In order to examine the possible involvements of plant hormones in actin filament organization in a garden pea shoot apex, the effect of growth promoting hormones such as IAA, GA3, brassinolide (BL), and zeatin (Zea) on the gene expression ofRho1ps was investigated by northern blot analyses. Compared with the level ofRho1ps transcripts in hormone-untreated segments of the apex, the application of 10-6 M IAA, GA3, and BL activated the accumulation ofRho1ps transcripts as much as 2, 2, and 4 times, respectively. Also the application of 10-6 M Zea showed accumulation of the transcripts 7 times higher than the control, which is about 2 to 3 times higher than that induced by the other three hormones. Thus, it is thought that actin filament organization mediated byRho1ps is closely associated with growth of the segments induced by the hormones tested, and that the organization is more necessary for a process related to cell division rather than that to cell expansion (elongation and/or enlargement) in the segment. The increase ofRho1ps transcripts induced by each hormone was additive when another hormone was simultaneously applied, indicating that the four hormones, IAA, GA3, BL, and Zea, participate independently in regulating the gene expression.