already known that they are regulated by phytohormones (i.e., jasmonic acid) and
triggered by specific signals which are further converted to large-scale biochemical and physiological changes, promoted by downstream signal events. Some
mechanisms previously reported involve membrane depolarization, ion flux, activation of mitogen-activated protein kinase (MAPK), changes in gene expression,
alteration of the plant proteome, and finally, production of specific specialized
metabolites [5, 29].
Dry leaves of Prosopis juliflora mainly present phenolic-derivatives [30] and
alkaloids, which were reported for the first time in the literature, being named after
the species, and are known as: juliflorine [31], julifloricine [31], julifloridine [32],
juliprosinene [33], juliprosine [34], juliprosopine [32], and mesquitol [35].
Previous research have attributed the allelopathy of P. juliflora to phenolic
compounds [36] as well as to aqueous extracts of P. juliflora leaves, suggesting
that the foliage species may contain water-soluble allelochemicals. These metabolites may be released into its vicinity by rain water from the leaves even before
hitting the ground [37]. These lixiviated chemicals present an inhibitory growth
effect on many plant parts and were isolated, evaluated, and identified as syringin,
(-)-lariciresinol, L-tryptophan, juliprosopine, juliprosine, and juliprosopinal, in which,
juliprosine and its derivatives demonstrated the most pronounced allelopathic activity (Fig. 1) [34, 38–40].
The metabolites from Prosopis juliflora with allelopathic properties result from
two major biosynthetic pathways (Fig. 2): shikimic acid metabolites [(-)-lariciresinol
for instance, is a lignoid (i.e., phenylpropanoids)] and piperidine alkaloids [secojuliprosopinal, derived from the acetic acid or polyketide metabolic pathway through
the lysine amino acid pathway] [41].
The composition of individual metabolites and their concentration are not static
but differs from organ to organ, not only within the developmental cycle of a plant
but also within and between populations [42]. This variation, which leads to
complex mixtures of secondary metabolites, is probably a strategy against the
selection of specialized herbivores or pathogens as well as other plants [43].
2.1.1 Prosopis juliflora Alkaloids
In Brazil, P. juliflora is widespread in the Caatinga biome, providing shelter, helping
to reduce soil erosion and improving the microclimate of these regions. In addition,
this plant has an important use for humans and animals, as a source of food, feed,
fuel, medicines, and cosmetics [24, 44]. However, the allelopathic potential of this
species is shown by inhibiting the germination or growth of other plants around it,
monopolizing spaces and nutrients [24]. This allelopathic potential might be caused
either by fallen leaves or plant leachates or root exudates, through the release of
allelochemicals into the environment [45, 46]. It is known that these allelochemicals
can induce toxicity to other biotas [47–49]. These toxic effects are commonly
reported in animals with free access to P. juliflora as a food source, and this toxicity
has been attributed to the presence of alkaloids in the algaroba pods ingested by
animals [50–52].
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G. A. de Brito Damasceno et al.
triggered by specific signals which are further converted to large-scale biochemical and physiological changes, promoted by downstream signal events. Some
mechanisms previously reported involve membrane depolarization, ion flux, activation of mitogen-activated protein kinase (MAPK), changes in gene expression,
alteration of the plant proteome, and finally, production of specific specialized
metabolites [5, 29].
Dry leaves of Prosopis juliflora mainly present phenolic-derivatives [30] and
alkaloids, which were reported for the first time in the literature, being named after
the species, and are known as: juliflorine [31], julifloricine [31], julifloridine [32],
juliprosinene [33], juliprosine [34], juliprosopine [32], and mesquitol [35].
Previous research have attributed the allelopathy of P. juliflora to phenolic
compounds [36] as well as to aqueous extracts of P. juliflora leaves, suggesting
that the foliage species may contain water-soluble allelochemicals. These metabolites may be released into its vicinity by rain water from the leaves even before
hitting the ground [37]. These lixiviated chemicals present an inhibitory growth
effect on many plant parts and were isolated, evaluated, and identified as syringin,
(-)-lariciresinol, L-tryptophan, juliprosopine, juliprosine, and juliprosopinal, in which,
juliprosine and its derivatives demonstrated the most pronounced allelopathic activity (Fig. 1) [34, 38–40].
The metabolites from Prosopis juliflora with allelopathic properties result from
two major biosynthetic pathways (Fig. 2): shikimic acid metabolites [(-)-lariciresinol
for instance, is a lignoid (i.e., phenylpropanoids)] and piperidine alkaloids [secojuliprosopinal, derived from the acetic acid or polyketide metabolic pathway through
the lysine amino acid pathway] [41].
The composition of individual metabolites and their concentration are not static
but differs from organ to organ, not only within the developmental cycle of a plant
but also within and between populations [42]. This variation, which leads to
complex mixtures of secondary metabolites, is probably a strategy against the
selection of specialized herbivores or pathogens as well as other plants [43].
2.1.1 Prosopis juliflora Alkaloids
In Brazil, P. juliflora is widespread in the Caatinga biome, providing shelter, helping
to reduce soil erosion and improving the microclimate of these regions. In addition,
this plant has an important use for humans and animals, as a source of food, feed,
fuel, medicines, and cosmetics [24, 44]. However, the allelopathic potential of this
species is shown by inhibiting the germination or growth of other plants around it,
monopolizing spaces and nutrients [24]. This allelopathic potential might be caused
either by fallen leaves or plant leachates or root exudates, through the release of
allelochemicals into the environment [45, 46]. It is known that these allelochemicals
can induce toxicity to other biotas [47–49]. These toxic effects are commonly
reported in animals with free access to P. juliflora as a food source, and this toxicity
has been attributed to the presence of alkaloids in the algaroba pods ingested by
animals [50–52].
524
G. A. de Brito Damasceno et al.