Genetics of Plant-Pathogen Interactions (Plant Immunity) Topics on Systemic Acquired Resistance (SAR)

Transcription

1 Genetics of Plant-Pathogen Interactions (Plant Immunity) Topics on Systemic Acquired Resistance (SAR)

2 Major types of plant pathogens - Bacteria - Fungi (>80% loss) - Viruses - Viroid Can you name a few important plant diseases?

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4 Most plants are resistant to most pathogens Basic fact: In nature, most plants are healthy most of the time. If disease occurs, it is actually restricted to a few plants and affects only small amount of tissue. So disease is the exception. Why?

5 Basic Terminology Pathogenesis: Process of infection, colonization and reproduction Pathogenecity: Ability to cause disease Virulence: Severity of disease Plant defense system: - Preformed (nonhost) - Activated or induced

6 Activated plant defense systems - Local responses: hypersensitive response (HR) - Systemic responses systemic acquired resistance (SAR)

7 Local responses: hypersensitive response (HR) HR is a pattern of localized cell death within the plant tissue at the site of infection. This local cell death blocks pathogen growth by depriving biotrophic pathogens of nutrition.

8 Systemic Acquired Resistance (SAR) - SAR + SAR

9 How is SAR established? How do you approach this question in the 1960s? Systemic resistance???? Necrosis?? Local HR

10 Pathogenesis-related (PR) proteins

11 SDS polyacrylamide gel electrophoresis Isoelectric focusing

12 Pathogenesis-related (PR) proteins PR-1: cysteine-rich proteins, with low antifungal activity PR-2 (β glucanases, BGL2): (1,3)-endoglucanase activity PR-3 (chitinases) PR-4 (chitin-binding) proteins: have antifungal activity PR-5 (TL): thaumatin-like proteins, sweet tasting, resistant to protease degradation, may affect membrane permeability How would you find PR proteins/pr genes today?

13 Microarray Hybridization 1. Synthesize gene specific probes for a large number of genes 2. Array the probes on a chip 3. Isolate RNA from a specific tissue/time 4. Make labelled cdna from the RNA 5. Hybridize the labelled cdna to the array of probes and detect which probes hybridize to the population of cdnas Ehlting et al., 2005, Plant J. 42: 618

14 RNA Seq 1. -isolate RNA from cells from different tissues or the same tissue at different time points. 2. -make cdna from mrna using reverse transcriptase. 3. -sequence the cdna population.

15 Can we enhance plant resistance to pathogens by overexpressing PR proteins? Systemic resistance??? PR gene expression? Necrosis?? Local HR

16 Are they all important for plant resistance to pathogens?

17 distance from the center What can you conclude from the data?

18 Is SA a signal molecule for SAR? How would you test it?

19 Is SA a signal molecule for SAR? Test for sufficiency: Is SA sufficient to induce SAR? Systemic resistance PR gene expression SA Local infection Necrosis? SA

20 Effects of 200 μm salicylic acid (SA) treatments and inoculation of Fusarium oxysporum f. sp. lycopersici (Fol) on hydroponically grown tomato. (a) Vascular browning; (b) Leaf yellowing wilting. From: Plant Physiology and Biochemistry Volume 47, Issue 7, July 2009, Pages

21 Is SA a signal molecule for SAR? Test for sufficiency: Is SA sufficient to induce SAR? Test for necessity: Is SA required for SAR? Systemic resistance PR gene expression SA Local infection Necrosis? SA

22 WT NahG NahG: salicylate hydroxylase from Pseudomonas fluorescens

23 Will SA save the day?

24 Ryals group, Novartis Bion TM

25 What is the mobile signal for SAR? Is SA the mobile signal? Systemic resistance PR gene expression SA? Local infection Necrosis SA?

26 Is SA the mobile signal? How do you test this hypothesis? Design an experiment to test this hypothesis Think about the expectations of the experiment Hint: - NahG plant can be used - Grafting in plants

27 nahg WT WT nahg What can we conclude from the data?

28 nahg transgenic plants can still produce the mobile signal for SAR. SA is not the mobile signal for SAR.

29 Summary on what we learned about SAR 1. SAR is a pathogen-induced defense response against broad-spectrum of pathogens. 2. PR genes are highly induced during SAR. They are used as molecular markers for defense responses. 3. SA level increases locally and systemically during SAR. 4. SA is required for SAR. 5. SA and its analogs are sufficient to induce SAR. 6. SA is most likely not the mobile signal for SAR. 7. Overexpression of a single PR gene is not sufficient to establish SAR.

30 Systemic resistance PR gene expression? SA?? Local infection Necrosis? SA

31 Proposed experiments to study SAR Question or hypothesis Proposed experiments/genetic screens 1. Can Arabidopsis be used for Test SAR responses in A.t. SAR studies?

32 Proposing experiments to address questions on SAR Hint: For each step in a signaling pathway, there are (almost always) not only positive regulators, but also negative regulators. Think about what kinds of phenotypes you would expect for l-o-f mutants of those regulators before you design a genetic screen.

33 Proposed experiments to study SAR Question or hypothesis Proposed genetic screens 1. Can Arabidopsis be used for Test SAR responses in A.t. SAR studies?

34 Proposed experiments to study SAR Question or hypothesis Proposed genetic screen 1. Can Arabidopsis be used for Test SAR responses in A.t. SAR studies? 2. What are the + regulators of SAR? Non-SAR mutants 3. What is upstream of SA synthesis? Low SA mutants (SA synthesis enzymes, + regulators) 4. What are the - regulators of SA synthesis? High SA mutants 5. What are the - regulators of HR? Contitutive HR mutants 6. What are the + regulators downstream of SA? Spray SA, no-sar mutants 7. What are the + regulators of HR? No-HR mutants

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