Examples of plant viruses, phytoplasmas and proteobacteria transmitted by different dodder species.
Parasitic plants obtain their nutrition from their hosts. In addition to this direct damage, they cause indirect damage to their hosts by transmitting various plant pathogens. There are some 4,500 species of parasitic plants known; out of them, nearly 60% are root parasites and the rest of them parasitise on the shoot parts. Orobanchaceae and Convolvulaceae are the two mostly studied families of parasitic plants; and the parasitic plants are the chief mode for transmission of the phytoplasmas. The parasitic plants have various modes of obtaining nutrition; however, the information about the mechanism(s) involved in the pathogen transmission by the parasitic plants is limited. The latest biotechnolgical advances, such as metagenomics and high througput sequencing, carry immense promise in understanding the host-parasitic plant-pathogen association in deeper details; and initiatives have indeed been taken. Nevertheless, compared to the other pests hindering crop productivity, parasitic plants have not yet been able to gain the needed attention of the plant scientists. In this chapter, we review and present some of the latest advances in the area of these important plant pests.
- parasitic plants
Parasitic plants, like microbes or pathogens, exploit other host plants for water and nutrients. They display a wide range of parasitic lifestyles, from obligate holoparasitism to facultative hemiparasitism . Parasitic flowering plants comprise of 4,500 species distributed in 280 genera in more than 20 plant families and represent roughly 1% of all angiosperm species [1, 2]. Out of total parasitic plants, 60% are root parasites, and the remaining 40% of the parasitic plants are stem parasites . Several well-known and agriculturally important parasitic plant species belong to the families of
Like fungi and oomycetes, parasitic plants develop specialised feeding structures called haustoria that establish intimate connections with host cells. A haustorium penetrates the vascular tissue of the host plant, forming a bridge between the parasitic plant and its host. The physiological conduit helps in redirecting resources from the host plant into the parasite . These include movement of water, carbohydrates, nutrients, small molecules (e.g., RNA and proteins) and microbes [7, 8, 9, 10]. Recent evidence suggested that the movement of biomolecules is bidirectional, which means exchange may occur from the host plant to the parasite and vice versa [11, 12]. Parasitic plants are reservoirs of various microbial groups belonging to bacteria, fungi, viruses and phytoplasmas [9, 13, 14, 15]. They can transmit many economically important plant viruses from infected hosts to healthy host plants. Several dodder plants, particularly,
2. Various modes of parasitism and nutrition of parasitic plants
Plant parasitism is a fascinating plant–plant interaction with the acquisition of at least some essential resources from the host plant. Parasitism exerts a strong impact on host growth, allometry, physiology, and reproduction . Parasitic plants can be broadly categorised into two groups based on their modes of nutrition: hemiparasites and holoparasites. The majority of the parasitic plants are hemiparasites, ca. 4100 species , which meet most of their photosynthetic assimilates using own photosynthetic machinery and the nutrients and water from their hosts. Three hundred ninety parasitic plant species are holoparasites that lack chlorophyll and, therefore, photosynthetically inept. They rely entirely on their host plants for nutrients and water . Both groups of parasites either connect to the host shoot (shoot parasites, or stem parasites, or aerial parasites) or to the root system of the host (root parasites). Majority of the parasitic angiosperm are root parasites (approximately 60%), while the rest are stem parasite , except the genus
Hemiparasites are predominantly xylem-feeders absorbing water and mineral nutrients from host plants. To ensure rapid intake of xylem solutes, hemiparasites undergo rapid transpiration to import hosts’ nutrients via the transpiration stream . In some cases, flux of organic carbon flow from host plant to the hemiparasite in the form of xylem-mobile organic elements . Hemiparasites can be further classified into two types based on their degree of dependency upon the host plant: facultative and obligate. Facultative hemiparasites can survive without a host and do not strictly require a host plant to complete their life cycle. Most studied root hemiparasites are facultative in nature . This includes parasitic plants from the families,
On the other hand, obligate hemiparasites need host plants for completion of their life cycles as these depend mainly on their hosts for essential resources. This includes stem parasites belonging to the families,
Parasitic plants have a broad host range and attack several co-occurring species, often simultaneously. Host range of parasitic plants is a function of the parasites’ feeding mechanisms (xylem- or phloem-feeder), distinct events of the evolutionary history of the species, and the biochemical compatibility with the host cells . However, host specificity is largely determined by the extent of reliance on the host plant and depends on the ability of the haustoria to functionally establish after invading the host. The most common potential hosts are from
3. Transmission of various pathogens by parasitic plants
Plant virus and phytoplasma diseases are major threats to modern agriculture and their management can be quite challenging. Different strategies have been developed to reduce the transmission of these pathogens. It is crucial to understand the various sources of contamination or inoculum during cultural practices to restrict the entry and thereby transmission of viruses in fields .
For the parasitic infection to initiate, it is important to understand the aetiology behind the transmission process. For infection in the above ground parts of the host, for instances,
The connection between host and the parasite is established with the development of ‘prehaustoria’ starting from the differentiation of a secondary meristematic tissue from epidermal and parenchymatous tissues of the parasite. Substances, such as pectins, facilitate the adherence and polysaccharides exuded by the prehaustoria and drives the host to produce factors for attachment and penetration [46, 50, 51]. After the process of penetration through a fissure in the host stem, the haustoria invades the epidermal and hypodermal tissue to develop inside the vascular bundle . While growing towards the xylem and the phloem tissues, they develop hyphal structures, similar to finger-like projections, also known as ‘absorbing hyphae’, which behaves like sieve element or transfer conduits for flow of nutrients between parasite and host [5, 38, 52, 53]. These multicellular haustoria functions with the aid of chemicals, also known as haustoria-inducing factors and some tactile cues . In such an interaction, it has been shown that in transgenic tobacco plants parasitised by
3.1 Transmission of viruses, phytoplasmas and proteobacteria in host plants by dodder
Majority of agriculturally important plant viruses and phytoplasmas are dodder transmissible and among which
Although the transmission of phytoplasma is quite similar to plant viruses, they are quite understudied. Most interactions of parasitic plants with phytoplasma necessarily are experimental in laboratory or greenhouse with special reference to dodder mediated transmission. Dodder acquires the phytoplasma cells from the infected plant via haustoria in the direction of the source of inoculum to the healthy host and progresses in the direction of the growing points . However, the efficiency of transmission depends on different combinations of phytoplasma and dodder species. In an experimental trial, it was seen that rubus stunt and cotton phyllody were transmitted in higher frequencies by
|Pathogen||Parasitic plant||Main host||Reference|
|Little cherry virus||Tobacco|||
|Apple mosaic virus||Apple|||
|Tobacco etch virus||Tobacco|||
|Mesta leaf curl virus||Mesta|||
|Tomato ringspot virus||Tomato|||
|Potato virus Y||Tobacco|||
|Dodder Latent Virus||Sugar beet|||
|Cucumber mosaic virus|||
|Tobacco mosaic virus||Tobacco|||
|Potato stem mottle virus||Tobacco|||
|Cuscuta Latent MLO||Periwinkle|||
|Oxtongues (experimental host)|||
|Cotton phyllody phytoplasma||Cotton (experimental host)|||
|Pear decline (||Pear (experimental host)|||
|Rubus stunt (||Different cultivated and wild |||
|European stone fruit yellows (||Plum & Apricot (experimental host)|||
|Sweet orange (experimental host)|||
|Tomato (experimental host)|||
4. Microbiomes of parasitic plants and their hosts
Microbiomes can expand the genomic potential of plants through efficient nutrients acquisition, promoting growth and development, and tolerance to biotic and abiotic stresses . Endophytic microbial communities of parasitic plants may affect parasitism and influence host microbial composition. Microbiota or microbial communities within a parasite can be divided into core- and transient-microbes. Core microbes are intrinsic to one or more developmental stages of a parasite that can vertically flow from parents to the offspring. Transient microbes are temporarily acquired by the parasite from their interacting hosts or environment . A study on microbial communities of parasitic weed,
Microbial communities of parasitic plants overlapped extensively with their parasitised host while still maintaining taxonomically distinct communities [67, 71]. For instance, bacteria communities of the root holoparasite,
5. Mechanism of pathogen transmission
Plant pathogens (mostly, viruses and phytoplasmas) are transmitted by parasitic plants by their twining stems. The parasite stem adheres to the host’s stem by exuding cutin as it wraps tightly around the stem of the host plant. Few species of parasitic plants like
The parasitic plants attach to the host plant through haustoria which originates at the site of association between the parasite stem coil and the host stem or leaf. The haustoria vary among different parasitic plant species, considerably in their anatomy and function, mostly by whether they form connections exclusively to the xylem only or both xylem and phloem . Initially, the haustorium enters the host tissue through the lower haustorium with the help of enzymes that break down cell wall connections. Cells then begin to elongate from the lower haustorium and traverse throughout the host tissue to reach the vascular system of the host which eventually leads to the formation of searching hyphae . These cells, termed searching hyphae, as it grows through the host cells, formation of new host cell wall occurs over the parasite cell wall, which appears to encase the hyphae over their entire surface. This formation of a new host cell wall around the parasite cell wall forms a host–parasite interface similar to that of neighbouring cells of the same species. The searching hyphae may develop as a xylem element when connections are made with the host xylem or it may differentiate into cells that are similar to sieve elements after contacting the host phloem.
The host–parasite cell wall is perforated by both simple and branched plasmodesmata, complete with desmotubules typical of normal plasmodesmata . The plant pathogens, mostly viruses are transmitted to the host plant through these plasmodesmata. The virus transmission through the plasmodesmata is felicitated by non-structural proteins, called movement proteins, which act to facilitate the movement of virus particles from cell to cell through these plasmodesmata .
Another mechanism of transmission of the virus from the infected parasite to the host is through the sieve element. The virus after being acquired from the vascular bundles of the infected host plant by the haustoria is transmitted in the food stream of the parasitic plant. After translocation through the parasite phloem, the virus is introduced to the next plant by the new parasite haustoria produced in contact with the vascular bundles of the inoculated plant. The parasitic plant absorbs phloem contents from the host, the searching hyphae of the parasite that contact host sieve elements grow around the element with finger-like projections. The parasite cell then differentiates like a sieve element, but with extensive development of smooth endoplasmic reticulum (ER) near the host cell and grows around the phloem cells of the host . These parasite cells then differentiate in a manner consistent with the development of sieve elements, although they also contain an elaborate network of smooth ER proximal to the host cell, a feature of transfer cells . In contrast to
The management of parasitic plants is difficult because there are few sources of crop resistance and is challenging to selectively kill the parasitic plants without damaging the host, as they are physically and biochemically attached to the host. The efficiency of the management of parasitic plants is also obstructed due to the dispersal efficiency, persistent seed bank, and quick responses to changes in agricultural practices. These qualities of the parasitic plants allow them to adapt to new hosts and manifest aggressively against new resistant cultivars. However, the management strategies of parasitic plants or crop resistance to parasitic plant infection can be classified as pre-attachment or post-attachment resistance according to whether the resistance occurs before or after the haustorium attaches to the host surface .
Mostly, the pre-attachment resistance or management includes the mechanisms that can be adopted by a host plant to prevent or avoid parasite attachment, this includes (
Post-attachment resistance occurs when the attached parasite haustorium attempts to penetrate host tissues to make connections with the vascular system. Substantial experimental evidence demonstrates that parasitic plants connect to the endodermis by activating the expression of genes encoding various cell wall degrading/softening enzymes such as pectate lyases, pectin methylesterase, polygalacturonase, endocellulase, β-xylanase, expansins. The expression of these enzymes assists the parasitic plants to penetrate the host endodermis through the epidermis and cortex . During this intrusive process, the host can succumb passively, rely on constitutively expressed general defence responses, or activate specific innate immune response cascades to fend off parasitic progress . Innate immunity can present as (
6.1 Use of herbicides as a strategy for parasitic plant control
The use of herbicides for management needs to be specifically designed depending on the target combination of the parasite-crop species and on the information available on the specific herbicide and the optimum herbicidal doses that have been proved to be sub lethal for the crop, on the other hand, it can be applied as lethal doses to the parasite, and the availability of crop varieties with herbicide resistance.
The systemic herbicide is applied to the crop foliage and delivered to the shoot or root parasites either via the haustorium or through exudation to the rhizosphere from the crop roots . The systemic herbicides used for parasitic weeds include inhibitors of aromatic (glyphosate) or branched-chain amino acid synthesis (imidazolinones and sulfonylureas), inhibitors of the vitamin folic acid (asulam), inhibitors of glutamine synthetase (glufosinate), or hormonal herbicides (2,4-D and dicamba) [89, 90].
Rationale and most effective control of parasitic plant disease is possible only if
the disease is correctly diagnosed,
the nature of transmission of the disease is known and
life cycle stages of the involved parasite, i.e., its mode of reproduction active structures produced under the favourable condition for repaid and wide dispersal and the structures produced to overcome adverse condition are known.
Parasitic plants are important hinderance in crop production and productivity, especially for perennial horticultural crops. In addition to their direct influence as a modulator of source to sink balance, they also are known vectors of obnoxious pathogens such as viruses and phytoplasmas. However, there seems not to have been equal, if not more, attention from the plant scientists on these multifaceted pests, as in case of other pests such as the pathogens and the insect-harbivores. Although there are at least 4,500 species of such parasitic plants forming some 1% of the angiosperms, very few of them have been studied in sufficient details. The extent of crop damage and their roles as pathogens vectors of most of them are not well-known. Considering the exploding population and its pressure on the limited resources of the planet, and the increasing demand for food and nutrition, harnessing each and every potential means of crop improvement and tackling all the potential causes of crop loss is the need of the hour. While the genetic potential of the important crops have reached near the maximum, sustainable management of the pests and pathogens is the most important step in this direction. Being a direct and indirect hinderance of crop production, as discussed in this chapter, the parasitic plants, therefore, demand further and deeper future research.
The research in the laboratory of BKB is supported by Department of Biotechnology, Govt. of India, Indian Council of Agricultural Research, Govt. of India and Department of Science and Technology, Govt. of India. AG is financed by a PhD Scholarship from the Norwegian University of Life Sciences, Norway.
Conflict of interest
The authors declare no conflict of interest.