How Pathogens Attack

1. How Pathogens Attack

2. Reading Assignment: Chapter 26

3. Pathogen Attack Concepts • Successful pathogens enter the host, overcome plant defenses and cause disease • Pathogens use strategies such as mechanical pressure, enzymes, toxins and hormones to attack plants and overcome plant resistant mechanisms.

4. Selection Pressure • Competition for nutrients is a source of selection pressure that promotes parasitism on plants • Microorganisms have evolved as saprophytes, facultative saprophytes, facultative parasites, endophytes, necrotrophs, and biotrophs. Antibiosis

5. Endophytes • Endophyte - microorganism living inside another one. The association runs from antagonism to mutualism. • Neotyphodium (Acremonium) species provide mutualistic benefits to their grass hosts – produce alkaloids and phenolic toxins that deter insects, nematodes and some fungal pathogens.

6. Hemibiotrophs Hemibiotrophs are microorganisms that have developed with plants where early in their life cycle they have a dependence on living cells but later can survive as facultative parasites or facultative saprophytes.

7. Sequence of Pathogenesis Events • Dissemination/ contact • Penetration • Establishment of food relationship within host • Invasion

8. How Pathogens Attack

9. Dissemination/ Contact • Propagules of pathogen must come in contact with host (efficient method of dissemination or produce high number of propagules. • Propagules must adhere to cell surface. The fungus Penicillium produces millions of conidia on a single orange.

10. Penetration • When spores of fungi germinate, the tip of the germ tube may swell and from a specialized structure known as an appressorium. • An appressorium adheres to the cell surface and has much higher turgor pressure than found within normal hyphae.

11. Penetration • From the appressorium, a penetration peg (pp) {much narrower than regular hyphae} and penetrates the cell walls. • Once the cell wall is breached, hyphae from the tip of the pp resume their normal size. • Penetration of the cell wall by the pp is aided by mechanical and biochemical mechanisms.

12. Establishment of food relationship within host • Haustorium – a specialized feeding cell that invaginates (does not rupture the cell membrane). • Digestive enzymes pass out of the haustorium, across the cell membrane and into the cytoplasm of the cell • Food passes across the cell membrane from the cell to the haustorium. Intracelluar hyphae of a downy mildew fungus with haustoria

13. Invasion Xylella bacteria colonizing xylem vessels. Soft rot of potato

14. Chemical Attack • Pathogens may use chemicals to overcome plant defenses to successfully penetrate, establish a food relationship and invade the host. • These chemicals include enzymes, hormones, toxins, and extracellular polysaccharides.

15. Enyzmes- used to attack host cells • Pathogens may mobilize numerous enzymes to attack walls of cells (primary and secondary) or the middle lamella. • These enzymes include cutinases, pectinases, cellulases, hemicellulases, ligninases, proteases, lipases, and amylases.

16. Hormones • Some genes involved with defense responses in plants are downregulated by hormones and susceptibility to some pathogens. • Hypersensitive response can also be inhibited by hormones.

17. Toxins • Phytotoxins are almost always secondary metabolites. • Selective toxins have a specific activity, have low molecular weights and are agents for virulence or pathogenicity.

18. Toxins • Sclerotium rolfsii produces a nonspecific phytotoxin, oxallic acid, that is kills cells prior to invasion by the fungus. • S. rolfsii is a necrotroph.