What do you recommend for isolating RNA from more than 100 mg of plant tissue?

You can use the RNeasy Midi Kit for up to 500 mg of plant tissue, and the RNeasy Maxi Kit for up to 1 g of plant material. Please follow the guidelines in Appendix D of the RNeasy Midi/Maxi Handbook. The samples will require rotor-stator homogenization, since QIAshredders are not available for the RNeasy Midi/Maxi format.