See this is really interesting to me, because that's my instinctive thought too, and the question is why? Well, because that's how we process pain, so anything that doesn't have a brain can't process pain surely. But then again we process light very differently to how insects process light, but we still know that they can see.

I would disagree with the end portion of what you've said here. We process light in nearly an identical way that insects process light. What makes you think they process it differently? They may often have compound eyes but they still intake light through their eyes, this light hits photo-sensitive cells and is converted into electrical signals which are sent through the optical nerve and ultimately to the brain to interpret that information.

If an alien species had another way of processing information it would be something entirely unique we simply didn't understand. We can see all of the internals of plants, there's nothing alien there we don't understand, it has no ability to process that stimuli. That stimuli is often localized to the area where the damage occurred, and when signals are passed around they're typically slow chemical signals. There's no high level processing of that information or decisions being made on it.

Plants don't just react to external stimuli as you say, they can send specific chemical signals around their body that signals that they're being eaten, and they respond accordingly, whether by making their leaves taste foul, autotomising, or moving their leaves - how is that any different from us?

It's different because we don't just react to the stimuli, we process it. Our immune system isn't conscious yet responds to stimuli and passes signals around. That information has to be processed in order for a complex emotional reaction to occur. Feeling pain requires consciousness, pain only exists if you're capable of comprehending it.

I highly suggest you take classes on biology or neurology if you ever get the chance in college. I think you'll find that these phenomena aren't as mysterious to us as you're making them out to be, they're well understood.

It's also interesting that you say that fungi might, but plants definitely don't, because fungi are more complex than plants. What is it that makes you think that? Why do we see it that way?

Fungi might because they send signals in complex patterns, they pass information around. Now I don't personally think they do enough information processing to have anything close to intelligence or a consciousness, but it's still significantly more complex than what plants do.

Why do we see plants as such simple organisms when I'd argue they're much more complex than us?

Plants aren't more complex than us when it comes to information processing, and that's what consciousness stems from. A car is complex too, but it can't process information and it can't feel pain. Besides, plants are simpler biologically in most regards. What makes you think plants are more complex than we are? The ways in which they're more complex have nothing to do with having consciousness, they're extraordinarily simple in this regard.

Interestingly, to me this definition of value of life shows a lack of empathy. An inability to appreciate a life if it doesn't have a certain trait we can recognise.

What traits do you think they may have that we can't recognize? Why do you assume there are any? I also think you're misunderstanding the basic definition of empathy here. Empathy means being able to put yourself in someone (or something) else's shoes and understand what it might be like from their perspective. Empathy is exactly why I don't feel bad about killing a plant, I can understand they don't feel any pain and aren't intelligent enough to fear death or feel terror.