structures known as hyphae. In a compatible host, the hyphae searching for vascular
tissue are able to expand from 800 to 2000 μm [1, 48], and their inter- and
intracellular expansion into the host tissue depends on the mechanical as well as
enzymatic processes [1]. These parasitic cells have been described as having ambivalent characters, functioning as both sieve elements and transfer cells [59, 63]. Interestingly, during this process, chimeric cell walls of host and parasite constituents are
formed, and interspecific plasmodesmata build up a cytoplasmic syncytium between
Cuscuta and its host plant [48, 64, 65]. To form a connection to the xylem, parasitic
and host cells of the xylem parenchyma commence a synchronized development,
fusing to build a continuous xylem tube from the host to the parasite [66]. With
functional connections to the xylem and phloem of its host, the parasitic plant is
supplied with water, nutrients, and carbohydrates [50, 58, 67].
4
Consequences of Field Dodder and Host Interaction
4.1
Impact on Host-Parasite Metabolites
After the establishment of a connection between host and parasite, the development
of the parasite is based on the exchange of nutrients. In the process of establishing
parasitic connections to its host, dodder uses a battery of hydrolytic enzymes,
primarily cell wall-modifying glycosyl hydrolases [68], which have been observed
directly through their activities [69] or indirectly through their structural
consequences during host-tissue invasion [48]. Further, dodder appears to induce
hydrolytic activities within its host [69, 70].
Transfer of fluids from the host to the parasitic plant occurs across a bridge created
between the two organisms utilizing the difference in water potential of cell sap
between the two plants. Parasitic flowering plants have a higher negative osmotic
potential of cell sap that allows them to uptake organic nutrients from the host plant or,
in other words, the phloems within vascular bundles of the parasite and the host
become connected, creating a “physiological bridge” between the two plants’ vascular
tissues [50]. As Cuscuta has no roots and no effective photosynthesis system, most of
the nutrients apparently come from the host phloem, but their haustoria reach into the
xylem too for nutrients such as calcium. This makes Cuscuta a phloem feeder, and
Haupt et al. [64] used fluorescent proteins to show a symplasmic connection with
companion cells of phloem. A lower phloem flux here causes a reciprocal interaction
between the host and the parasite. In certain cases, Cuscuta can be a mediator of virus
infection for the host plant. Apoplasmic and symplasmic connections are found case
by case. The presence of a plasmodesmata connection between Cuscuta and host plant
was shown by Birschwilks et al. [65].
The connection between host and dodder vascular systems is continuous [65] and
facilitates transport of not only water and minerals but also viruses, proteins [64],
and mRNAs [71] from host to the parasite. Because plants possess hundreds of
different phloem-mobile proteins and RNAs that play important roles in regulating
plant development and stress responses [72], it is expected that the development and
108
M. Sarić-Krsmanović
tissue are able to expand from 800 to 2000 μm [1, 48], and their inter- and
intracellular expansion into the host tissue depends on the mechanical as well as
enzymatic processes [1]. These parasitic cells have been described as having ambivalent characters, functioning as both sieve elements and transfer cells [59, 63]. Interestingly, during this process, chimeric cell walls of host and parasite constituents are
formed, and interspecific plasmodesmata build up a cytoplasmic syncytium between
Cuscuta and its host plant [48, 64, 65]. To form a connection to the xylem, parasitic
and host cells of the xylem parenchyma commence a synchronized development,
fusing to build a continuous xylem tube from the host to the parasite [66]. With
functional connections to the xylem and phloem of its host, the parasitic plant is
supplied with water, nutrients, and carbohydrates [50, 58, 67].
4
Consequences of Field Dodder and Host Interaction
4.1
Impact on Host-Parasite Metabolites
After the establishment of a connection between host and parasite, the development
of the parasite is based on the exchange of nutrients. In the process of establishing
parasitic connections to its host, dodder uses a battery of hydrolytic enzymes,
primarily cell wall-modifying glycosyl hydrolases [68], which have been observed
directly through their activities [69] or indirectly through their structural
consequences during host-tissue invasion [48]. Further, dodder appears to induce
hydrolytic activities within its host [69, 70].
Transfer of fluids from the host to the parasitic plant occurs across a bridge created
between the two organisms utilizing the difference in water potential of cell sap
between the two plants. Parasitic flowering plants have a higher negative osmotic
potential of cell sap that allows them to uptake organic nutrients from the host plant or,
in other words, the phloems within vascular bundles of the parasite and the host
become connected, creating a “physiological bridge” between the two plants’ vascular
tissues [50]. As Cuscuta has no roots and no effective photosynthesis system, most of
the nutrients apparently come from the host phloem, but their haustoria reach into the
xylem too for nutrients such as calcium. This makes Cuscuta a phloem feeder, and
Haupt et al. [64] used fluorescent proteins to show a symplasmic connection with
companion cells of phloem. A lower phloem flux here causes a reciprocal interaction
between the host and the parasite. In certain cases, Cuscuta can be a mediator of virus
infection for the host plant. Apoplasmic and symplasmic connections are found case
by case. The presence of a plasmodesmata connection between Cuscuta and host plant
was shown by Birschwilks et al. [65].
The connection between host and dodder vascular systems is continuous [65] and
facilitates transport of not only water and minerals but also viruses, proteins [64],
and mRNAs [71] from host to the parasite. Because plants possess hundreds of
different phloem-mobile proteins and RNAs that play important roles in regulating
plant development and stress responses [72], it is expected that the development and
108
M. Sarić-Krsmanović