Moving in straight lines (left), CESA (green) is coincident with microtubules (red).

EHRHARDT/AAAS

Cellulose synthase (CESA) tracks along paths coincident with microtubules, say Alex Paredez, Chris Somerville, and David Ehrhardt (Carnegie Institution, Stanford, CA). The resulting parallel cellulose fibrils constrain cell expansion so that plants elongate primarily along a single axis.

A transmembrane CESA complex takes cytoplasmic substrates and turns them into 36 extracellular glycan chains. At some distance from the complex, the extruded chains crystallize into a cellulose microfibril.

CESA's relationship to plant cortical microtubules has been difficult to determine given the microtubules' dynamic nature. Through rapid treadmilling and turnover, the microtubules bump into each other and realign, thus helping create a parallel array that is perpendicular to the axial direction of plant growth. In static pictures CESA was often nowhere near a microtubule, leading some to suggest that CESA was channeled between microtubule tracks rather than interacting with them directly.

Using live, single-particle imaging, however, the Stanford group saw that CESA was often coincident with microtubules, tracked along the microtubules, and reoriented in response to reorientation of the microtubule arrays by light.

When a microtubule treadmilled away from CESA, the CESA complex kept going in a straight line as defined by the now-absent microtubule. This is consistent with the team's belief that most if not all of the motive force comes from cellulose polymerization rather than a cytoskeletal motor. Extruded cellulose microfibrils bond to other cell wall polymers, so it is the CESA that must move forward as more cellulose is created. Any link between CESA and microtubules is yet to be determined.

Reference:

Paredez, A.R., et al.
2006
.
Science.
doi:.