We know for a fact that for humans consciousness only exists in parts of the brain and can be turned off using drugs, sleep and accidents.

It is almost certainly the same for all animals with a brain.

Given the fact that plants have no brain it is a reasonable assumption they have no consciousness. It may be wrong, but given all evidence it is, so far, the best assumption.

We usually call assumptions based on our best current understanding scientific facts.

Yes, there is no evidence to believe they are “conscious” (well, not exactly sure what is meant by conscious here since that word is closely related to religious beliefs in many cases, but assuming it means sentient), they lack all known biological features that could possibly lead to sentience, and there has been no evidence found so far to suggest there is any sentience among plants.

Also, considering they’re rooted to the ground and cannot move there is no evolutionary advantage to sentience (and actually there are massive disadvantages since it will use energy for sentience which serves no purpose). Now, just because there isn’t an evolutionary advantage, or even if it’s an evolutionary disadvantage to a property doesn’t automatically mean it doesn’t exist, but given that there’s no other reason to believe it does exist this is just another piece of the puzzle that shows it doesn’t even need to exist.

It’s a fair assumption because there’s no genetic benefit to being a smart plant. Why would nature make that?

Opposable thumbs mean we have the ability to use smarts, so genetics walked the path to consciousness over many generations. A smarter horse is fine and all but not that beneficial. Being faster or stronger or sexier is probably better. Same with a plant: get more nutrients or sunlight. There are cheaper ways than being smart to do that.

The scientific consensus is that, given that plants don’t have a nervous system, that they can’t process sensory input that is integrated and that produces some experience, coupled with the lack of subjective experience and the chemistry-based explanations for observed behaviors (such as following light etc), under the current definition of “consciousness”, plants are not conscious.