In Jacobaea species, all PAs except senecivernine are derived from senecionine
N-oxide; senecionine N-oxide is synthesized in the roots, transported to the shoots
via the phloem, and diversified into other PA structures in the shoots [28, 31].
Aside from structural diversification, PAs do not undergo any turnover or degradation [32].The diversity from senecionine N-oxide to other PAs comprises simple
one-step or two-step reactions such as hydroxylations, epoxidations, dehydrogenations, and O-acetylations, as well as the more complex conversion of the
retronecine into the otonecine base moiety [32]. The first specific compound of
PA biosynthesis was identified as homospermidine, which is turned into a basic PA
molecule with the enzyme homospermidine synthase (HSS) [33]. It was shown that
the HSS-encoding gene originated by gene duplication [34], independently in
unrelated angiosperm families [35]. The enzymes responsible for the PA diversification are not identified yet. It suggested that the genes encoding PA pathwayspecific enzymes are regulated by a transient switch-off and switch-on mechanism
rather than gain and loss, since PA distribution appears to be largely incidental in
Senecio species [30]. Structures of PA detected in the Jacobaea hybrid system
used in this study and a schematic diagram representing putative PA biosynthetic
pathways are shown in Figs. 1 and 2.
PA accumulation in a particular tissue is caused by a number of interacting
processes: (i) synthesis of senecionine N-oxide in roots, (ii) continuous longdistance translocation of senecionine N-oxide into shoots, (iii) differential
senecionine N-oxide transformations in different plant organs, (iv) continuous
allocation of PAs in the plant, and (v) tissue selective vacuolar storage of PAs
[reviewed by 32]. In Jacobaea erucifolia (syn. Senecio erucifolius), a closely
related species of J. vulgaris, PA biosynthesis occurs mainly in the root apex and
thus coincides with the site of active root growth [37]. This coincides with the
finding that in young J. vulgaris plants, the total PA amount in plants was
positively correlated to root biomass but negatively correlated to shoot to root
ratio, which suggested that PAs are produced by roots at a root-biomass-dependent
rate, and the greater the shoot to root ratio, the greater the overall dilution of
alkaloids [36]
3
Leaf-Tissue PA Variation in the Jacobaea and Senecio
Plants
3.1
Interspecies Variation
Large variations of PA profiles were found among Senecio species [24]. PA profiles
are species-specific [32]. For instance, jacobine-like PAs are very rich in some plants
of J. vulgaris, and erucifoline-like PAs dominate in J. erucifolia. However, PA
profiles do not represent the phylogenetic relationships between the Senecio and
Jacobaea species [24, 30]. The difference between these findings on the evolutionary compared to an ecological time scale indicates that PAs probably are under
selection and that it is easy to change the PA profile of plants.
252
D. Cheng
N-oxide; senecionine N-oxide is synthesized in the roots, transported to the shoots
via the phloem, and diversified into other PA structures in the shoots [28, 31].
Aside from structural diversification, PAs do not undergo any turnover or degradation [32].The diversity from senecionine N-oxide to other PAs comprises simple
one-step or two-step reactions such as hydroxylations, epoxidations, dehydrogenations, and O-acetylations, as well as the more complex conversion of the
retronecine into the otonecine base moiety [32]. The first specific compound of
PA biosynthesis was identified as homospermidine, which is turned into a basic PA
molecule with the enzyme homospermidine synthase (HSS) [33]. It was shown that
the HSS-encoding gene originated by gene duplication [34], independently in
unrelated angiosperm families [35]. The enzymes responsible for the PA diversification are not identified yet. It suggested that the genes encoding PA pathwayspecific enzymes are regulated by a transient switch-off and switch-on mechanism
rather than gain and loss, since PA distribution appears to be largely incidental in
Senecio species [30]. Structures of PA detected in the Jacobaea hybrid system
used in this study and a schematic diagram representing putative PA biosynthetic
pathways are shown in Figs. 1 and 2.
PA accumulation in a particular tissue is caused by a number of interacting
processes: (i) synthesis of senecionine N-oxide in roots, (ii) continuous longdistance translocation of senecionine N-oxide into shoots, (iii) differential
senecionine N-oxide transformations in different plant organs, (iv) continuous
allocation of PAs in the plant, and (v) tissue selective vacuolar storage of PAs
[reviewed by 32]. In Jacobaea erucifolia (syn. Senecio erucifolius), a closely
related species of J. vulgaris, PA biosynthesis occurs mainly in the root apex and
thus coincides with the site of active root growth [37]. This coincides with the
finding that in young J. vulgaris plants, the total PA amount in plants was
positively correlated to root biomass but negatively correlated to shoot to root
ratio, which suggested that PAs are produced by roots at a root-biomass-dependent
rate, and the greater the shoot to root ratio, the greater the overall dilution of
alkaloids [36]
3
Leaf-Tissue PA Variation in the Jacobaea and Senecio
Plants
3.1
Interspecies Variation
Large variations of PA profiles were found among Senecio species [24]. PA profiles
are species-specific [32]. For instance, jacobine-like PAs are very rich in some plants
of J. vulgaris, and erucifoline-like PAs dominate in J. erucifolia. However, PA
profiles do not represent the phylogenetic relationships between the Senecio and
Jacobaea species [24, 30]. The difference between these findings on the evolutionary compared to an ecological time scale indicates that PAs probably are under
selection and that it is easy to change the PA profile of plants.
252
D. Cheng