Disclosed are: a substance which has an activity of inhibiting the intracellular signaling from a plant-derived cytokinin receptor and which can control the growth or differentiation of a plant. Also, disclose is a method for searching for a chemical substance capable of promoting the growth of a root of a plant, which comprises measuring the level of intracellular signaling from the receptor in a system where a cell having the receptor is contacted with a chemical substance having an agonistic activity on the receptor and a substance to be tested, comparing the level of intracellular signaling measured in the preceding step with a level of intracellular signaling measured in the absence of the chemical substance, and determining the chemical substance as being a chemical substance capable of promoting the growth of a root of a plant based on the difference obtained by the comparison; and others.