The insights come from studies of Arabidopsis thaliana, which normally grows straight. Hashimoto's team isolated lefty1 and lefty2—which have left-handed helical growth—as suppressors of an existing right-handed helical growth mutant. The new mutants have an identical change in either TUA6 or TUA4—two of the plant's α-tubulin genes.
The change is near the interface with β-tubulin. The disturbed interface may produce the altered angle of microtubules seen in the mutants. Cortical microtubules are normally found running directly across the cells, but the mutant microtubules form in a skewed right-handed helix. This should alter the direction of growth, as cellulose- forming enzymes are thought to use microtubules as train tracks to lay down new cellulose fibers, and these circumferential fibers then constrict lateral growth and force all growth in a perpendicular direction.
Although the microtubule bending might determine the direction in which helical growth twists, the extent of helical growth is probably determined by the instability of the aberrant microtubules. Hashimoto has previously shown that treatment with antimicrotubule drugs affects the growth of inner cells disproportionately, leading to more rounded growth in these cells. Near-normal growth continues in outer cells, however. To prevent these more elongated cells from getting too far ahead of the inner cells, the outer cells skew their growth pattern to form a helix. Thus, two aspects of microtubule behavior determine both the direction and extent of helical growth. ▪Reference: