Left: the bizarre-looking dodder (Cuscuta, family
Convolvulaceae) is an obligate parasite of a wide range
of plants (it has low host specificity) including sugar
beet, tomato, cucumber, potato, asters, Corylus (hazel)
and onions. Obligate means that it cannot survive and
complete its life cycle without a host. It is a holoparasite,
meaning that it is more-or-less totally reliant on
photosynthesis in the host plant as a source of fixed
carbon. Photosynthesis in dodder is reduced or absent
and the leaves are reduced to tiny scales and the stem
is a reddish colour.
When Cuscuta germinates, the seedling root (if present)
emerges followed by a single reddish, apparently
leafless (except for the minute scale leaves) shoot. This
shoot undergoes circumnutation - it waves about in a
circular motion (reportedly anti-clockwise) searching for
a host plant. It will also bend towards the odour of a
suitable host species (chemotropism). If a host is not
found, then the root and the seedling will die after about
one month. If, on the other hand, a host is found then
the shoot will entwine around the host stem (probably
using the sense of touch which has been demonstrated
in a number of climbing plants, see The sensitive Plant).
The epidermal cells of the entwining shoot will put out
hair-like elongations called trichomes, which are
secretory. These trichomes have flexible walls and bend
to press against the contours of the host surface and
then cement the dodder to the host by secreting a
biological glue consisting of de-esterified pectins
(see below). A sucker-shaped protuberance, initially
called an appressorium develops at each such point of
adhesion between the dodder stem and its host. This
develops into an invasive feeding structure called an
haustorium (plural: haustoria).
Once an haustorium is established, the dodder's own root rots away - it is now dependent entirely on its host for water and
nutrients. The haustoria (particularly the lowest one) will penetrate the host by first creating a break or fissure in the covering
tissues of the host (it is not clear whether this is by mechanical pressure or enzyme action). The haustorium epidermal cells
then put-out invasive elongations called hyphae which may extend up to 800 micrometres into the host, and search around
inside the host tissues in order to find the host's vascular tissue with which they make contact. Once contact is established,
the vascular system of the dodder then grows into the haustorium and makes direct contact with the host's vascular system.
Host nutrients can then be transported directly into the vascular system of the dodder. The host phloem connects to the
dodder phloem and the host xylem connects to the dodder xylem. The dodder siphons off water, nitrogen, organic carbon,
RNA, proteins and also viruses are transferred from the host to the dodder. An enzyme of dodder produced in the haustoria,
a protease called cuscutain, has been shown to be essential for efficient establishment of feeding haustoria.
Pectins play an important role in the invasion process. Pectins form a biological glue which glues plant cells together (it
forms the middle lamella between adjacent cell walls) and are integral cell wall components. Pectins (or pectic
polysaccharides) are chains of sugar molecules rich in galacturonic acid. About 80% of the organic acid (carboxyl) groups of
the galacturonic acid are usually esterified with methanol (and sometimes ethanol). (An ester is a chemical formed when an
organic acid binds to an alcohol). The plant enzyme pectinesterase de-esterifies pectin, removing the methanol to reform
the acid groups, which makes the pectins more gel-like. This occurs, for example, in fruit ripening, pollen-tube elongation and
elongation of growing plant parts. It makes cell walls more extensible and stretchable. These gel-like de-esterified pectins are
produced by the haustoria, and by the hyphae in particular, and serve to glue them to the host.
In dodder, there are two growth modes for the invading hyphae - they may grow between host cells (extracellular hyphae)
or they penetrate through host cells (intracellular hyphae). However, it is now known that the cell walls of the intracellular
hyphae are surrounded by apparently newly synthesised, and modified host cell wall (and so are strictly extracellular). This
coating of the hypha in host cell wall materials is believed to disguise the hypha such that host defences are minimally
activated. The dodder does not want the plant to start reacting and cutting off its access to the host plant vascular system!
Extracellular hyphae have few or no plasmodesmata in their cell walls (or ectodesmata as plasmodesmata-like organelles are
called when occurring in the outer wall of epidermal cells, such as those used by the leaves of some plants to absorb water
and nutrients). In contrast, the growing tips of intracellular hyphae have numerous plasmodesmata which form connections
with plasmodesmata in the host cell wall layer around the hypha, presumably allowing the parasite to communicate with its
host in some unknown way. The outermost, host-derived wall of the hypha is also rich in de-esterified pectins, accounting for
its ability to elongate, though evidence suggests that it is not simply pre-existing cell wall material that has been stretched,
but is, at least in part, newly synthesised. The extracellular hyphae enzymically degrade the pectin middle lamella between
the host plant cells to forge a path to the vascular tissues. The dodder may also entwine around other non-host species for
support, but no haustoria will form in these regions.
Dodders are either annuals or perennials and eventually they put out flowers in tight clusters or spikes, each flower up to
one centimetre in diameter. The five petals fuse basally to form a trumpet-shaped flower tube or corolla. The petals vary in
colour from white to pink to yellow. Fused to the five petals are the filaments of the four or five stamens which project from
the corolla. The ovary has two compartments and puts out two styles and stigmas. Upon successful pollination, a seed
develops in each ovary compartment. Situated beneath the stamens, inside the corolla, are four or five petal scales (scales
borne on the petals) or infrastaminal scales. These scales, thought to be outgrowths of staminal tissue fused in the petal
wall, may connect to form a continuous ring in some species. They are variously divided into terminal hairlike appendages
called fimbriae. The tip of each fimbria usually bears a laticifer. Laticifers have soft cell walls and are swollen with a
secretion containing latex. These cells typically burst open when touched by insects and are often involved in plant defence
against herbivorous insects. In Cuscuta they apparently do not function as nectaries and may serve to protect the ovary and
developing seeds from herbivorous insects. Alternatively, the scales may trap falling pollen to reduce self-pollination.
Nectaries have been reported in some Cuscuta, consisting of a band of modified secretory stomata around the ovary,
sometimes pigmented orange or red. However, it is not clear whether or not all Cuscuta species have functional nectaries.
Left: entwining stems of dodder. Photo by
Silvae (via Wikimedia Commons).
Infection by Cuscuta has been shown to
dampen or weaken the host's defencive
responses against insect pests. Certain of
these defences have been shown to be
effective against dodder, including salicylic
acid and jasmonic acid.
Right: a flower cluster of Cuscuta europeae. Photo
credit: Michael Becker (via Wikimedia Commons).
Haustoria are also clearly visible above the flower
Pollination in dodders is thought to be primarily by
means of insect vectors, although self-pollination
possibly occurs in some species. The flower spikes
develop in the axils of scale leaves. The seeds may
last up to ten years in the soil in nature, in
unsuitable conditions, before germinating.
Mistletoe, Viscum album
Left: Mistletoe, Viscum album. Photo
credit: Alexbrn (via Wikimedia Commons).
Mistletoe is a perennial evergreen shrub
and a hemiparasite of a variety of tree
species, including firs (Abies), pine and
oak. Hemiparasites are only partially
reliant on their host for nutrients.
Misteletoe has a low photosynthetic rate
but a high transpiration rate. The roots of
mistletoe penetrate the wood of the host,
as a haustorium, with the xylem of the
mistletoe establishing direct contact with
the xylem of the host through which the
mistletoe draws off water and mineral
salts. The high rate of transpiration likely
ensures that the mistletoe sequesters
enough nitrogen from the host's xylem
sap for construction of proteins and
nucleic acids. Mistletoe apparently does
not cause significant damage to its host.
Mistletoe is dioecious (separate sexes) and insect pollinated. The white berries are dispersed by certain birds, including the
mistle thrush (Turdus viscivorus). The berries contain a sticky mucilage called viscin, which helps ensure that any seeds
dropped or regurgitated by the feeding birds adheres to tree bark. The seeds are dispersed in winter and germinate in
spring. Mistletoe is slow growing, after all penetrating a woody host is no easy matter, but is persistent and can live as long
as its host.
The seedling attaches to a branch of the host tree initially by a disc-like holdfast. A primary haustorium forms and then the
mistletoe overwinters. On fir at least, the cycle continues as follows: in the second year the first leaf pair develops and the
primary haustorium reaches the host cambium. In the third year, strands of root tissue extend through the cortex (outer
layers) of the host and develops secondary haustoria. In year 4, the first dichasial shoots develop. Dichasial means two
symmetrical axes fork outwards, this branching into two is characteristic of mistletoe (as can be seen in the photo above).
Flowers form in late winter of the fourth year and the berries ripen by the following autumn. The flowers are small, the male
flowers having no sepals and four small yellowish sepal-like petals. The female flower has similar petals but also has four
very reduced sepals. The stamens of the male flower lack filaments (anther stalks) and open by pores.
Left and below: Hay (Yellow) Rattle (Rhinanthus minor)
The yellow rattle, Rhinanthus minor, is a root hemiparasite
of grasses and legumes which taps into the host's xylem,
principally to steal nitrogen. (Other species of Rhinanthus
may tap into the phloem or parenchyma of the host). It has a
wide host range of over 20 species. It is a facultative
parasite, meaning that it can grow and complete its life-cycle
without a host, though in nature it thrives only where the host
is abundant. The fruit form the familiar hay rattle as they
rattle with the seeds they contain, gradually dispersing the
winged seeds as the plant blows about in the wind. The seed
must be chilled over winter for it to germinate the following
Striga (witchweeds) and broomrape (Orobanche) are two
more root parasites of the Broomrape family
(Orobranchaceae). To assist in finding a suitable host these
plants produce thousands of tiny 'dust seeds' which are
easily dispersed by wind and rain. The seeds of broomrape
can also survive more than 50 years in the soil, in unsuitable
conditions, before germinating. Both these plants pass
through a subterranean stage after germinating. Striga, an
obligate hemiparasite of important crops e.g. in Africa,
responds to host root chemicals and germinates. The radicle
emerges first and the tip of this seedling root develops hairs
which glue it to the host root.
The tiny dust seeds characteristic of orchids carry little in the way of food reserves and yet many orchids live below ground
for several years before emerging. How do they do this? They form symbioses with fungi which infiltrate the roots and
supply the embryonic orchid with nutrients the fungus has acquired, whilst the fungus receives nothing in return. Once
they emerge, most adult orchids will then produce their own food by fixing carbon dioxide by photosynthesis. Now they may
supply sugars to their fungal partner in return for soil nutrients obtained by the fungus - a mycorhiza which benefits both
plant and fungus. However, some orchids remain pale as adults and lack chlorophyll, either completely or nearly so, and
are not capable of any effective photosynthesis. These fungi emerge only to flower and scatter seed, they are
mycotrophic, eating the fungi that live inside the subterranean stem or rhizome.
For example, the Ghost Orchid (Epipogium aphyllum), the Coralroot Orchid (Corallorhiza trifida) and the Bird's-Nest Orchid
(Neottia nidus-avis) fall into this category. Neottia nidus-avis occurs in mature temperate beechwoods, Epipogium aphyllum
in oak woods and beechwoods, Corallorhiza trifida in willow, alder or pine woods. All three feed upon fungi associated with
the roots of trees, that is fungi which form mutually beneficial mycorhiza with the host tree. The fungus gives the tree soil
minerals in exchange for fixed carbon. The orchid, however, contributes nothing and is thus secondarily or indirectly
parasitic upon the trees and their fungi. The fungi are usally basidiomycetes and form classic ectomycorhizas with their
host trees, in which fungal hyphae wrap around the tree root to form a mantle and then penetrate the tissues of the outer
root, to one to three cell layers in depth (forming a Hartig net) but without invading the individual cells.
Neottia nidus-avis associates with a fungus called Sebacina, Epipogium aphyllum associates with the fungus Inocybe, most
(75%) of the time (else with other fungi), whilst Corallorhiza trifida associates with fungi of the family Thelephoraceae. The
rhizomes of Epipogium aphyllum and Corallorhiza trifida have no roots as such (the roots are highly modified), but consist
of masses of short, stubby branches infiltrated by the fungus. These rhizomes resemble coral in form.
Above: The bizarre-looking Ghost Orchid (redrawn from Roy et al., 2008; Irmisch 1853; Taylor and Roberts, 2011)!
The coral-like branches of the coralloid rhizomes contain fungal hyphae inside the cells (presumably enclosed in host cell
membrane) as coiled tangles called peletons. Each peleton lasts only a few days before being digested by the host cell.
The epidermis and outermost few cell layers are not infiltrated, but many or most of the remaining root cortical cells are.
The Ghost Orchid can reproduce asexually by putting out thin stolons from the rhizomatous mass, along which bulbils
develop at intervals. The stolons may be up to 0.5 m in length. Neither the stolons nor the bulbils are infiltrated by fungus
and thus the new plant must acquire fungus, as must the germinating seed. The scarcity of the required fungus, which is
possibly also susceptible to atmospheric pollution may account for the present rarity of the ghost Orchid in certain regions,
such as the United Kingdom. In England, for example, it hasn't been recorded for about 20 years, however, it can survive
for many years beneath the soil without emerging and is also often hard to see amongst leaf litter and so it may not be
extinct in England.
A haustorium forms, linking to the host's vascular system.
The subterranean seedling lacks chlorophyll and has scale
leaves and produces adventitious roots (roots originating
from stem tissue) which produce secondary haustoria. The
infecting root stimulates host root development, ensuring a
good supply of nutrients. Eventually it emerges from the
soil, producing chlorophyll and flowers. Broomrape is an
obligate root holoparasite and passes through a similar
Rhinanthus minor beginning to form the 'hay rattle' fruit.
Rhinanthus has been classified in the Figwort family (Scrophulariaceae), along with the semi-parasitic Eyebrights
(Euphrasia) and Cow-Wheats (Melampyrum) but these have all also been classified in the same family as the parasitic
Eyebrights are much smaller plants than Yellow-Rattles, and are difficult to classify as the various species form many wild
hybrids. Eyebrights are also hemiparasitic, forming haustoria that attach to the young living roots of host plants, such as
grasses. By the time the Eyebright flowers, the parasitised roots of the host are dying and both Eyebrights and
Yellow-Rattles allegedly continue to feed saprophytically on the decaying remains. Melampyrum attaches its haustoria to
the roots of shrubs and trees as well as to dead organ matter, feeding both parasitically and saprophytically.
Above and below: Eyebright (Euphrasia sp.) growing on chalk grassland. The flowers are tiny,
29 Nov 2014
6 June 2015
29 Aug 2015
9 May 2016
14 May 2016
Toothwort (Lathraea squamaria) a member of the
Broomrape family (Orobanchaceae) is found in woods on
rich or calcareous soils and is parasitic primarily on hazel
and elm (these were found at the base of an oak tree). The
flower spike is the only above ground structure this plant
produces and has a white or pale pink stem bearing flowers
on one side. The plant does not photosynthesise (at least
not to any significant degree) and the inflorescence bears
white or pinkish tooth-like bracts. The calyx (sepals) is
yellowish-wgite and has sticky hairs and the corolla
(petal-tube) is pink.
The main body of the plant is a chlorophyll-lacking
branching rhizome with white scale leaves which decussate
(they are arranged in a criss-cross pattern). The rhizome
puts out lateral roots which end in sucker-like haustoria
which attach to the roots of the host tree and access the
host's vascular tissue. The undersurface of the rhizome's
scale-leaves possess dome-shaped hydathodes
(water-secreting glands, modified trichomes). Similar
hydathodes are seen in a number of parasitic and
hemiparasitic plants, including Rhinanthus, Pedicularis
(Lousewort) and Odontites. In Pedicularis palustris, the
dome-shaped glands consist of 4 cells: a basal cell, a collar
cell and two terminal cells enclosing a pore which excretes
water and mucilage (though water secretion seems to be
their main function). The glands in Lathraea (and
Rhinanthus) are reportedly similar in basic plan, but
consisting of 9 cells (4-1-4) rather than 4 (1-1-2). (See:
Groom, P. 1897. On the leaves of Lathraea squamaria and
of some allied Scrophulariaceae, Annals of Botany 9:
385-398). Xylem tracheids pass close beneath the glands,
delivering water to them.
Capitate trichomes (hairs with a swollen terminus) are also present in the same regions, though these do not appear to be
water-excreting (reports disagree on this issue). In Lathraea, the closely-packed rhizome leaf scales have branched
pocket-like cavities which open to the underside of the leaf. These cavities are lined by epidermis containing the capitate
and dome-shaped glands. (In Rhinanthus these glands lies along the veins of the leaves and are less numerous and have
a less obvious function than in Lathraea). Copious quantities of water can be excreted, apparently from these glands (the
amount of water excreted correlates with the number of dome-shaped glands present).
Toothworts can complete their whole life-cycle underground: they can produce subterranean flowers which are
cleistogamous (do not open) to enable self-fertilisation. Toothworts apparently lack stomata on their subterranean parts,
and being subterranean in any case raises problems with generating sufficient transpiration to draw-off enough host sap
to extract the necessary nutrients. The hydathodes (whatever their mechanism) presumably serve to draw enough host
sap through the parasite to extract sufficient nutrients. It has been suggested that the pockets in the rhizome leaf-scales of
Lathraea may also trap and digest tiny soil organisms, digesting them and absorbing the released nutrients. Water
secertion appears to be their primary function, but it would be interesting to see to what extent, if at all, lathraea may be
supplementing its intake of nitrogen by carnivory. Asexual reproduction can occur by fragmentation of the rhizome.