There were many different kinds of environments in the Arena. Deserts, largely (or at least Soumaw had mostly experienced deserts. It was hard to tell the real composition of such a large plane), but Soumaw had come across ruins, mysterious grandiose flights of stairs, towers and more, and now he encountered some manner of jungle. Unlike many parts of the Arena, this area was luscious and full of great trees and other manner of plant, but more importantly, there were creatures here. With few exceptions, the animals were enormous in this area regardless of how they were commonly seen in other planes. And yet, they still ran from Soumaw.
Soumaw leaped over a fallen tree trunk, diagonal to the floor as it had dropped to rest against another tree. A tiger-like creature sprinted ahead of the planeswalker, who roared in its direction, sending the animal's fur into a craze and its step to be even faster. Shifting the energy It'd accumulated on Its hands to keep up, Soumaw stopped and stood, throwing an ethereal projectile at the trees ahead. A green hue spread from within the branches, causing them to suddenly curl in a seemingly impossible twist and stabbing the enormous tiger's back. The branch's form continued to consolidate into a single body, an enormous wooden spider with the green glowing tint of the soul that had been infused into it.
Soumaw approached the fallen animal, climbing onto its back. Its claws dug into the wounds and it bent down, biting the tiger's neck until it stopped moving, soul completely drained.
Golden Hind was the comparison that first came to mind for me, but Deathbloom Gardener is definitely the more apt comparison. While your card has 1 more power, that card can generate a mana of any color, which I think is definitely more powerful especially considering both of these have deathtouch. Still, while your card is comparatively weaker, that is certainly not something to discount it over, it's certainly good design from a balance perspective.
That being said, official card that did it or not the fact remains that mana generation and deathtouch are counter-syngergistic. Your thing will die and no longer give mana if you try to use the deathtouch.
Furthermore, the card doesn't feel like a good response to my own. I guess technically deathtouch responds to a creature, but such a universal response as just "removal" is a little lacking in my view. How a toadstool would protect forest life is also not something I'm seeing (giant or not) but I suppose that's less a matter of the card's flavor and more the RP itself.
Overall, I'll give this card a 7/10. Not bad, but could use a bit more work and creativity, particularly oriented towards making a better response.
PS: As you didn't specify how a mushroom is supposed to "protest the forest life" from a giant possessed tree spider and Soumaw, I have taken the liberty of trying to envision a way. I apologize in advance if it's not what you had in mind.
Suddenly, a shadow was cast over Soumaw and the spider, an enormous mushroom cap covering the two like a dome. Though the arena was strange, areas like these tended to be consistent. Was this a creature or plant waiting in ambush? It lacked a soul of its own, so not likely a creature. Perhaps it was conjured? It had appeared so suddenly, so that was a possibility. Either way, it seemed relatively harmless to Soumaw, but nonetheless there was nothing wrong with a bit of caution. With a glance, the planeswalker commanded the spider It summoned to make their way out of the mushroom.
The tree-turned-monster advanced towards the cap, and attempted to pierce through. Giant or not, reinforced or not, it was still a mushroom. Hardly the kind of thing that could stop a creature like that from tearing an opening and opening it like one would a tent. However, as Soumaw walked past it, It noticed something dripping onto the spider from the cap. It seemed to be spreading, causing cracks to form, cracks which shed light from the soul infused into the creature. A toxin. A strong one at that. However, the enormous spider withstood it, continuing the bear the mushroom cap. Even with its form cracked and decaying, the creature persisted in its mission and stood just as fierce.
Out of the cap, Soumaw glanced around in search of the one who caused that, if anyone had. It did sense a clear soul nearby, though it had yet to pinpoint its exact location.