What is the symbolism of Estragon and Vladimir eating the radish-like plant?
Your question reminds me, first off, of a famous quotation of Beckett's: "What do I know about Man's destiny? I could tell you more about radishes."
A radish, as a food, is spicy, biting, and clears your nasal passages. A radish is a product of the land and of the soil. Man is also a product of the soil from which he springs. The fact that a radish is biting and powerful might be symbolic of man's ability to be like a "radish" or it could imply Beckett's harsh and truthful message: man's plight is a bitter and acidic affair since we have to make our way in a chaotic universe....
In the play there's a point where Estagon and Vladimir are concerned with carrots and turnips. Estragon requests a carrot, is passed a vegetable, takes a bite and is surprised who angrily declares, "It's a turnip!" (14). Vladimir apologizes and "could have sworn it was a carrot" (14). This might suggest that we are never really sure of things and oftentimes the most banal and commonplace can shock or surprise us. It might be symbolic of how we "rummage" through our own lives to find things that sustain us or keep us alive. We could also quote Forrest Gump here too, "..you never know what you're gonna get..."