Pleco's don't eat plants. What you do see is on some of the broader leaf plants like Amazon Swords or large Anubia a pleco will go and rasp on the leaves to remove algae and biofilm which also scrapes some of the leaf tissue off making you think they are eating the plant.
If your looking for a food item for your pleco I cut a 1/4" thick slice of Zucchini, microwave it for about 10 seconds, then immediately rinse under cold tap water (blanching) to stop the cooking process. Then either tie it on a rock and lay it on the substrate or I put it on a veggie clip and hang it on the side of the glass after lights are off for the pleco to eat on. My pleco's know right where to go and wait for me to hang it.