need to rotate planes 90 degrees in start function

Hello, what i need to do is rotate a plane 90 degree's but i have to do it in the start function. I'm asking on here because everything i seem to find is always turning over time, but i need to do it instantly in the start function. The reason is i made a prefab for a segment of a grid and the planes are rotated 90 in the wrong way so there sideways :P Thanks! :D