Sucrose synthase localizes to cellulose synthesis sites in tracheary elements

The synthesis of crystalline cellulose microfibrils in plants is a highly c oordinated process that occurs at the interface of the cortex, plasma membr ane, and cell wall. There is evidence that cellulose biogenesis is facilita ted by the interaction of several proteins, but the details are just beginn ing to be understood. In particular. sucrose synthase, microtubules, and ac tin have been proposed to possibly associate with cellulose synthases (micr ofibril terminal complexes) in the plasma membrane. Differentiating trachea ry elements of Zinnia elegans L. were used as a model system to determine t he localization of sucrose synthase and actin in relation to the plasma mem brane and its underlying microtubules during the deposition of patterned, c ellulose-rich secondary walls. Cortical actin occurs with similar density b oth between and under secondary wall thickenings. In contrast, sucrose synt hase is highly enriched near the plasma membrane and the microtubules under the secondary wall thickenings. Both actin and sucrose synthase lie closer to the plasma membrane: than the microtubules. These results show that the preferential localization of sucrose synthase at sites of high-rate cellul ose synthesis can be generalized beyond cotton fibers, and they establish a spatial context for further work on a multi-protein complex that may facil itate secondary wall cellulose synthesis. (C) 2001 Elsevier science Ltd. Al l rights reserved.