It seems narrow and premature to assume that organisms without a brain or nervous system like ours most likely can't be conscious or intelligent on some level (or to act as if one has perused and analyzed ALL the evidence and arguments on this subject). I once heard an interesting story which I'll paraphrase, because it highlights how bizarre and contradictory our perspective can be in this regard. If you created a robotic beetle that could sense and respond to the environment, make decisions, fly, metabolize, reproduce, and so on, those in the artificial INTELLIGENCE community would be absolutely baffled at its capabilities upon witnessing it in action. Yet a real living beetle, far more complex and advanced than the robotic beetle, is somehow less remarkable to them and not worthy of the word.
As history has shown time and time again - and this includes the evolution of science - the universe is always smashing and transforming our conceptual models of how things work, and the limits we impose on how nature works are constantly being redefined (for example, it seems like every month there is research on an 'impossible new galaxy'...or 'impossible star defies physicists models'...etc). Science fiction can quickly become science fact, and what you might call supernatural one decade could easily become consensus reality the next. We should be careful of thinking that we have something ever truly nailed down either way.
I find it odd that people dismiss the idea of plants having some level of consciousness or intelligence by reducing all the baffling things they do to being '
merely' biochemical reactions (or X set of physical processes) crafted through nothing more than the mindless process of natural selection. We know that they can sense light, moisture, gravity, temperature, water, chemicals, touch, sounds, infections, parasites, sense unstable branches (like vines on a tree), ward of predators, attract beneficial organisms, warn other species, heed warnings from others, and so on, and respond to this stimuli through a wide variety of behaviors. We can't see it, but standing in the woods we are immersed in a biochemical soup of pant communication with the environment and symbiotic relationships.
If all we do is reduce everything we discover about them, or ourselves, in this way, then you might as well deduce that human consciousness and conscious human communication is merely an illusion in this regard as well...An illusion generated by neuro-chemical processes crafted through natural selection that give the illusion of consciousness or intelligence, but which is ultimately
merely X sets of physical processes. And indeed, there are many reductionists who jump on that philosophical bandwagon...This branch of left-brained reductionist "merely-ism" has existed for centuries and often fails to look at the wider context, and how the whole is greater than the sum of its parts.
Similarly, to think that the fact that DMT is in the human body/brain and found throughout the planetary ecosystem is not remarkable because we understand the chemical pathways that lead to its creation is to completely miss the point entirely IMO. I'm not saying I believe 100% that it is there for inter-species/planetary communication, or that plants have consciousness, or any of that; I'm saying that this reductionist approach does not suck the fascination out of things like people try to portray it as doing (an approach which seems to be partially a reaction to the suppression of the church long ago). We don't even know what DMT is DOING in our own bodies in the first place, much less what it is doing in the thousands or millions of species throughout the biosphere. Understanding the metabolic pathways that lead to its creation does not make the experience or it's ubiquitous presence throughout nature any less remarkable or mysterious. If you think it does, I'd suggest a re-acquaintance with that dimension sometime soon

...the true profoundness of the implications of that experience fade in our awareness as the time between the last deep experience grows.