Precisely how the pla[cebo]s work should not be of as much concern to you as what they can do for you.
One of the most puzzling facts of medicine is the placebo effect: namely, that a substantial proportion of patients report feeling better after receiving a "sugar pill," or some other treatment with no known benefit for their illness. Between 30 - 60% of patients with illnesses ranging from arthritis to depression report a substantial improvement in their symptoms after receiving a placebo. It is not clear that placebo can "cure" any illness, but the power of the placebo effect in improving symptoms and reducing suffering is impressive.
You use armchair skepticism, but you never bothered to experiment with the plate...
There is no way to use "placebo" as an explanation with plants.
Furthermore, one's own experience cannot be reduced to neural signals; consciousness is irreducible; kindly reference again Nagel's arguments and the latest research in mind science:
http://www.firstthings.com/blogs/firstthoughts/2013/03/thomas-nagel-and-the-anathema-of-questioning-materialismhttp://www.noetic.org/library/book-reviews/irreducible-mind/