Hi everyone,
I study powdery mildew-arabidopsis interaction. I have screened three mutants which show susceptible to powdery midew. But when I stain the infected mutants leaves with DAB, I found more H2O2 in interaction site than Col-0. H2O2 is supposed to be a product to restrict fungi growth. So it is very surprised that also fungi grow more happily on mutants leaves with more H2O2.
My view is that my mutants may shut off some important defense pathway. And the plant detect this shut-off upon fungi attack and make more H2O2 to compensate. But the overall effect is more susceptible.
What do you think about this?

I often got some bizarre results.

Thomas

Your interpretation seems to be sensible, it is more or less is what I would have guessed. I suspect, however that the H₂O₂ production is in response to the mildew infection rather than as a response to a down regulation of another defense mechanism. I would also guess that resistance to a fungal infection like this involves a multiple response rather than just one gene, so it would be fairly logical to see a single gene knockout also producing other responses.

Bob, I just think about maybe it matters the way I quantify the H2O2 production.
My method is to count about 60 powdery mildew colonies. If I see DAB staining there--even a tiny dots, I count it as "+", So Finally, I make a graph on the percentage of +/60. I think there may be better way to quantify. Do you have any idea?

There is probably a commercial test for peroxide available, I suppose you could use something like that.

The other thing to do would be to get a similar tissue from another plant that doesn't express peroxide, and stain that with DAB at the same time as you do your staining, so you can see what a background/non-specific staining looks like.

some parasitic plants need H2O2 as a germination stimulant, if that helps you somehow.