This all comes from delusion of SELF. Plant does not have a self, human does not have a self. But consciousness experiences the human condition, consciousness does not experience a plant condition. People look at a plant and they imagine that there is a self to it, and that it is ultimately identical to their self. There is no self to the plant, it is your own mind that is playing tricks on you.
If a plant is sentient, when is its consciousness born? When does it pass away? How does the plant "sense" or "feel" (in Buddhism, feeling is a requisite of sentience--sorry)? What are the perceptions of a plant? What are the intentional formations of a plant like, how does it even get those formations?
A plant touches water, it absorbs water, all plants behave in this way and chemically they grow in response. Is a dried sponge sentient because it absorbs water and expands? Sometimes a tree will sprout other trunks with identical DNA, sometimes spawning entire forests of connected trees, all reacting in a passive and non-intentional way. Even if you were to say that a plant is sentient, it cannot experience its own world and if you cut off its life, what happens? A chemical reaction ensues, not a conscious one. It is similar to removing the leg from a table, the table falls every time. It does not intend to fall or not fall, it does so inanimately. Similarly, plants respond inanimately, simply by stimuli.
Plants are organisms just like those found in the human body. When something happens to a system in the body, chemical reactions take place and the body reacts accordingly regardless of "intentional formations" or "perception" in the being. Consciousness is aware of this functions, because it is experiencing them, because of birth due to karma. The reason consciousness doesn't normally experience being a plant is because it can't experience without feelings and perceptions.
Consciousness can't become a plant because karma doesn't lead to non-karmic existence, that would be the cessation of karma and thus the being would be released from existence, and therefore could not be a sentient plant. By that logic, if a plant is sentient and thus released from existence, therefore it is not a living being and cannot experience suffering as a plant. The same applies to rocks, tables, cups, or any inanimate objects. Again, in a plant where does the consciousness begin? There is a seed which contains chemicals and nutrients required for growth and sustenance, which cannot grow without soil and water. Fire starts with wood, it needs air and dryness, and an igniting force (for a plant, fertilization of the seed). Then the seed grows, as a fire grows. There needs to be an understanding of karma and consciousness in relation to sentient beings. To call a plant sentient is okay, but where a plant differs from the requirements of a being with consciousness, perceptions, volitional formations, and sensations, united in a living way, this needs to be accepted. The problem with calling plants "living beings" Buddhistically is that it leads to delusions and misunderstandings of the Dharma. That is why it is wrong view. By definition, if it didn't affect at all to enlightenment whether or not plants were sentient, it wouldn't matter.
Plants need to exist because in OUR world, there has to be organic life in order for us to exist. Karma leads to this existence, and since there is the possibility of plantlife, we as sentient beings realize that possibility because our karma leads us to rely on plantlife. In modern society, ideas of plant sentience are the results of preconceived notions based on assumptions about cells and scientific discoveries. Plants don't need to be sentient, they have no karma--no intentional action. It is natural for us to identify ourselves with plants because of how we see them, that it because of our own karma, our own existence in this world, and our inherent reliance on plantlife. In biology we see that all life has common origin and is interdependent. This is in no way whatsoever against ancient Buddhist teachings. Of course we share common origin, how could our human world possibly exist for us--the beings who have the karma for it--if there were no plants? The karma that led us to human lives relies on plants, and when plants come to be it is because beings are being born into worlds where plants are a biological phenomena derived from the processes of Biological Law (Bija Niyama), one of the five sets of law, which describe the consistency of reality. The others are Seasonal Law, Karmic Law, Natural or Dharmic Law, and Psychological Law.
The mind-matter reality exists dynamically according to these five orders. What sentient being has no perception and no karma, has no feeling? How can a being without karma "experience" the life of a plant? This all comes from delusion of SELF. Plant does not have a self, human does not have a self. But consciousness experiences the human condition, consciousness does not experience a plant condition. People look at a plant and they imagine that there is a self to it, and that it is ultimately identical to their self. There is no self to the plant, it is your own mind that is playing tricks on you.