Mode of action of herbicides that specifically
disrupt extracellular matrix assembly

Cell wall biogenesis is inexorably linked to plant developmental processes and the plant ECM also has a critical role in environmental adaptation and stress responses. The typical higher plant ECM consists primarily of polysaccharide, with high tensile strength cellulose microfibrils aligned in a matrix of hemi-cellulose and pectins. Plant growth and development are dependent on assembly and alterations/maintenance of the ECM. These processes require extrusion of ECM components into the apoplast with concomitant or subsequent assembly into the complex, interactive cell wall matrix. Controlled cell expansion is essential for all morphogenetic processes in plants. The orientation of newly deposited cellulose microfibrils determines the direction of cell expansion and controls this aspect of plant development. Cell division patterns determined by cell plate and subsequent cross wall formation are amplified and modified by cell enlargement. Cellulose synthesis appears to be critical to proper formation of the cell plate and the new cross wall between dividing cells. In addition, plant development also encompasses aspects of cell specialization and differentiation. Differentiation processes have been shown to be dependent on cellulose synthesis for structural integrity and in mediating the patterning of other wall components critical to successful function of mature cells.

One approach to critical evaluation of ECM participation in the above processes and to dissect the mechanisms of ECM biogenesis is the application of specific chemical inhibitors. Two herbicides, isoxaben and dichlobenil (DCB) specifically disrupt ECM synthesis in a variety of higher plants and algae. In higher plants, the effect of DCB treatment is to specifically inhibit cellulose synthesis. The effect of DCB in some marine algae which do not have cellulose is to modify or eliminate biosynthesis of ECM polysaccharides. This differential effect provides an opportunity to gain significant insight into the mode of action of DCB and related herbicides and to apply this knowledge to discern details of ECM biogenetic sequences in very distinct systems. We have recently synthesized a fluorescent derivative of DCB (DCBF) that allows for intracellular localization of sites of herbicide actions as well as identification of DCB-binding polypeptides. DCBF represents a powerful new probe to investigate the mode of action of DCB and related compounds on polysaccharide synthesis. We have applied this probe to higher plants and algae and have identified several DCB binding proteins in the molecular mass range of 18 kD. We are currently in the process of characterizing these proteins. Using three-photon microscopy, we are localizing herbicide movements and the site of DCB action with our fluorescent analog.