Lecture 28: Bacterial disease of plants (and animals)

1. Saprophytes - grow in soil on dead organic matter 2. Epiphytes - grow on external plant surfaces 3. Pathogens - grow on internal plant surfaces causing disease a. Biotrophs - keep host alive b. Necrotrophs - kill host as a way of life c. Hemibiotrophs - switch from biotrophy to hemibiotrophy

• Pseudomonas • Xanthomonas • Erwinia • Agrobacterium These are “facultative pathogens”, I.e: a. Exist as saprophytes or epiphytes b. Migrate into plant tissues, replicate, and cause disease Disease e.g; fruit rot, wilt, leaf blight, stalk root

• Disease occurs naturally • Isolate and grow in pure culture • Inoculate healthy plant • Development of original disease • Re-isolate bacterium

Necrotrophic: • Erwinia carotovora a. Host range: Wide b. Disease: Soft rots Biotrophic: • Erwinia amylovora a. Host: Rosacaeae b. Disease: fire blight • Ralstonia solanacearum a. Host range: wide b. Wilts • Xanthamonas campestris pv. vesicatoria a. Host: pepper, tomato b. Disease: leaf spot

Virulence: The severity of an infection 1. Inoculate host (via; syringe infiltration, spraying, or dripping) 2. Allow disease development 3. Take samples 4. Grind in buffer 5. Make serial dilutions 6. Plant, incubate, and count colonies Example: avrRPM1 gene from Pseudomonas syringae pv. maculicola is required for full virulence on Arabidopsis

• T2SS is a two-step process: a. SecYEG or Tat system transport proteins into periplasm b. Dodecameric secretin GspD transports proteins across the outer membrane • Four components a. Cytoplasmic ATPase (GspE) b. IM platform (4 membrane proteins - GspC, GspF, GspL, GspM) c. Periplasmic pseudopilus (GspG) d. OM complex (GspD) aka ‘secretin’ IM = inner membrane OM = outer membrane Tat system = twin-arginine translocation Gsp = general secretion pathway

• Free-living bacteria; T2SS secretes enzymes that degrade compounds in the environment (incl. proteases, lipases, Chitinases) • Pathogens that require T2SS e.g., a. Yersinia enterocolitica yts1E (ATPase-) mutants show reduced infection in mouse

T2SS secretion of cell wall degrading enzymes by “necrotrophic” Erwinia species (soft rots). Best studied: E. chrysanthemi and E. carotovora • Pectate lyase, endo- and exo- enzymes (at least 9) • Pectin acetylesterase • Pectin methylesterase (2) • Polygalacturonase • Cellulase (2) • Endoxylanase • Protease (at least 4)

• T3SS a. Multi-protein nano-machine, structurally related to bacterial flagella b. Transfers “effector proteins” from bacterium “into host cytoplasm” • Components (in salmonella) include: a. Hexameric ATPase (InvC) b. Sorting platform (SpaO, OrgA, OrgB) c. Secretin (InvG) d. Needle (Prgl)

T3SS required for virulence of some animal/human pathogens. E.g., Shigella flexneri T3SS effector IpaB induces apoptosis in macrophages

• Bacterial Hrp proteins form a T3SS • Several Hrp proteins form a pilus through which effectors (incl. Avrs) are translocated

1. AvrBs3, PthA, AvrXa7, and related proteins from Xanthomonas species make up TALE family 2. TALE: "transcription activator-like effectors" 3. Exhibit sequence specific DNA binding 4. Act as transcriptional activators

• Suppresses PAMP-triggered immunity (PTI) a. FLS2 recognition of flagellin b. FLS2 binds to flagellin (a protein of bacterial flagella) c. Binding causes intracellular signalling, turning on defence against bacteria

• “Ti” plasmid is inserted into plant cell target containing “tDNA”, which gets incorporated into plant chromosome • EXAMPLE: “Crown Gall” a. Caused by Agrobacterium tumefaciens

• aux - auxin biosynthesis genes • cyt - isopentyl transferase (involved in cytokinin biosynthesis) • tm1 - tumour size regulation • ocs - octopine synthase

• Exopolysaccharides - extracellular polysaccharide (EPS) • Form a capsule around most bacterial cells

• Exopolysaccharides - extracellular polysaccharide (EPS) • Form a capsule around most bacterial cells

• Req. for virulence of some animal pathogens. E.g., a. Klebsiella pneumoniae K antigen (capsular polysaccharide)

1. e.g., Xanthomonas campestris pv. campestris (Xcc): 2. EPS suppresses Ca2+ signalling, and thus, the expression of plant defence genes like "PR1"

• B subunit of toxin binds to GM1 gangliosides on surface of gut epithelial cells • A subunit is internalised and activates adenylate cyclase • increase intracellular cyclic AMP levels over activate protein kinase A • PKA phosphorylates cystic fibrosis transmembrane conductanse regulator (CFTR) causing loss of Cl-, Na+, and HCO3- and H2O from the cell

• coronatine a. mimics the plant hormone “jasmonic acid” b. suppresses stomatal closure (a consequence of PTI)