PROBLEM TO BE SOLVED: To change plant cell walls by a chemical substance.SOLUTION: A plant cell wall thickening promoter and a plant cell wall degradation promoter contains either: lasalocid A or a salt thereof, or a culture of an actinomycete of Streptomyces or a variant thereof or a processed product thereof which contains them ; or a compound represented by the formula (I) in the figure (where R, R, Rand Rare identical or different and each represent a hydrogen atom or a Calkyl group) or a salt thereof.