。
Mr. Tatsuya deals in grain.
Literal
Tatsuya-Mr. [topic-は] grains [object-を] deals-in.
商う (あきなう, to deal in / to trade) is a somewhat literary verb for conducting business in goods. 穀類 (grains/cereals) is a formal word. 氏 (し) is a formal honorific used in written contexts, like 'Mr.' in formal prose.