivity (Patra and
Saxena 2011 ). PAs are also being studied because
of the proposed role of polyphenols in the pre-
vention of cardiovascular disease when included
in the human diet (Quinones et al. 2013 ). Meta-
bolic engineering of PA content in various crop
plants through transgenic approaches requires a
detailed understanding of the PA biosynthetic
genes and their regulation, and considerable pro-
gress has been made in this area (Bogs et al. 2007 ;
Dixon et al. 2013 ).
Leaf PA content of 31 Lotus species and
accessions was determined by Gruber et al. ( 2008 )
and varied from undetectable in seven species
(includingL. japonicusGifu B129) to very high in
L. unifoliolatus(4.1 % FW). InL. japonicus,PAs
accumulate infloral organs, seeds, and stems, but
not in leaves (Skadhauge et al. 1997 ). There was
no relationship between PA content and mor-
phological traits or geographic location. A wide
variation in chemical composition, mean degree
of polymerization (mDP), and degree of unifor-
mity of PAs was found in twelveLotusspp. by
Sivakumaran et al. ( 2006 ). The mDP varied from
8 to 97 units, although most species had a mDP of
less than 20, with only a high mDP found in
L. pedunculatusandL. americanus. There was
considerable variability in the hydroxylation pat-
tern of the B-ring of theflavan-3-ol PA polymer
extension units. The dihydroxylated procyanidin
(PC) units or trihydroxylated prodelphinidin (PD)
units were predominant in some species and
approximately equal in others. Regarding the
stereochemistry at the C-ring, in all species, the
2,3-cisisomers epicatechin and epigallocatechin
were predominant as extension units, while the
2,3-transisomer catechin was the typical terminal
unit of the polymer.Fig. 14.1 Example compounds of classes of plant-
specialized metabolites present inL. japonicus. The
polymeric proanthocyanidins (a schematic structure is
drawn), the isoflavonoid vestitol, the triterpenoid lupeol,
and the cyanogenicα-hydroxynitrile glucoside lotaustralin
and the non-cyanogenicγ-hydroxynitrile glucoside rhod-
iocyanoside A
150 A.M. Takos and F. Rook