Question

if a plant cell doesn't have centrioles then how do they produce spindle fibers during mitosis?

Answer 1

I have to admit, I never thought about it! But this is what Wikipedia had to say about it "The cells of higher plants (such as the flowering plants) lack centrioles; instead, microtubules form a spindle on the surface of the nucleus and are then organized into a spindle by the chromosomes themselves, after the nuclear membrane breaks down"

Answer 2

Which occurs in Preprophase: http://en.wikipedia.org/wiki/Preprophase