There is only one scene from the manga I remember. It was two people, probably brothers, wandering around. They were hungry and stumbled upon a giant humanoid shaped plant laying on the ground. In a daze, they ate it and became sleepy after. The plant disappeared when they woke up and both of them became immortal?

The characters mentioned were not the main characters of the manga. The plant/humanoid is supposed to be like the progenitor of all the rest of the plants. All of them have a unique name, number and ability associated with it. The humanoid one was numbered 0 or 000 and its ability was implied that it gave the people who ate it immortality.

It was probably a fighting/shonen fantasy adventure manga. I think one of the people who ate the original plant and became immortal was a villain in the story. It was possible he made all the other plants numbered after 0 and 0 was the only one that was in the world naturally.