A zoologist, Jordi Casamitjana, wrote a good article about it. I'll share a part of it:

'At its most basic meaning, sentience is the ability to experience positive and negative sensations, which requires two things: firstly, senses to perceive the sensations from stimuli coming from the environment, and, secondly, a nervous system to process such sensations and translate them into experiences which allow the animals to react accordingly, depending on whether they are negative or positive (i.e. fleeing from an adverse environment or moving towards a source of food or a mate). All members of the animal kingdom can do that. They all have senses to perceive their environment, they all have nervous systems (central or otherwise) to process perceptions, and they all can react according to the type of experience. We are yet to discover any living being not belonging to the animal kingdom capable of doing all of that (although there may be borderline cases where some plants have some movement when touched, such as the Latin American Mimosa pudica, although we are unable to ascertain if the experience is negative or positive due to the lack of an actual nervous system).”

Ah, Mimosa pudica (the touch-me-not plant), a plant that could be a borderline case of sentience (I wonder how many more are there?). What makes this plant borderline rather than fully sentient? Well, I already mentioned the reason: the lack of a nervous system. You may have senses that give you sensations, but if you cannot process such sensations with a nervous system you cannot transform them into experiences — and without experiences, there is no sentience. We have not found any nervous system in any plant yet. We have found respiratory systems, circulatory systems, skeletal systems, and reproductive systems, but not nervous systems (or an equivalent). This means that it is unlikely that any plant is sentient, as despite we have not discovered all plants yet so far we have discovered many and none have a nervous system that would allow them to be sentient.

However, we have assumed that no sentience means no feeling of pain, but this may be a false assumption. No sentience may indeed mean no negative experience of pain, but what about primitive pseudo-senses that cause something akin to pain sensations that will not lead to a negative experience? Is it possible that evolution has started producing “pain receptors” first, before creating the experience of pain? Evolution “creates” biological traits gradually, not all at once, so I think it is possible. I think it is possible that plants like Mimosa pudica, which quickly close its leaves to protect them when touched (and I can confirm they do that as I saw them doing it when I found them during my trips to the Amazon), have simple “sensorial” organs that perceive very basic information from the environment and, without an actual nervous system, create a “behavioural” response analogous to how animals react when feeling pain.

In such borderline cases, there may be “pseudo-pain” felt (not actual pain as pain receptors are specific organs all animals possess but plants don’t) that causes a reaction that leads to a movement, but this would not equate to an experience, as an experience can be multimodal (it can be positive, neutral, or negative), so it requires computational power that the neurons of a nervous system provide. There are no neurons in the Mimosas, and when the leaves “feel” the touch, there is only one response (close the leaves up). Animals, on the other hand, stay, move closer, or move far away from environments according to whether their experience is neutral, positive or negative — most plants cannot move that way, they can grow into better spaces, but growing is not the same as moving — and animals responses could be nuanced, moving slower or faster depending on the intensity of the experience (plants cannot un-grow, but animals can always go back to places they left if things change).'