whom可以在定语从句中充当主语吗??

请看这句话：

Presenters at the seminar, one of whom is blind, will demonstrate a daptive equipment that allows visally impaired people to use computers.

one of whom是做这个定语从句的主语的啊，为什么whom可以充当呢？

想不通阿，请大牛指点，谢谢~

在“介词+whom”引导的定语从句中，只能用whom,不能用who.