So what you're really talking about, then, is not consciousness, but conscious choice.
You can't make conscious choices without consciousness.
Plants can't make conscious choices. Plants are not conscious.
But that wasn't the original proposition, here. And there is nevertheless debate on the fact that we even have a choice. (I believe that we do.) Also, at what point do we determine that conscious choice is taking place?
When it is preceeded by some kind of analysis of the situation to some extent or another, which involves several options on how to move forward being explored, and then chosing one of those options.
When a plant turns towards the sun, it is not first pondering the situation and reflecting on its options.
When my dog turns his nose up at something edible is that a choice? Or is he smelling something that is "automatic" brain is telling him is bad for him to eat? When a cat stops playing with a toy is that a "choice"? or is it some automatic subconscious mechanism?
It might be in some situations and it might not be in others.
We humans also have "automated" / instinctive responses. Like jumping up when hearing a loud unexpected noise.
I think the lines here are far more obscure than our definitions would imply.
I agree the line is obscure. But that is only a problem to really determine from which point on, organisms can no longer be called conscious. To identify that exact "border", if you will.
But I don't think it's hard to identify on the extreme ends.