The physiological effects of an experimental herbicide and cellulose synthesis inhibitor, N2-(1-ethyl-3-phenylpropyl)-6-(1-fluoro-1-methylethyl)-1,3,5-triazine-2,4-diamine, called AE F150944, are described. In the aminotriazine molecular class, AE F150944 is structurally distinct from other known cellulose synthesis inhibitors. It specifically inhibits crystalline cellulose synthesis in plants without affecting other processes that were tested. The effects of AE F150944 on dicotyledonous plants were tested on cultured mesophyll cells of Zinnia elegans L. cv. Envy, which can be selectively induced to expand via primary wall synthesis or to differentiate into tracheary elements via secondary wall synthesis. The IC50 values during primary and secondary wall synthesis in Z. elegans were 3.91 x 10(-8) M and 3.67 x 10(-9) M, respectively. The IC50 in suspension cultures of the monocot Sorghum halapense (L.) Pers., which were dividing and synthesizing primary walls, was 1.67 x 10(-10) M. At maximally inhibitory concentrations, 18-33% residual crystalline cellulose synthesis activity remained, with the most residual activity observed during primary wall synthesis in Z. elegans. Addition to Z. elegans cells of two other cellulose synthesis inhibitors, 1 microM 2,6-dichlorobenzonitrile and isoxaben, along with AE F150944 did not eliminate the residual cellulose synthesis, indicating little synergy between the three inhibitors. In differentiating tracheary elements, AE F150944 inhibited the deposition of detectable cellulose into patterned secondary wall thickenings, which was correlated with delocalization of lignin as described previously for 2, 6-dichlorobenzonitrile. Freeze-fracture electron microscopy showed that the plasma membrane below the patterned thickenings of AE F150944-treated tracheary elements was depleted of cellulose-synthase-containing rosettes, which appeared to be inserted intact into the plasma membrane followed by their rapid disaggregation. AE F150944 also inhibited cellulose-dependent growth in the rosette-containing alga, Spirogyra pratensis, but it did not inhibit cellulose synthesis in Acetobacter xylinum or Dictyostelium discoideum, both of which synthesize cellulose via linear terminal complexes. Therefore, AE F150944 may inhibit crystalline cellulose synthesis by destabilizing plasma membrane rosettes.