Invasion of plant cells by fungal pathogen are accompanied by extensive metabolic changes reflected in well defined morphological modifications. The major changes following fungal attacks are: (a) major rearrangements of the cytoskeleton, (b) migration of the cytoplasm and the cell nucleus to the fungal penetration site, (c) formation of rigid cell-wall as a result of mobilization of suitable material around this site. If the first line of defense does not work, host cells react in a different way to halt the progression of pathogen attack. Infected plant cells become hypersensitive and they die. As a result, the penetrating fungus starves and it perishes. In any case, the initial defense response is of much greater importance. The speedier is the response, the better are the chances of the containment of the pathogen.
In the September 2002 issue of Trends in Plant Science (vol 7:411-415), Elmon Schmelzer at Max Planck Institute for Plant Breeding Research, Köln, Germany, discusses the current status of our knowledge about the mechanisms that resistant plant species adopt in order to foil the attack of fungal pathogens.
In his introductory remarks, the author describes how a plant resistant to a specific pathogen counters the attack of an invading pathogen. The gene(s) conferring resistance encode receptor-like or receptor-related proteins. These proteins, being receptor-like in nature, bind either directly to pathogen-derived molecules (avirulence proteins, specific or unspecific elicitors) or they act as constituents of larger protein complexes transducing signals. If the plant successfully thwarts the attack of the invading pathogen, damage (also called necrotic) is confined only to a few scarcely visible necrotic spots. On the other hand, susceptible plants fail to elicit defense responses and as a result the pathogen quickly spreads all over the plant body, killing the infected plant.
Pathogen attack activates numerous defense-related genes, leading to the synthesis of antimicrobial substances such as phyto- alexins and other secondary metabolites at the sites of infection. Defense-related proteins move throughout the plant body preventing secondary infection called SAR (systemic acquired resistance).
Unlike mobile animals, plants have developed a different strategy to thwart the attack of pathogens. They put up physical barriers to contain pathogens, denying them nutrient access. The author describes how a resistant potato epidermal cell generates a massive dark brown plug called the papilla at the site of infection. Thereafter, the wall undergoes further thickening, cutting off the supply of nutrients to the invading pathogen. As a result, the infected cells die and the pathogen perishes from starvation.
Cytoplasmic and cytoskeletal reorganization:Following the fungal attack, profound reorganization of the cytoskeleton takes place. For instance, there is an alignment of actin filaments and cytoplasmic strands directed towards the site of the attack in the proximity of the cell wall. The filament number increases depending upon the intensity of attack. Simultaneously, the role of microtubules is brought into play by their appearance below the appresorium. A chain of events follow in which the nucleus with an envelope of actin bundle, begins to move towards the penetration site. An actin filament bundle appears between the nucleus and the penetration site, interconnecting the cytoplasmic bridge. Further study on the movement of the nucleus revealed that while actin filaments persist until the hypersen-sitive cell death, microtubules disappear much sooner. The study also showed that often a collar like structure called papilla is formed around the invading hyphae, thereby destroying them. Interestingly, once the fungus is dead, the nucleus moves away from the penetration site.
The cytoskeleton – a key player in cellular defense: The author provides proof about the crucial role, the cytoske- leton plays in cellular defense. For instance, it has been shown that actin inhibitors such as cytochalasin destroys defense responses such as nuclear movement, papilla formation, callose deposition. On the other hand, microtubule inhibitors hardly interfere with defense responses Interestingly, a parallel chain of events take place in a root epidermal cell (trichoblast), destined to become an unicellular root hair structure. As in a pathogen-infected cell, actin inhibitors stop the growth of root epidermal cells, while microtubule inhibitors have no effect on the root hair formation. Thus there is a striking similarity between the 2 seemingly unrelated systems.
Potential involvement of actin binding proteins: Regarding the potential involvement of actin binding proteins, the author considers that both profilin (PFN) and actin depolymerizing factor (ADF) may be implicated in regulating the rapid production of actin filaments at the fungal penetration site.
On the basis of several observations, the author believes that villin and actin-related protein 2 (ARP2) may work in unison with ADF to produce rigid cables of interconnected actin filaments in response to fungal attack concomitant with their orientation towards the penetration site. If such structures are conserved in plants, the author thinks, they (ARP 2/3) may participate in the initiation of intensive actin polymerization. In light of some recent findings, it is presumed that pathogen attack is perceived by means of physical contact between the cell wall and the plasma membrane. As in mammalian cells, this may occur as a result of interactions between RGD and integrin/spectrin homologues that link the intracellular cytoskeleton of cells with the extracellular matrix through recognition of the RGD motif. The latter is the single letter code for arginine-glycine-aspartate and found in proteins of the extracellular matrix. RGD-binding sites at the plasma membrane have been reported in Arabidopsis also.
The author considers that Ca2+ influx via ion channels within the plasma membrane is vital as it is linked with receptor-mediated pathogen recognition. In this connection, the author mentions about broad-spectrum mildew resistance in barley. Levels of resistance are reduced in proportion to free Ca2+ in the cytoplasm; furthermore, it has been shown that in order for calmodulin to function as a negative regulator of defense, the MLO protein requires Ca2+ to interact.
The author lists a number of recently developed techniques such as video- and computer- confocal laser scanning technology and a few more including the use of molecular non-invasive tools for monitoring gene expression. He also mentions about the recent development in microscopic monitoring technique. The application of these powerful tools will unravel a wealth of information on temporal and spatial changes that take place in the infected cells of resistant plants; microscopic monitoring has enabled the cell biologists to gain valuable information about the in vitro rearrangements of actin filaments during root hair-, and trichome cells development in normal – and the corresponding mutant cells. Because of technological advance- ments, the author concludes with a confident note that the next decade will witness solution of many unanswered questions centering round defense-related mechanisms brought into play in resistant plants in response to a pathogen attack.