BOTANY, VOLUME 2 - PLANT PHYSIOLOGY - 2007

9. ALLELOPHYSIOLOGY

9.3. Pathogens

As the primary link in the food chain, photoautotrophic plants serve as a source of Nutrition for a vast number of other organisms and are thus constantly exposed to their adverse effects (Table 9.2).

Class="center">Table 9.2. Proportion of plant pests within specific Organism groups

A plant disease is defined as any abnormal physiological condition or structural damage in a plant accompanied by characteristic symptoms caused by abiotic or biotic factors. An abiotic cause of plant disease is the deficiency of essential mineral nutrients (see 6.2.2). The aforementioned parasites (9.1.1) are also classified as causal agents of plant diseases and therefore fall within The Scope of phytopathology.

Biotic causes of disease are referred to as pathogens (causal agents of disease). The majority of pathogens are microorganisms (bacteria, fungi, and certain Protozoa), but this category also includes VIRUSES AND VIROIDS (see 1.2.5). Damage caused by plant "eaters" (herbivores) or other plants is generally not classified as a plant disease and is considered separately (see 9.4, 9.5).

9.3.1. Basic Concepts of Phytopathology

Estimates suggest that each plant species is exposed to up to 100 potential pathogens. In other words, every plant species is immune to the vast majority of potential disease-causing agents, and each pathogen typically possesses The ability to successfully infect only a narrow range of potential hosts. When a disease breaks out, the affected plant is considered susceptible, the pathogen is virulent, and the host-pathogen interaction is described as compatible. Conversely, if no disease develops, the host plant is termed resistant, the pathogen avirulent, and the host-pathogen relationship incompatible. In either case, the outcome of the interaction is determined by the genotypes of both the host and the pathogen. On the pathogen's side, two groups of genes are distinguished: those encoding pathogenicity factors, which determine the manifestation of disease symptoms in host plants, and those determining the host range, which serve for host recognition. All species that cannot be recognized are non-hosts, while all others can potentially be infected. Some pathogens have a broad host range, whereas others infect only specific species or even distinct races within a species. Race-specific interactions are particularly well studied due to their major agricultural significance. It has been demonstrated that these strictly specific host-pathogen relationships are based on a Gene-for-gene interaction between both partners: on the pathogen side, these involve avirulence genes (avr genes, named thus because their presence leads to a loss of virulence, while their absence or functional loss through mutation results in virulence), whereas on the host plant side, they involve resistance genes (R-genes).¹ If a matching R-gene is present, resistance to the pathogen carrying the corresponding avirulence gene is conferred; if it is absent, the plant is susceptible (Fig. 9.13). The avr genes encode race-specific elicitors (derived from Latin elicere — to entice, to induce) that trigger plant defense responses against the pathogen (see 9.3.4); R-genes encode the corresponding elicitor receptors. When a race-specific elicitor binds to its matching receptor, a robust defense response is triggered in the affected plant Cells, manifesting as hypersensitive Cell death, during which the invading pathogen is eliminated alongside the locally dying plant cells. In recent years, several plant resistance genes as well as their corresponding pathogen avirulence genes have been identified (see 9.3.4).²

¹ From the English word "resistance." — Editor's note.

² In Russian literature, METABOLISM/2.html">THE CONCEPT OF vertical resistance is introduced for specific plant defense reactions. — Editor's note.

Fig. 9.13. The gene-for-gene model of race-specific interaction between a pathogen and a host plant. Coevolution of the pathogen and the host plant leads to The Emergence of resistance and avirulence gene groups. The plant defense response is activated only when a compatible combination of the avirulence gene (avr) and resistance gene (R) is present. Plants carrying multiple diverse resistance genes are particularly well protected against various pathogen races; conversely, a pathogen can infect a greater number of host races the fewer avirulence genes it carries. The host range is determined by host recognition genes and is generally very narrow in race-specific pathogens. Disease symptoms are determined by pathogenicity genes or their respective gene products.

Alongside local defense reactions such as hypersensitive cell death, pathogens also induce so-called system-

ic (whole-organism) responses in many plants. For instance, inoculating a tobacco leaf with tobacco mosaic virus results, within a few days, in enhanced resistance of the entire plant (i.e., including virus-free Organs) to numerous pathogenic bacteria and fungi. This pathogen-non-specific whole-organism defense is termed systemic acquired resistance (SAR). This response demonstrates that, In addition to race-specific mechanisms, plants possess numerous less specific, broad-spectrum defense mechanisms that are pathogen-inducible and Complement constitutive defenses—those measures present even in the absence of a pathogen (see 9.3.4). The inducible spectrum of defense mechanisms¹ is triggered by non-race-specific elicitors, which are often low-molecular-weight degradation products of bacterial, fungal, and/or Plant Cell Walls, or membrane components released by lytic processes at the site of pathogen entry (e.g., oligogalacturonides from the primary plant Cell wall, bacterial flagellin fragments, chito-Oligosaccharides from fungal cell walls, fungal Steroids such as ergosterol, or glycopeptide fragments of fungal Glycoproteins).

¹ Russian literature discusses mechanisms of horizontal resistance, which encompass a broad array of non-specific defense reactions. — Editor's note.

9.3.2. Microbial Pathogens

The vast number of known plant pathogens (see Table 9.2) corresponds to a multitude of plant diseases, which can only be outlined schematically here. The largest group of pathogens consists of fungi, which include both obligate and facultative parasites. Because obligate parasites grow exclusively in the presence of their hosts rather than on artificial media, they are also referred to as biotrophic. Pathogenic fungi land on the plant surface as spores and germinate there—likely stimulated by chemical substances produced by the host plant. Depending on the pathogen, the mycelium penetrates the plant either through natural openings (Stomata, lenticels, or hydathodes), through wounds or cracks (e.g., where lateral roots emerge), or directly. In the latter case, surface structures are first broken down by fungal Enzymes (cutinases, cellulases), allowing haustoria (penetration hyphae) to grow through.

Necrotrophic fungi are parasites that invade the plant, kill and destroy cells in the region of the growing mycelium, and absorb nutrients from these "devastated" areas.

Most phytopathogenic bacteria are facultative parasites (they also grow on artificial nutrient media) and infect plants, depending on the species, through wounds, stomata, hydathodes, or the duct systems of nectaries. The major bacterial plant pathogens belong to the Gram-positive genera Agrobacterium, Erwinia, Pseudomonas, and Xanthomonas; these are flagellated, rod-shaped bacteria. This group also includes species of Clavibacter (Gram-negative, flagellated or non-flagellated rods) and Streptomyces. Agrobacterium tumefaciens, the causal agent of crown gall tumors, has been studied in particular detail and is widely used for introducing Genetic information into many (though not all) plant species (see Box 9.2; 7.3).

It was only in 1967 that Electron Microscopy revealed a novel class of disease agents—highly simplified bacteria lacking a cell wall, which either exhibit a helical Structure (referred to as spiroplasmas) or form spherical to rod-shaped cells (referred to as phytoplasmas). They have been identified as the causative agents of over 200 distinct plant diseases (e.g., in pear, apple, peach, corn, tomato, and coconut palm) and induce symptom profiles similar to those caused by many viruses (such as leaf yellowing, pathological shortening of internodes, and disruption of apical dominance). The taxonomic placement of spiroplasmas and phytoplasmas remains uncertain; they are typically grouped near the Mycoplasmas (see 11.2).

Viruses (see 1.2.5) are complex infectious nucleoprotein particles specialized for obligate Replication within host cells, consisting minimally of PROTEIN AND NUCLEIC acid. In most plant viruses, the nucleic acid is single-stranded RNA (e.g., tobacco mosaic virus); in some (40 species), it is double-stranded RNA, single-stranded DNA (50 species), or double-stranded DNA (30 species, e.g., cauliflower mosaic virus). Viruses are generally classified among microbial pathogens, although they lack cellular or organismal Organization. Viruses infect plants via wounds typically inflicted by insect vectors, or they are transmitted during Fertilization via infected pollen. Viral replication occurs exclusively in living cells and yields between 10⁵ and 10⁷ virus particles per cell. Viruses spread to uninfected cells through plasmodesmata; remarkably, only the viral nucleic acid travels from Cell to Cell aided by viral movement Proteins, utilizing the plant's own machinery for transporting cellular proteins and mRNA molecules (see 7.4.4). In this manner, a virus traverses 8 to 10 cells (~1 mm) daily. Once viruses reach the sieve tubes, propagation accelerates dramatically, allowing a locally infected plant to become systemically infected within 3 to 4 weeks. Like Agrobacterium tumefaciens, cauliflower mosaic virus is utilized for introducing genetic information into plant cells, and its replication cycle has been thoroughly investigated (Box 9.1).

The remarkable mechanism of natural gene transfer from a prokaryote into The Genome of a eukaryotic plant cell has been investigated intensively. Once it was demonstrated that the transfer mechanism Functions even in the absence of oncogenes, and that the T-DNA can be truncated (deleted) down to 25-bp terminal sequences at its left and right ends (= left and right border sequences) while accommodating virtually any foreign genes in between, Agrobacterium tumefaciens became the most prominent vector organism for generating Transgenic Plants (see Box 7.3). Arabidopsis thaliana, the model organism of higher plant developmental biology (see Box 7.1), can also be successfully transformed using Agrobacterium tumefaciens.

Box 9.1. Cauliflower Mosaic Virus

Cauliflower mosaic virus (CaMV), a member of the caulimovirus group, is transmitted by aphids and causes mosaic symptoms on infected plants, which are further characterized by stunted growth, reduced yield, and lower quality. The virus spreads via plasmodesmata in the parenchyma and subsequently systemically through the phloem, i.e., throughout the entire plant. The Cytoplasm of infected cells is frequently densely packed with replicated Viral Particles (viroplasm).

Caulimoviruses are isodiametric particles approximately 50 nm in diameter (Fig. A), whose protein coat (capsid) is built from a single subunit with a molecular mass of 42 kDa. The cauliflower mosaic virus genome (Fig. B) is approximately 8 kbp in size and consists of a circular double-stranded DNA assembled from 3 single-stranded DNA molecules—the α-, β-, and γ-strands—which are non-covalently linked to each other. It contains 6 genes with known functions (I–VI, Fig. B) and 2 shorter genes with as yet unknown functions (VII, VIII), as well as 2 promoters with strong activity in plants: the 19S promoter (p19S) and the 35S promoter (p35S) (named after the Svedberg sedimentation coefficients of the mRNA molecules whose synthesis is controlled by these promoters, see below). Upon entry of the virus into the host cell, the DNA is released and transported into the Cell Nucleus. There, single-stranded breaks are first repaired by plant ligases. The covalently closed, circular DNA molecule then associates with Histones to form a "minichromosome-like" structure, both promoters of which are efficiently recognized by plant DNA-dependent RNA polymerase II (see 7.2.2.2), leading to intensive Transcription of its genes. Two transcripts are produced: the 19S mRNA, which encodes gene VI, and the polycistronic 35S mRNA, which spans the entire viral genome. The α-strand (minus strand) serves as the template for this process. The 19S mRNA and a portion of the 35S mRNA (the latter presumably only after splicing) are translated in the cytoplasm of the host cell, with the protein encoded by the 19S mRNA enhancing the Introduction/27.html">Translation of the 35S mRNA. The Functions of the individual gene products are presented in the table in Fig. B. Gene product I functions as a "viral movement protein" (VMP), which mediates the symplastic cell-to-cell transport of the virus (for the mechanism, see 7.4.4.1). The Reverse Transcriptase encoded by gene V transcribes the portion of the 35S mRNA not required for translation into DNA, initially synthesizing the α-strand. The transcriptase halts at two purine-rich Regions of the 35S mRNA and cleaves the mRNA sequences on both sides of these regions; primer-like RNA-DNA double-strand regions form over these sites—which correspond to the later Δ2 and Δ3 discontinuities between the β- and γ-strands—following the degradation of the remaining mRNA. DNA polymerase then binds to these regions to synthesize both complementary strands (β and γ). Viral replication thus concludes, and the packaging of DNA into capsids begins.

Fig. A. General appearance of caulimoviruses

Fig. B. Genome of cauliflower mosaic virus

Fig. C. Nucleotide sequence and STRUCTURE OF THE 35S promoter. Domain boundaries are indicated in the sequence by arrows and nucleotide numbers

Cauliflower mosaic virus serves as an effective vector for introducing foreign DNA into plant cells, although it is currently less convenient compared to the Agrobacterium tumefaciens vector system (see 9.3.3, see boxes 7.3; 9.2). The viral promoter regulating 35S mRNA production (referred to briefly as the 35S promoter) is one of the strongest plant promoters (Fig. C). Consequently, it is frequently utilized for the overexpression of foreign genes in plants. The 35S promoter lacks tissue Specificity and is therefore highly active in almost all plant cell types. The region from -46 to +8 (with transcription start designated as +1) constitutes the minimal promoter, which contains a TATA box (see 7.2.2.1, 7.2.2.2). The remaining promoter regions (domains A1 and B1–B5) function as potent enhancers (enhancers are transcription-activating cis-elements that operate independently of their orientation and position relative to the minimal promoter, see 7.2.2.1, 7.2.2.2). Each of these enhancer sequences exhibits a certain degree of independent tissue specificity; the summation of sequences within the complete promoter influences its activity throughout the entire plant.

Box 9.2. Biology of Agrobacterium-induced tumors¹

¹ In Russian literature, Agrobacterium-induced tumors include crown gall disease (A. tumefaciens), hairy ROOT disease (A. rhizogenes), and crown tumor (A. rubi). The latter two diseases are not discussed here. — Translator's Note.

Crown galls occur naturally worldwide as a symptom of infection by the rod-shaped, peritrichously flagellated soil bacterium Agrobacterium tumefaciens (a member of the Rhizobiaceae family, like the related genus Rhizobium), primarily on woody plants and shrubs such as rosaceous species, willows, and grapevines. Under artificial conditions, tumor formation can be induced in numerous species across more than 60 families, particularly among dicotyledonous angiosperms (Fig. A). Infection occurs through wounds in the transition zone between the SHOOT and the root (the root crown) and depends on the presence of a bacterial virulence plasmid, the so-called Ti plasmid (tumor-inducing plasmid), which is approximately 0.2 Mbp in size and present in various bacterial strains in several structurally similar variants (Fig. B).

Fig. A. A tumor several weeks old, experimentally induced on a Lycopersicon esculentum shoot

Fig. B. Structure of the Ti plasmid. In the example shown, the complete T-region is present. Alternatively, it may be split into up to three segments. The individual operons of the vir region are indicated with the number of genes per Operon in parentheses

During Pathogenesis, a portion of the plasmid DNA—the T-DNA region (transferred DNA)—is transferred into The plant cell and stably integrated into the nuclear genome as one or multiple (up to 20) copies. The transformed plant cell initiates tumor growth, meaning that uncontrolled Cell Division ensues, leading predominantly to The formation of undifferentiated and unstructured callus tissue (crown gall). Once the agrobacteria are eliminated, crown gall tissue can be propagated indefinitely in sterile culture without The addition of Auxins and Cytokinins to the nutrient medium (which is impossible for untransformed tissue, cf. 7.6.2.3, Fig. 7.47); the tumor grows independently of Hormones. Meanwhile, the tumor tissue synthesizes opines. Opines (Fig. C) are Condensation products of α-keto acids (Pyruvate, 2-oxoglutarate) with Amino Acids (e.g., Lysine, Arginine) that the plant cannot further metabolize. However, they serve as the sole source of carbon and nitrogen for the agrobacteria residing within and around the tumor tissue in the soil. Each tumor produces a specific opine depending on the inducing agrobacterium strain.

Fig. C. Biosynthesis of opines by bacterial opine synthases

Both the genes responsible for auxin and cytokinin independence and the opine synthesis genes are located on the T-DNA, whereas the genes responsible for opine Catabolism, virulence, and transformation reside on the non-transferred portion of the Ti plasmid (see Fig. B). The hormone-independent growth of the tumor is governed by three genes (onc genes)¹ whose functions have been elucidated. Gene 1 encodes Tryptophan monooxygenase, gene 2 encodes indoleacetamide hydrolase, and gene 3 encodes isopentenyl transferase. These enzymes catalyze the Synthesis of the auxin IAA (see 7.6.1.2) and the initial intermediate in plant cytokinin biosynthesis, isopentenyladenosine 5'-monophosphate (see 7.6.2.2) (Fig. D). Their production and activity are not regulated by the plant cell. Consequently, crown gall tumor cells produce large amounts of auxin and cytokinin, behaving essentially like plant cells supplied with these hormones exogenously in culture (see Fig. 7.47).

¹ These plant oncogenes are also designated as iaaH, iaaM, ipt, or tms1, tms2, tmr. — Translator's note.

Fig. D. Enzymes of auxin and cytokinin biosynthesis encoded by onc genes (for structural formulas of the isopentenyl transferase reaction, see Fig. 7.46)

Numerous (approximately 600) VirE2 proteins bind to the excised single-stranded T-DNA, forming a filamentous T-complex about 3.6 µm in length. This complex consists of single-stranded DNA (ssDNA) covalently linked at the 3' end to the VirD2 protein, enveloped by a sheath of VirE2 protein (molecular mass ~50,000 Da); the latter exhibits similarities to simple viruses that utilize single-stranded DNA as their genetic material. The T-complex then exits the bacterial cell through a pilus constructed from numerous distinct VirB proteins. This transport pilus shows striking similarities, on the one hand, to F-pili, which mediate DNA transport during bacterial conjugation, and on the other hand, to the type III secretion systems of other pathogenic bacteria used for injecting toxins into host cells (e.g., in Bordetella pertussis, Yersinia pestis, or the phytopathogen Xanthomonas campestris). The VirE2 protein fulfills another function during The entry of the T-complex into the plant cell: it integrates into the host cell Plasma Membrane and forms (presumably via oligomerization) a pore through which the T-strand enters the plant cell. The details of this process remain to be fully elucidated.

Within the plant cell, the T-DNA is protected from attack by plant Nucleases by the proteins coating it. Furthermore, both the VirD2 and VirE2 proteins possess distinct nuclear localization signals (NLS1, NLS2) for protein import into The Cell nucleus (nuclear import, see Fig. 7.17): the T-complex is bound by plant importins and translocated into The Nucleus. The subsequent integration of T-DNA into the nuclear genome is not yet fully understood. It occurs apparently at random sites¹; therefore, suitable T-DNA constructs (see Box 7.3) can also be employed for insertional mutagenesis in plants. Following genomic integration, the T-DNA genes are efficiently transcribed by plant DNA-dependent RNA polymerase II. Once the synthesis of IAA and cytokinins sufficiently elevates the cellular levels of these hormones, transformed cells transition from the G₀ phase back into the G₁ phase and thus into the active Cell Cycle (see 7.3.2). Stimulated by the released hormones, neighboring untransformed cells are similarly induced to resume division, As a result of which tumors generally represent a mosaic of transformed and normal cells.

¹ In recent years, data indicating a more specific insertion of T-DNA have emerged. — Ed. note.

Fig. E. Interaction of Agrobacterium tumefaciens with a damaged host cell: 1 — production of the wound substance acetosyringone and activation of the acetosyringone receptor (VirA, VirG); 2 — bacterial chemotaxis; 3 — contact connection with the plant cell (the exact process is unknown, but agrobacteria can synthesize Cellulose and thus firmly bind to cell walls); 4 — activation of inducible vir operons by the phosphorylated VirG protein; 5 — biosynthesis of inducible Vir proteins; 6 — excision of the T-strand; 7, 8 — formation and export of the T-complex, respectively; 9 — translocation of the T-complex into the plant cytoplasm; 10 — binding of plant NLS receptors to NLS1 and NLS2 (NLS stands for nuclear localization signal); 11 — nuclear import. Subsequent steps prior to the integration of T-DNA into the nuclear genome remain unknown (12)

When using Ti-Plasmids lacking three *ops* genes, morphologically normal transgenic plants can be regenerated. Tumor formation is thus suppressed, and it is caused exclusively by the presence of the three *ops* genes. If either gene is deleted, teratomas (organized tumors) are formed. When gene 1 (and/or gene 2) is absent, the additional synthesis of IAA drops, while a large amount of extra cytokinin is produced: the tumor grows as numerous abnormal shoots.¹ If gene 3 is deleted, so that only cytokinin synthesis is suppressed, the tumors grow as teratomas with multiple abnormal roots.² This Organogenesis is comparable to that which occurs when the auxin/cytokinin ratio in the nutrient medium is altered to regenerate plants from callus tissue (see Fig. 7.47).

¹ Based on this symptom, the Mutations are designated *tms1* and *tms2*: tumor Morphology shooty. — Ed. note.

² Similarly, the mutation is designated *tmr*: tumor morphology rooty. — Ed. note.

The course of interaction between Agrobacterium tumefaciens and its host plant has been elucidated in considerable detail, though not yet completely. Of particular importance in this regard are the GENES OF THE vir region of the Ti-plasmid (vir — virulence). The bacterium locates a wounded site on the host plant via chemotaxis. As "wound factors"

phenols, for instance Flavonoids (as in Rhizobium), as well as phenylpropanoid degradation products (see 6.16.1; 6.17.2) have been identified, among which acetosyringone is of particular importance (Fig. E). Acetosyringone binds to one of the two constitutively expressed products of the vir genes — the VirA protein. This is a receptor protein of a typical bacterial two-component regulatory system (cf. 7.6.5.3; Fig. 7.63). This binding affects the phosphorylation of the VirA protein, whose phosphate group is transferred to the second constitutively expressed Vir protein, VirG, which is the regulatory protein of the two-component system. In its phosphorylated form, VirG acts as an active transcription factor that, in turn, activates the transcription of all other vir genes. Upon the combined action of the VirD1, VirD2, and VirC1 proteins at the 5' end upstream of the right-border sequence, a single-stranded nick is introduced into the DNA, with VirD2 + VirD1 acting as sequence- and DNA-strand-specific endonucleases. VirD2 covalently binds to the newly generated 5' end. The T-strand is then cleaved at the left-border sequence and separated from the opposite strand, while the resulting "gap" in the Ti-plasmid is filled in the 5' —> 3' direction via repair synthesis, thereby regenerating a fully intact double-stranded closed plasmid.

All known viroids are phytopathogenic. These are very small (see Fig. 1.10) circular RNA molecules whose replication likely occurs via RNA-RNA replication, and which spread from infected plants to healthy ones mainly as a result of human activity and agricultural practices (such as propagation via cuttings). The Mechanism of viroid-induced disease remains unknown. It is hypothesized that viroids activate certain plant enzymes (e.g., protein Kinases), thereby disrupting Protein Synthesis. A disruption of the interaction between mRNA and Ribosomes is also suspected.

9.3.3. Mechanisms of Pathogenesis

From the pathogen's perspective, two processes are crucial for successful host colonization: (a) host recognition and (b) ESTABLISHMENT OF THE pathogen by bypassing the plant's defense mechanisms or neutralizing them, often accompanied by the weakening of the host plant due to pathogenicity factors produced by the pathogen. From the plant's perspective, successful defense against a pathogen involves (a) "recognizing" the pathogen and (b) the subsequent pathogen-induced defense, which reinforces constitutive structural and chemical defense barriers. This defense against pathogens is discussed in the next section (see 9.3.4).

The pathogenesis scheme characteristic of many compatible host-pathogen interactions, especially those involving fungi, is presented in Fig. 9.14. The pathogen must overcome both race-specific and general defenses against pathogens, or otherwise prevent the activation of the plant's inducible defense responses. The mobilization of race-specific defense does not occur in the presence of incompatible combinations of avirulence genes and host plant resistance genes (see 9.3.1, Fig. 9.13). Host plant substances (and possibly also surface structures) ensure host recognition by the pathogen, which triggers the activation of (typically numerous) pathogenicity genes. These gene products include many lytic enzymes, such as cutinases, cellulases, and polygalacturonases, which serve to penetrate the cuticle, cell walls, and middle lamellae. Enzymatic preparations from phytopathogenic fungi (e.g., *Trichoderma viride*) are used commercially for The production of protoplasts (plant cells lacking a cell wall). Many pathogens produce toxins, among which both host-specific and numerous non-host-specific ones have been found (Fig. 9.15).

Fig. 9.14. Schematic diagram of The Development of a compatible interaction between a pathogen and its host, leading to plant disease

Fig. 9.15. Examples of non-host-specific phytotoxins (A — courtesy of C. Oecking): A — fusicoccin, a wilting-inducing toxin from *Fusicoccum amygdali*; B — coronatine, a chlorosis-inducing toxin from phytopathogenic strains of *Pseudomonas syringae*

Host-specific toxins include the chlorine-containing pentapeptide victorin from the fungus *Cochliobolus victoriae*, which specifically attacks the oat cultivar 'Victory'. Victorin blocks oat mitochondrial Glycine decarboxylase (see Fig. 6.75) and thereby disrupts Photorespiration. Non-host-specific toxins include, for example, the aforementioned fusicoccin (see Fig. 9.15, A) from the fungus *Fusicoccum amygdali*, a potent activator of the P-type H⁺-ATPase in the Plasmalemma, which thus stimulates proton Transport Across the membrane and, consequently, strong stomatal opening. Fusicoccin therefore acts as a toxin that induces wilting and general weakening of the host plant. Coronatine, a chlorosis-inducing toxin from phytopathogenic strains of *Pseudomonas syringae* (Fig. 9.15, B), has proven to be a structural analogue of jasmonates (see 7.6.6.2). It stimulates — like jasmonic acid administered at very high doses — enhanced

Ethylene production and, consequently, senescence. Since coronatine is not transported within the plant, chlorotic spots develop at the sites of bacterial proliferation. The pathogen can multiply extensively in these weakened areas (typically on leaves).

Some microbial pathogens produce phytohormones and thereby influence plant developmental processes. Cytokinin-induced "witches' brooms" were mentioned above (see 7.6.2.3). In some cases, auxins are involved in gall formation (e.g., during cyst and root nodule formation upon nematode infection, see 9.1.1, as well as in clubroot of crucifers caused by *Plasmodiophora brassicae* and tumor formation on infected young maize kernels caused by *Ustilago maydis*). However, it remains unclear whether auxins are supplied by the pathogen or produced by the host plant itself in response to a pathogen signal.¹ Gibberellins, secreted by certain fungi and causing excessive internode elongation in host plants (e.g., *Gibberella fujikuroi* in rice, *Sphaceloma manihoticola* in cassava), have already been mentioned (see 7.6.3). The Role of auxins and cytokinins in tumor formation and hairy root disease induced by *Agrobacterium* is particularly well studied (see Box 9.2).

¹ In the case of *Pseudomonas syringae* infection, it has been conclusively demonstrated that auxins are synthesized within the bacterial cells. — Ed. note.

Finally, some pathogens release suppressors of the plant's inducible general defense (see Fig. 9.14). This general defense is triggered by non-host-specific elicitors (see 9.3.4), at least some of which are recognized by specialized plant receptors. The fungus *Mycosphaerella pinodes* produces glycopeptides that bind to the pea glycoprotein elicitor receptor, thereby preventing pathogen recognition by the host plant.

Alongside the pathogenesis mechanisms shown in Fig. 9.14, many pathogens employ more specialized strategies that, however, cannot be

covered within the scope of this book. The best-studied host-pathogen interaction is that between *Agrobacterium tumefaciens* and its host plants, leading to the formation of crown galls and hairy root disease (see Box 9.2). Today, *Agrobacterium tumefaciens* is typically used for the introduction and stable Integration of Foreign genes into the plant genome (see Box 7.3).

9.3.4. Pathogen Defense

In addition to pre-formed (constitutive) defense mechanisms against microbial pests (such as the cuticle, cell walls — especially when lignified —, and the accumulation of toxic substances in cell walls and vacuoles, e.g., saponins, phenols, and Quinones; cf. 6.16), plants possess numerous inducible defense responses that differ in their Mechanisms of action as well as in the structural and Chemical Components of their response to pathogen invasion. Invasion-induced antimicrobial Organic compounds are termed phytoalexins; many phytoalexins are derivatives of terpene or phenylpropanoid metabolism (see 6.16.1; 6.16.2; Fig. 9.16). Inducible structural components include: the deposition of callose — a (β1 —> 3)-glucan — at the site of pathogen penetration, increased cell wall thickening, and enhanced lignification.

Fig. 9.16. Examples of phytoalexins. The antimicrobial activity of phytoalexins was demonstrated particularly clearly using pisatin. The virulence of Nectria haematococca, a fungus pathogenic to peas, depends on its ability to enzymatically detoxify pisatin (the structural formula of the soybean phytoalexin glyceollin is shown in Fig. 6.118)

As already mentioned, plant defense against pathogens involves both general, non-pathogen-specific and (partially race-) specific defense mechanisms (see Fig. 9.14; 9.17). A matching gene-for-gene combination between a plant resistance gene and a pathogen avirulence gene (race-specific incompatible interaction, see Fig. 9.17) triggers a hypersensitive response in the plant. This response begins with the rapid and massive production of toxins (especially phenols), the release of reactive oxygen species (such as O^-₂) and H₂O₂, and results in localized, programmed cell death, which also kills the pathogen. The hypersensitive cell death manifests as small necrotic (dead) tissue spots, frequently on the leaves.

General defense is triggered by non-host-specific elicitors, which are generated as degradation products of pathogen cell walls or membranes and/or plant cell walls during pathogen penetration. Numerous elicitors have already been identified (e.g., oligogalacturonides, oligoglucans, glycopeptides, oligochitosans, fungal steroids such as ergosterol, Glycolipids, etc.). Sometimes, concentrations of less than 10^-9 mol/L are sufficient for elicitors to induce general defense responses against pathogens. These include, on the one hand, local processes such as phytoalexin formation, cell wall thickening, and lignification, and on the other hand, systemic processes such as the production of special PR proteins (PR, pathogenesis-related), the exact function of which, however, remains unclear.¹

¹ PR proteins include plant chitinases and glucanases. One of the functions of these enzymes is to degrade the pathogen's cell wall to release elicitors. In addition, PR proteins include SAR short Peptides (systemins), which act as signaling molecules responsible for systemic acquired resistance throughout the entire plant (see 9.3.1). — Note by the Editor.

Fig. 9.17. Diagram of the development of an incompatible interaction between a pathogen and its host leading to pathogen defense. Left: race-specific incompatibility (cf. Fig. 9.13), resulting in hypersensitive cell death. Right: non-host-specific resistance, stimulated by the release of elicitors and (in some cases) jasmonates, which involves general local plant defense reactions as well as systemic defense responses

It is assumed that elicitors bind to plant receptors located in the plasmalemma, initiating a signaling cascade that leads to pathogen-inducible genes. The Nature and mode of action of these receptors are still poorly understood. Some elicitors appear to be capable of direct integration into cell membranes, acting as ion-conducting pore formers and thereby reducing the plant Membrane Potential. How this leads to the activation of defense genes remains unknown, as is The Nature of the systemic signals that originate from the site of pathogen infection and induce specific defense reactions throughout the entire plant.

In many cases, it has been demonstrated that jasmonates (see 7.6.6.2) serve as a link in the signaling chain connecting elicitor perception to defense Gene Expression. Arabidopsis thaliana mutants defective in jasmonate biosynthesis were found to be more susceptible to microbial pathogens. In other cases (e.g., viral infections), the involvement of salicylic acid (see Fig. 6.114) is discussed. However, it likely functions as a phytoalexin rather than a signaling molecule. Salicylic acid could potentially play a hormonal role in systemic acquired resistance (SAR, see 9.3.1).