In the 2015 Fantastic Four film,
five people are transformed by Planet Zero. We see how four of them are transformed. Von Doom observes that green lava is alive, like neural tissue that makes the planet a sort of organism. Then Von Doom touches the green neural lava and blends with the planet to the point where he can control matter with his thoughts. Then the others gain lesser powers as part of the storm that is provoked in the wake of this. Ben has rocks fall onto him, so he becomes rock. Jonny gets torched, so he becomes flames. And Sue gets hit by electromagnetic energy, so she gains the ability to control electromagnetic energy. ... Each of those four get hit by specific aspects of the planet, and they each become manifestations of the aspect that hit them individually.
But I do not recall seeing Reed during the act of his transformation. Instead, we just see Reed afterwards in the form in which we would expect if we saw any of the other movies.
So what caused Reed to become elastic in the 2015 film?