80
H. Zhu et al.
research studies on the antiviral activities of cytochalasans have been carried out
in vitro, and there are no in vivo data available as yet. Moreover, the exact mechanisms involved in the antiviral activities of the cytochalasans are not yet understood.
The effects on HIV-1 protease, integrase, and CCR5 imply that there seems to be a
mechanism or mechanisms of action independent of microfilaments, but there is a
lack of in-depth research on this area and no definitive conclusions can be drawn.
Considering that some research has been conducted in cells infected with a virus,
it may be surmised that the microfilament-targeting effects of cytochalasans still
play an important role in their antiviral activities: they may affect microfilaments
of the host cells and then interfere with the process of the virus infecting the host
cells, and thereby inhibit the activity of virus-related enzymes and hence overall viral
replication.
3.6 Phytotoxic Effects and Ecological Role
Early studies demonstrated that several plant cells are susceptible to cytochalasan
treatment. The addition of cytochalasin B (110) to the pollen tubes of Lilium longiflorum resulted in immediate cessation of cytoplasmic streaming and eventually led to
inhibition of tip growth. Similar effects were observed in Caulerpa prolifera rhizoids
and Acetabularia mediterranea [1]. Then, researchers discovered that low-molecularweight compounds secreted from phytopathogenic fungi will weaken or kill the
plant, inclusive of cytochalasans. In particular, the genus Phoma includes several
pathogenic representatives, which are responsible for plant diseases with characteristic symptoms, such as lesions on leaves, stems, blossoms, and pods. Among other
secondary metabolites, cytochalasans have been proposed as mediators of virulence.
Several cytochalasans have been isolated from plant-pathogenic fungi. Cytochalasin A (109) and B (110) can be isolated from Phoma exigua var. exigua, the causative
agent of potato gangrene. Evidente and coworkers purified several cytochalasans
from a culture of Phoma exigua var. heteromorpha grown on wheat kernels and
determined their phytotoxic effects on germinating tomato seedlings [75]. Later on,
cytochalasins B (110), F (111), Z2 (115), Z3 (116), and desoxaphomin (104) were
found to be produced by Phoma exigua var. exigua isolated from Cirsium arvense
(Canada thistle or creeping thistle) and Sonchus arvensis (perennial sowthistle), and
their phytotoxic activity against both plants was investigated. The results suggested
that a carbocyclic or a lactone macrocycle fused to an unaltered perhydroisoindolyl
residue as well as a secondary hydroxy group at C-7 are essential to confer biological
activity [88].
Targeting the early stages of weed growth is a strategy of herbicidal development that may be applied to natural metabolites of microbial and plant origin.
Cytochalasins E (145) and B (110) were tested on Striga seed germination, and
the results showed only cytochalasin E (145) to inhibit Striga germination by 50%,
while cytochalasin B (110) had no inhibitory effects. The herbicidal effects mediated
by cytochalasins A (109) and B (110) in inducing necrosis in the tissues of weeds
H. Zhu et al.
research studies on the antiviral activities of cytochalasans have been carried out
in vitro, and there are no in vivo data available as yet. Moreover, the exact mechanisms involved in the antiviral activities of the cytochalasans are not yet understood.
The effects on HIV-1 protease, integrase, and CCR5 imply that there seems to be a
mechanism or mechanisms of action independent of microfilaments, but there is a
lack of in-depth research on this area and no definitive conclusions can be drawn.
Considering that some research has been conducted in cells infected with a virus,
it may be surmised that the microfilament-targeting effects of cytochalasans still
play an important role in their antiviral activities: they may affect microfilaments
of the host cells and then interfere with the process of the virus infecting the host
cells, and thereby inhibit the activity of virus-related enzymes and hence overall viral
replication.
3.6 Phytotoxic Effects and Ecological Role
Early studies demonstrated that several plant cells are susceptible to cytochalasan
treatment. The addition of cytochalasin B (110) to the pollen tubes of Lilium longiflorum resulted in immediate cessation of cytoplasmic streaming and eventually led to
inhibition of tip growth. Similar effects were observed in Caulerpa prolifera rhizoids
and Acetabularia mediterranea [1]. Then, researchers discovered that low-molecularweight compounds secreted from phytopathogenic fungi will weaken or kill the
plant, inclusive of cytochalasans. In particular, the genus Phoma includes several
pathogenic representatives, which are responsible for plant diseases with characteristic symptoms, such as lesions on leaves, stems, blossoms, and pods. Among other
secondary metabolites, cytochalasans have been proposed as mediators of virulence.
Several cytochalasans have been isolated from plant-pathogenic fungi. Cytochalasin A (109) and B (110) can be isolated from Phoma exigua var. exigua, the causative
agent of potato gangrene. Evidente and coworkers purified several cytochalasans
from a culture of Phoma exigua var. heteromorpha grown on wheat kernels and
determined their phytotoxic effects on germinating tomato seedlings [75]. Later on,
cytochalasins B (110), F (111), Z2 (115), Z3 (116), and desoxaphomin (104) were
found to be produced by Phoma exigua var. exigua isolated from Cirsium arvense
(Canada thistle or creeping thistle) and Sonchus arvensis (perennial sowthistle), and
their phytotoxic activity against both plants was investigated. The results suggested
that a carbocyclic or a lactone macrocycle fused to an unaltered perhydroisoindolyl
residue as well as a secondary hydroxy group at C-7 are essential to confer biological
activity [88].
Targeting the early stages of weed growth is a strategy of herbicidal development that may be applied to natural metabolites of microbial and plant origin.
Cytochalasins E (145) and B (110) were tested on Striga seed germination, and
the results showed only cytochalasin E (145) to inhibit Striga germination by 50%,
while cytochalasin B (110) had no inhibitory effects. The herbicidal effects mediated
by cytochalasins A (109) and B (110) in inducing necrosis in the tissues of weeds