In the years 1902-1913 assistant, assistant professor of the Kazan University. He was a professor of the Voronezh Agricultural Institute (1913-1931). In 1917-1918, Keller was a teacher in the Voronezh provincial assembly. Professor of Voronezh State University (1919-1931). Director of the Botanical Institute (1931-1936), the Soil Institute (1935-1936), the Moscow Botanical Garden of the USSR Academy of Sciences (1937-1945). Chairman of the Turkmen branch of the Academy of Sciences of the USSR (1941-1945).
Research interests: plant ecology, classification of the flora of the Russian steppes. The author of the books: "The Flora of the Russian Steppes, Semi-Deserts and Deserts" (Voronezh, 1923-1926, Vol. 1-2.), Selected Works (Moscow, 1951). Author of popular science books for children.