Gravitropic microtubule reorientation can be uncoupled from growth

The causal relationship between gravitropic growth responses and microtubul e reorientation has been studied. Growth and microtubule reorientation have been uncoupled during the gravitropic response of maize (Zea mays L.) cole optiles. Microtubule orientation and growth were measured under three diffe rent conditions: (i) a gravitropic stimulation where the growth response wa s allowed to be expressed (intact seedlings were displaced from the vertica l position by 90 degrees), (ii) a gravitropic stimulation where the growth response was suppressed (coleoptiles were attached to microscope slides and kept in a horizontal position), (iii) suppression of growth in the absence of gravitropic stimulation (coleoptiles were attached to microscope slides and kept in a vertical position). It was found that (i) gravitropic stimul ation can induce a microtubular reorientation from transverse to longitudin al in the upper (slower growing) flank of the coleoptile, and an inhibition of growth, (ii) the reorientation of microtubules precedes the inhibition of growth; (iii) the gravitropic response of microtubules is weaker, not el evated, when the inhibition of growth is artificially enhanced by attaching the coleoptiles to a slide; and (iv) artificial inhibition of growth in th e absence of gravitropic stimulation cannot induce a microtubular response. Thus, the extent of microtubule reorientation is not correlated with the e xtent of growth inhibition. Moreover, these findings demonstrate that micro tubules do not reorient passively after growth changes, but actively in res ponse to gravitropic stimulation.