On the alignment of cellulose microfibrils by cortical microtubules: a review and a model

The hypothesis that microtubules align microfibrils, termed the alignment h ypothesis, states that there is a causal link between the orientation of co rtical microtubules and the orientation of nascent microfibrils. I have ass essed the generality of this hypothesis by reviewing what is known about th e relation between microtubules and microfibrils in a wide group of example s: in algae of the family Characeae. Closterium acerosum, Oocystis solitari a. and certain genera of green coenocytes and in land plant tip-growing cel ls, xylem, diffusely growing cells, and protoplasts. The salient features a bout microfibril alignment to emerge are as follows. Cellulose microfibrils can be aligned by cortical microtubules, thus supporting the alignment hyp othesis. Alignment of microfibrils can occur independently of microtubules. showing that an alternative to the alignment hypothesis must exist. Microf ibril organization is often random, suggesting that self-assembly is insuff icient. Microfibril organization differs on different faces of the same cel l, suggesting that microfibrils are aligned locally, not with respect to th e entire cell. Nascent microfibrils appear to associate tightly with the pl asma membrane. To account for these observations, I present a model that po sits alignment to be mediated through binding the nascent microfibril. The model, termed templated incorporation, postulates that the nascent microfib ril is incorporated into the cell wall by binding to a scaffold that is ori ented; further, the scaffold is built and oriented around either already in corporated microfibrils or plasma membrane proteins, or both. The role of c ortical microtubules is to bind and orient components of the scaffold at th e plasma membrane. In this way, spatial information to align the microfibri ls may come from either the cell wall or the cell interior, and microfibril alignment with and without microtubules are subsets of a single mechanism.