Helical microtubule arrays and spiral growth.

Asymmetric cell shape depends on the ability of oriented layers of cellulose microfibrils to channel the nondirectional drive of turgor pressure. Tough microfibrils encircling the cell resist increases in girth and redirect the swelling force to a direction perpendicular to the alignment of the microfibrils. This transformation of an isotropic force into directional growth is crucial for setting up the main axis that allows an organism to project its photosynthetic and reproductive organs into the environment. But what controls the orientation of the microfibrils in the first place? Cortical microtubules beneath the plasma membrane are thought to guide the directed extrusion of new microfibrils from cellulose-synthesizing machinery embedded in the plasma membrane. Newly laid cellulose microfibrils often are aligned above microtubules, which look like tracks for the cellulose-synthesizing enzymes. However, the molecular details of this hypothetical transmembrane system are scant, and there are now sufficient examples of the lack of coalignment to raise questions about the basic microtubule-microfibril paradigm. Before we can arrive at new models to explain phenomena as broad as “cell shape,” we need to answer more fundamental questions. Where are the microtubules formed? How do they move and link up to form a dynamic entity that covers the inner face of the plasma membrane? How can arrays form and reform so rapidly, and what controls their stability? Several recent articles have provided a quantum leap in our understanding: one group of articles begins to show how the plant cell’s unique cortical microtubule array is constructed, and the other shows that mutations in the microtubule’s subunit protein—tubulin—affect the orientation of the entire microtubule array, resulting in changes in the direction of spiral growth. MICROTUBULES BEGET MICROTUBULES