3.1
Seed Germination and Searching for a Host Plant
The life cycle of Cuscuta, as in other angiosperms, begins with seed germination.
Germinating Cuscuta seedlings depends on limited seed reserves; they are unable to
survive alone for a long time and must find an appropriate host plant stem within a few
days [29]. Cuscuta seedlings normally live less than 3 weeks before becoming parasitic.
Seed dormancy is an important feature of C. campestris that ensures its survival
as a parasite of crops [30]. There are three different types of seed dormancy
(morphological, physical, and physiological), at least two of which have evolved
on several separate occasions [31]. Dormancy of C. campestris occurs owing to its
hard seed coat [32]. The percentage of hard seeds at dispersal varies among
C. campestris [33] and C. chinensis plants [34]. Dormancy can be broken by the
activity of soil microorganisms or by tillage, causing scarification of seed coat [35],
etc. The dynamics of germination of C. campestris depends on a double mechanism
of dormancy. After a period of primary dormancy (additional maturation caused by
coat impermeability), the seed goes into an annual cycle of secondary dormancy. In
C. campestris, secondary dormancy occurs at the end of summer, and it prevents
germination during the following autumn and winter in order to avoid the season in
which potential hosts of the temperate region would be scarce due to low temperatures. Secondary dormancy ends at the end of winter when temperature begins to grow
and overall conditions for germination and growth of host plants improve [25]. Physical dormancy has been reported for seeds of several Cuscuta species: C. campestris
[25, 30], C. trifolii [36], C. monogyna and C. planiflora [37], C. chinensis [34],
C. gronovii, C. umbrosa, C. epithymum, and C. epilinum [38]. However, it is not
common for Cuscuta pedicellata [39] because seeds of that species are readily water
permeable due to a specific structure of their epidermis and endosperm.
To find and catch potential hosts, Cuscuta plants recognize plant volatiles as
chemoattractants which guide seedling growth and increase the chances of successful establishment of a connection [29]. However, expert options vary as what is the
necessary impulse for germination of field dodder seeds. Some researchers [40, 41]
believe that Cuscuta spp. do not require host-root exudates to stimulate germination,
similar to some important holoparasitic weeds of the genus Orobanche and some
hemiparasitic weeds in the genus Striga. Field dodder as a stem parasite is strongly
impacted by light signals, which stimulate germination of its seeds [42–44]. Field
dodder seedlings tend to grow in the direction of light source, primarily red/far-red
light, which help them find hosts, while far-red and blue light have a significant role
in prehaustorium formation. Recognition of a host occurs through phototropic
mechanisms, and some authors claim that chemotropism (movement induced by
chemical stimulus) and thigmotropism (movement induced by mechanical stimulus,
i.e., by touch) have equally important roles in host recognition process [45]. Mechanical stimulus, following initial contact with the host plant, induces cell differentiation
and haustorium formation, and its subsequent penetration into the host stem. This is
facilitated by the recruitment of stress-responsive and defense genes for host recognition and activity of cell wall-modifying enzymes [46–48]. Runyon et al. [29] found
that volatile chemical substances were also important for movement of Cuscuta
5 Field Dodder: Life Cycle and Interaction with Host Plants
105
Seed Germination and Searching for a Host Plant
The life cycle of Cuscuta, as in other angiosperms, begins with seed germination.
Germinating Cuscuta seedlings depends on limited seed reserves; they are unable to
survive alone for a long time and must find an appropriate host plant stem within a few
days [29]. Cuscuta seedlings normally live less than 3 weeks before becoming parasitic.
Seed dormancy is an important feature of C. campestris that ensures its survival
as a parasite of crops [30]. There are three different types of seed dormancy
(morphological, physical, and physiological), at least two of which have evolved
on several separate occasions [31]. Dormancy of C. campestris occurs owing to its
hard seed coat [32]. The percentage of hard seeds at dispersal varies among
C. campestris [33] and C. chinensis plants [34]. Dormancy can be broken by the
activity of soil microorganisms or by tillage, causing scarification of seed coat [35],
etc. The dynamics of germination of C. campestris depends on a double mechanism
of dormancy. After a period of primary dormancy (additional maturation caused by
coat impermeability), the seed goes into an annual cycle of secondary dormancy. In
C. campestris, secondary dormancy occurs at the end of summer, and it prevents
germination during the following autumn and winter in order to avoid the season in
which potential hosts of the temperate region would be scarce due to low temperatures. Secondary dormancy ends at the end of winter when temperature begins to grow
and overall conditions for germination and growth of host plants improve [25]. Physical dormancy has been reported for seeds of several Cuscuta species: C. campestris
[25, 30], C. trifolii [36], C. monogyna and C. planiflora [37], C. chinensis [34],
C. gronovii, C. umbrosa, C. epithymum, and C. epilinum [38]. However, it is not
common for Cuscuta pedicellata [39] because seeds of that species are readily water
permeable due to a specific structure of their epidermis and endosperm.
To find and catch potential hosts, Cuscuta plants recognize plant volatiles as
chemoattractants which guide seedling growth and increase the chances of successful establishment of a connection [29]. However, expert options vary as what is the
necessary impulse for germination of field dodder seeds. Some researchers [40, 41]
believe that Cuscuta spp. do not require host-root exudates to stimulate germination,
similar to some important holoparasitic weeds of the genus Orobanche and some
hemiparasitic weeds in the genus Striga. Field dodder as a stem parasite is strongly
impacted by light signals, which stimulate germination of its seeds [42–44]. Field
dodder seedlings tend to grow in the direction of light source, primarily red/far-red
light, which help them find hosts, while far-red and blue light have a significant role
in prehaustorium formation. Recognition of a host occurs through phototropic
mechanisms, and some authors claim that chemotropism (movement induced by
chemical stimulus) and thigmotropism (movement induced by mechanical stimulus,
i.e., by touch) have equally important roles in host recognition process [45]. Mechanical stimulus, following initial contact with the host plant, induces cell differentiation
and haustorium formation, and its subsequent penetration into the host stem. This is
facilitated by the recruitment of stress-responsive and defense genes for host recognition and activity of cell wall-modifying enzymes [46–48]. Runyon et al. [29] found
that volatile chemical substances were also important for movement of Cuscuta
5 Field Dodder: Life Cycle and Interaction with Host Plants
105