pledi

One root, and shorter than it looks. Pledi is not pl- plus anything and not related to pleni or plendi, which is the near-miss worth flagging: plendi is to complain and sits one letter away. A plendo is a grievance you voice; a pledo is a case you argue for somebody. Both happen in a courtroom and they are not the same act.

A hidden relative you already know. Plaĉi at rank 477, to please, comes from the same Latin placere as pledi does. The route is worth a sentence because it explains the strange semantics: a placitum was originally the thing that pleased the ruler, which became his decree, which became the hearing where decrees were made, which became the speech you make at one. Four steps from please to plead, all of them documented, none of them guessable.

How to use it. Pledi takes por for the side you argue for and kontraŭ for the side you argue against: pledi por la akuzito, pledi kontraŭ la nova leĝo. It is not limited to law - pledi por is standard Esperanto for advocating a position in a debate, an article or a congress speech, and the movement's own prose is full of it. The person is the advokato in court and the pledanto in general, the speech is the pledo, and the room is the juĝejo. The opposite motion in a trial is akuzi, to accuse - which is, as it happens, the verb behind akuzativo eleven ranks back.