It is not wrong to eat plants for the same reason that it is not wrong to kill bacteria. Every time I brush my teeth, I kill thousands of bacteria, yet I see nothing wrong with doing so. 

The distinguishing characteristic between animals and bacteria is that animals are conscious beings capable of feeling pain and suffering.  Animals are capable of feeling happiness, joy, and sorrow.  Animals are capable of feeling a desire to live.

Bacteria, on the other hand, just like plants, and just like rocks, do not possess anything similar to a nervous system, nor do they exhibit any behavior which would indicate that they possess consciousness. Bacteria are not capable of feeling pain or suffering. Bacteria are not capable of feeling a desire to live. Bacteria, just like plants, and just like rocks, are not capable of feeling anything whatsoever.

When you kick a rock, you do not have to worry about whether or not you are inflicting suffering on the rock, or interfering with the rock's desire to live, since rocks are incapable of such feelings. It is for this reason that it is not immoral to kick rocks, and it is for this same reason that it is not immoral to eat plants.