In One Piece, many characters wield strange powers gained from eating one of the mysterious Devil Fruits, unique, one-of-a-kind fruits that give whoever eats one a supernatural power. These abilities come at the cost of the user's ability to swim, though the potential powers one could receive usually make the sacrifice worth it. Strangely though, after more than 20 years, fans still have little information on the fruits themselves. Most of what's known about the Devil Fruits comes from tales told by characters or descriptions of how strong certain ones are. Even why Devil Fruit users lose their ability to swim isn't fully confirmed, and is instead just an assumption based off of an old legend. Obviously, this has led many fans to speculate over just what their origins could be.
One theory, posted by Reddit user u/Anima4, suggests that the origin of Devil Fruits may have a connection to medicine. u/Anima4, looking at the Devil Fruits as plants, suggests that the reason almost all Devil Fruits have stems of some kind whenever they are seen or reincarnate is because all of them originally grew from a tree somewhere in the world. With multiple trees in One Piece holding significance, such as the Treasure Tree Adam or the Sunlight Tree Eve, this is quite possible.
From there, u/Anima4 suggests that the doctors of One Piece have special knowledge of and connections to plants, which is actually true -- many doctors in the series are even named after plants. As seen with Chopper, knowledge of plants and medicine can even have an effect on Devil Fruits, as he was able to enhance his Zoan-type Devil Fruit to provide him with multiple forms and specialized transformations, more than any other Zoan user. This is further supported by Caesar Clown's experiments on both himself and his pet gas monster, Smiley.
From there, the theory even explains how this medical knowledge could be responsible for how Marshal D. Teach, a.k.a Blackbeard, is able to wield the power of two fruits. Blackbeard has always been portrayed as a man with a plan as well as someone who wanted to remain hidden until the time was right. Taking that into account, him and his crew attacking the Drum Kingdom on a whim doesn't make a lot of sense.
With the doctors from the Drum Kingdom renowned throughout the Grand Line, it's possible that they knew more about Devil Fruits than previously thought. It's never elaborated on what exactly the doctors from the Drum Kingdom are famous for accomplishing, so this theory is surprisingly likely, opening the door to many possibilities. The theory suggests that Doc Q advised Blackbeard to attack the island to acquire their secret knowledge of Devil Fruits as well as their possible connection to the Will of D.
The theorist points out that they believe there is a further connection to the Will of D. and doctors. Throughout the series, doctor characters have brought up the Will of D., most notably Dr. Kureha, who also happens to be from the Drum Kingdom. Also, every pirate captain with the "D" initial in their name has had a doctor, including Blackbeard, Roger and Luffy.
u/Anima4 believes that, when Doc Q was recruited by Blackbeard, he felt it was fate that he was able to meet a D., upon whom he could bestow his medical secrets. It is through their meeting, as well as the hidden information they potentially stole from the Drum Kingdom, that allowed Blackbeard to steal the Tremor-Tremor Fruit's powers from Whitebeard. They believe that this is something that only someone from the D. clan can do, so with the right medical understanding of the situation, it's clear why Blackbeard was able to do what should have been impossible.
Another thing u/Anima4 makes sure to note is that there hasn't been a canon plant-based Devil Fruit yet. With plant-based powers, it is theoretically possible that a user could control other Devil Fruits and their users, as well as give themselves certain health boosts and enhancements. Until one is presented in the story though, fans are probably best off not taking these ideas with more than a grain of salt.
With so little confirmed information about Devil Fruits, it's no wonder that a fan spent so much time trying to crack the code. With One Piece moving more and more away from Devil Fruits and instead focusing much more on Haki recently, it's hard to know when or if an answer will ever come. However, it should be remembered that One Piece has rarely introduced a concept without returning to it later with a greater explanation, with the aforementioned Haki being a prime example. With enough patience, it's likely that fans will eventually get the answers they are seeking.