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

Abstract

The hypothesis that microtubules align microfibrils, termed the alignment hypothesis, states that there is a causal link between the orientation of cortical microtubules and the orientation of nascent microfibrils. I have assessed the generality of this hypothesis by reviewing what is known about the relation between microtubules and microfibrils in a wide group of examples: in algae of the family Characeae, Closterium acerosum, Oocystis solitaria, and certain genera of green coenocytes and in land plant tip-growing cells, xylem, diffusely growing cells, and protoplasts. The salient features about microfibril alignment to emerge are as follows. Cellulose microfibrils can be aligned by cortical microtubules, thus supporting the alignment hypothesis. Alignment of microfibrils can occur independently of microtubules, showing that an alternative to the alignment hypothesis must exist. Microfibril organization is often random, suggesting that self-assembly is insufficient. Microfibril organization differs on different faces of the same cell, suggesting that microfibrils are aligned locally, not with respect to the entire cell. Nascent microfibrils appear to associate tightly with the plasma membrane. To account for these observations, I present a model that posits alignment to be mediated through binding the nascent microfibril. The model, termed templated incorporation, postulates that the nascent microfibril is incorporated into the cell wall by binding to a scaffold that is oriented; further, the scaffold is built and oriented around either already incorporated microfibrils or plasma membrane proteins, or both. The role of cortical microtubules is to bind and orient components of the scaffold at the plasma membrane. In this way, spatial information to align the microfibrils may come from either the cell wall or the cell interior, and microfibril alignment with and without microtubules are subsets of a single mechanism.

Isolation and handedness of helical coiled cellulosic thickenings from plant petiole tracheary elements

“…In general, the orientation of cellulose microfibrils in the cell wall governs many plant functions (Burgert and Fratzl 2009). Again, there is much evidence that the major factor in controlling microfibril orientation during cell growth is the orientation of the cortical microtubules, but some other factor must also be involved (Baskin 2001).…”

Two Leucine-Rich Repeat Receptor Kinases Mediate Signaling, Linking Cell Wall Biosynthesis and ACC Synthase in Arabidopsis

“…Genetic analysis and inhibitor studies indicate that cytoplasmic microtubules play an important role in guiding the orientation of the deposition of cellulose microfibrils (reviewed in Baskin, 2001), and the cellulose synthase rosette was found to move along the plasma membrane in tracks that largely coincided with the cortical microtubules (Paredez et al, 2006).…”

Dynamics of Actin Filaments in Epidermal Cells of Azuki Bean Epicotyls under Hypergravity Conditions

“…The shape of whole plant body is predominantly regulated by anisotropic growth of individual cells. Cortical microtubules, a characteristic cytoskeleton in interphase cells of plants, are likely involved in the modification of growth anisotropy of cells through the regulation of the orientation of cellulose microfibrils (Shibaoka, 1994;Baskin, 2001;Paredez et al, 2006). We have shown that the gravitational force affected the orientation of cortical microtubules during modification of anisotropic growth of stems.…”