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The fruit grower grafted the apple scion onto the rootstock.
Literal
Fruit-tree-cultivator [subject-が] apple [gen-の] scion [obj-を] rootstock [on-に] grafted.
A neutral SOV sentence full of technical horticultural vocabulary: 果樹栽培者 (fruit grower), 接ぎ穂 (scion, the cutting being attached), 台木 (rootstock, the stock it's attached to), 接ぎ木 (the grafting operation itself). The particle pattern が/を/に maps cleanly onto agent / theme / goal of a transitive action. 接ぎ穂 also has a figurative life outside horticulture, where it means a 'conversational graft' — a topic prompt to keep dialogue going.