Generally, plecs leave plants alone, as far as food goes... but large specimens can still damage them nonetheless. Young plants and cuttings are uprooted, for example. Partly it's because these fish are big, but also because they are incredibly clumsy.
My Panaque does further damage when eating algae. It has very specialised teeth, and these scratch anything, even the acrylic casing of the filter. Needless to say, the stems and leaves of any plant it latches onto gets hammered. Surprisingly, the plants that work best with it are those species too flimsy to hold its weight, eg., Vallisneria and Cabomba. Anything that is sturdy it will try and rasp algae from, destroying the plant in the process. Examples are Amazon swords, Java ferns, and Anubias.
Cheers,
Neale