Several kinds of host-pathogen interactions have been observed in the environment, of which parasitic relationship is a kind. Many of the parasitic associations are interesting since some major food crops are involved in these interactions and the parasitic organisms are responsible for causing devastating diseases on them.
A plant parasite relationship may be biotrophic, hemi-biotrophic or necrotrophic in nature. The obligate biotrophic parasites survive in the living hosts and complete their lifecycle with minimal damage to the host over a long period of time. In contrast to necrotrophic and hemibiotrophic fungal and oomycete pathogens which have little or no biotrophic phase, the obligate biotrophs are entirely dependent on the living plant tissue for their growth. These obligate biotrophic parasites are thus characterized as true biotrophs with the following criteria: (a) highly differentiated infection structures; (b) limited secretory activity; (c) a narrow contact zone separating fungal and plant plasma membranes; (d) long term suppression of host defense responses; (e) the formation of haustoria (Mendgen & Hahn, 2002).
One of the most striking evidence of obligate biotrophs are the establishment of haustoria within living plant cells. The haustoria produced by the biotrophic fungi and oomycetes are extension of the pathogens into the living host cells. This has been a subject matter of great interest to study the lifestyle of obligate biotrophic fungi. However, knowledge about haustoria is scarce since the obligate biotrophic fungus are unable to form the haustoria in culture. According to the criteria formed to mark off the true obligate biotrophic pathogens from hemibiotrophs and necrotrophs, the true obligate biotrophs comprises the downey mildews, the powdery mildews, and the Rusts.
The Haustoria:
The literal meaning of the term haustorium is ‘drawer of water’. The fungal haustorium is defined as a specialized organ which is formed inside a living host cell as a branch of an extracellular (or intercellular) hypha or thallus, which terminates in that host cell, and which probably has a role in the interchange of substances between host and fungus. They are specialized terminating organ evolved in fungi to contend with the rigid thick wall of the plant cell, bringing the fungus into close contact with the host protoplast. In contrast, many intracellular hyphae are able to grow from cell to cell but still possess several functional and structural similarities to haustoria (Sarah & Jonathan, 2001). The terminal characteristics of haustorium distinguishes it from intracellular hyphae as in smut fungi. Similarly, the attachment to a hypha or thallus outside the host cell differentiates it from thalli of parasites that are entirely intracellular like Plasmodiophora spp.
The haustorium is one of the hallmark of true obligate biotrophic fungi. The idea that this typical infection structure is not formed in the artificial cultures does not merely indicate the lack of specific nutrients, but rather one or more signals from the host are missing in culture either to induce or to complete the differentiation of haustoria in vitro.
The fungal haustorium may be of varying shapes and sizes; short or long, simple unbranched or branched, intercellular or intracellular, epicuticular etc. However, the haustoria are not truly intracellular: they are separated from the host cytoplasm by a specific derivative of the host plasma membrane, which is formed upon haustorium formation and tightly surrounds this fungal organ. This structure is often referred to as Extra-haustorial membrane (EHM). Undoubtedly the extra-haustorial matrix represents a place for the exchange of nutrients and information (Heath & Skalamera, 1997).
Development of Haustorium:
The formation of haustorium is the last step of the infection process; spore landing, adhesion, spore germination, appressorium formation, germ tube initiation etc. This process occurs in compatible host pathogen interaction but may be impeded in incompatible host pathogen interaction.
The haustorium is formed inside the living host cell as a branch of the fungal thallus. Although the haustorium is located within the host cell, it is not located directly into the cytoplasm but rather is surrounded by an extrahaustorial membrane. So, they remain outside the physiological barrier of the host cell. The extrahaustorial membrane is an extension of the host plasma membrane that surrounds the entire intracellular haustorium. An extrahaustorial matrix, a gel like layer enriched in carbohydrates with varying dimension and density, occurs between the haustorial body wall and the extrahaustorial membrane. The extrahaustorial matrix resembles an amorphous mixture of components, mainly carbohydrates and proteins, partly of fungal but primarily of plant origin.
The morphological spectrum of a haustorium is best exemplified by the rust fungi which has monokaryotic and dikaryotic stages with very different morphologies of haustoria. Haustoria in rust fungi produced by monokaryotic mycelium (M- haustoria) appears as intracellular extensions of intercellular hyphae as they exhibit only a few structural modifications during growth in the host cells. On the other hand, the dikaryotic haustoria develops from an external structure called Haustorial Mother Cell (HMC).
Ultrastructural studies of dikaryotic rust infection process showed that haustorium formation begins when a haustorial mother cell (HMC) differentiates from intercellular hyphae by laying down a septum near hyphal tip. During haustorium formation, the cytoplasmic contents of the HMC, including the two haploid nuclei, migrate into the haustorium through the neck structure, leaving the HMC enucleate and highly vacuolated.
Functions of the haustorium:
Several studies have been conducted to study the functions of the haustorium. The major role of haustorium since its identification has been thought to be uptake of nutrients from the infected plants. In addition to their role in nutrient uptake, it is hypothesized that haustoria are actively involved in establishing and maintaining the biotrophic relationship.
Role of Haustoria in nutrient uptake:
Since its early identification, haustoria have been thought to be the organ by which obligate biotrophs derive nutrients from their hosts. Although most of the parasitic mycelium of rust fungi consists of intercellular hyphae, the ultrastructure and location of haustoria within host cells suggest their special role in nutrition.
HXT1 gene, expression and function:
In search of genes involved in nutrient uptake during biotrophic stage of rust fungus Uromyces fabae, a gene was identified exhibiting high similarity to hexose transporters from a variety of organisms. This gene, HXT1, encodes for a polypeptide of 522 amino acids with a calculated molecular mass of 56.8 kDa. It is more closely related to the recently identified monosaccharide transporter AmMst1 from the ectomycorrhizal fungus Amanita muscaria.
HXT1 expression was analysed in in-vitro grown infection structures and was found unexpressed up to the haustorial mother cell (HMC) formation. However, a strong hybridization signal was detected with RNA from isolated haustoria thereby suggesting that HXT1 gene is either exclusively or at least preferentially expressed in haustoria.
HXT1p was completely characterized on a biochemical level by heterologous expression of HXT1 in S. cerevisiae glucose uptake mutant. This strain is unable to grow on a D-glucose substrate as its sole carbon source. Transformation of this strain of S. cerevisiae using a plasmid pDR195::HXT1 complemented the glucose uptake defect of this mutant. This hence adds to the proof for the uptake of sugar by the HXT1p.
Sucrose is the primary sugar that is transported in plants and, therefore, it has been speculated that the haustoria may import sucrose directly. Data from Voegele et al. (2001) reveals the fact that HXT1p is a proton-motive-force driven monosaccharide transporter and exhibits specificity for D-glucose and D-fructose. In addition, studies with powdery mildew fungus indicate that glucose, rather than sucrose, may be the sugar imported. These findings support the idea that glucose/fructose but not sucrose are the primary sugars imported by haustoria.
Source of Hexoses for Hexose Transporter:
Elucidating the mechanism and specificity for carbohydrate uptake in U. fabae has made it easier to understand the biotrophic relationship between the host and the parasite. However, these findings also have added some query.
The disaccharide sucrose is the major long-distance transport form for assimilates in higher plants. Also it has been shown that the levels of free hexoses (mainly D-glucose and D-fructose) in Vicia faba are rather low & present in abundance is the disaccharide, sucrose. Therefore, sucrose-cleaving enzymes, such as invertases, play a pivotal role in carbon partitioning in these organisms.
A gene, INV1, has been identified with homology to invertases in U. fabae. Localization of the gene product was found in the periphery of the haustorium. It has been suggested that the gene product of INV1, INV1p, produces invertase enzyme which serves as key steps in carbon partitioning in higher plants thereby making the substrate available for the hexose transporter.
Sugar transport mechanism:
Plasma membrane H+– ATPase plays a key role in active nutrient uptake. An increase in the plasma membrane H+-ATPase activity for haustorial membranes compared with membranes from other fungal structures was reported by Struck et al. (1998). This proton gradient generated by this ATPase was suggested to drive secondary active transport systems engaged in nutrient uptake by the parasite. Thus it is believed that the haustoria plays a special role in the uptake of nutrients from an infected host cell.
AAT2 gene:
Another study conducted by Matthias Hahn et al. (1997) have shown that a protein PIG2p (now AAT2p), located on the haustorial plasma membranes & encoded by gene PIG2 (now AAT2), is amino acid permease. This protein, similar to other fungal amino acid permeases, is supposed to be a transporter of amino acid and is only found in the haustorial plasma membrane which further adds to the function of haustoria in nutrient uptake.
Conclusion:
Several recent researches have proven that haustoria are in fact, the nutrient uptake devices. However with the advance in the research at deeper level, new facets of haustoria are being identified. Much molecular work on these structures are necessary to fully understand the obligate biotrophic lifestyle.
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