。
The leaves rustled in the wind.
Literal
Tree-[possessive-の]-leaves [subject-が] wind [in-に] rustled.
そよぐ is a verb specifically for the gentle rustling, swaying motion of leaves in a light wind — a delicate, almost poetic word. The に here marks the wind as the medium causing the rustling, a locative-dative use rather than a clear cause. Japanese has a whole family of verbs for subtle plant motion (そよぐ, 揺れる, なびく…) that English covers with generic verbs plus adverbs.