The roles of microtubules in tropisms

Plant tropisms, or growth towards or away from a stimulus, usually involve the bending of shoots or roots which reorient growth in a newdirection. Plant responses to tropic cues, especially gravity and light, have been active areas of investigation for many years. Despite all of this attention we still do not understand how these responses are regulated. In this review possible roles for microtubules in tropisms are discussed. Tropisms occur in a series of steps; directional cues are perceived and converted into biochemical signals that induce bending in roots and shoots. One model suggests that microtubules function late in the response pathway, during organ bending. Microtubules have been linked to organ bending by virtue of their role in regulating the direction of cell elongation. In elongating cells microtubules appear to function as guides for the deposition of cellulosemicrofibrils into the cell wall and the placement of the microfibrils in the wall is thought to constrain the direction of cell elongation. According to the model bending occurs when different microtubule/microfibril alignments across the organ cause cells on the outer flank to elongatemore than cells on the inner flank. In support of this idea is the observation that tropic signals can induce the appropriate changes inmicrotubule orientations across a bending organ. However, attempts to validate the hypothesis have produced conflicting results and the idea that microtubule alignment regulates cell expansion during organ bending is controversial. Microtubules have also been linked to organizational events associated with the plasma membrane. Although speculative, one possibility is that microtubules influence tropisms by positioning regulatory proteins and/or complexes in the plasma membrane. Two possible mechanisms by which microtubules could contribute to organizational events associatedwith the plasmamembrane are outlined. In addition to cell expansion, microtubules are postulated to have roles in the perception of touch and gravity signals. Although microtubules are associated with touch sensing in animals, we know very little about the relevant receptors in plants. One way to assess how microtubules function during tropisms is to identify and study proteins that function in concert with microtubules. In particular, the analysis of microtubuleassociated proteins whose mutant forms confer defects in tropic responses promises to provide additional insights into the roles of microtubules in tropisms. 2008 Elsevier Ireland Ltd. All rights reserved.