The cellulose-biosynthesis inhibitor (CBI) herbicides all selectively inhib
it the synthesis of cellulose despite significant chemical differences. Wit
h the exception of quinclorac they are most effective in inhibiting cellulo
se synthesis in dicot plants. Dichlobenil and isoxaben are the oldest and b
est studied of these herbicides, whereas flupoxam is a more recent introduc
tion and acts in many ways similarly to isoxaben. Quinclorac is unusual in
that it seems to act as a cellulose inhibitor in grasses but as an auxinic
herbicide in dicots. These herbicides inhibit cell plate formation at one o
f two relatively late stages without affecting microtubule function. The ef
fects of dichlobenil are different from other CBI herbicides; dichlobenil i
nhibits cellulose synthesis but promotes callose synthesis in its place. Su
spension cells of both Lycopersicon esculentum and Nicotiana tabacum can be
come habituated to normally inhibitory concentrations of dichlobenil or iso
xaben by replacing the normal cellulose network in their walls with pectin
and extensin. Natural resistance to CBI herbicides is rare and has only bee
n found in red algae species. Arabidopsis lines produced by mutagenesis all
share changes in active site rather than alterations in uptake, translocat
ion, or metabolism of these herbicides. The lack of cross-resistance to dif
ferent CBI herbicides of these mutants indicates that no fewer than three d
ifferent sites in the cellulose biosynthesis pathway are affected by the di
fferent herbicides in this class.