The influence of light on microtubule dynamics and alignment in the Arabidopsis hypocotyl.

Light and dark have antagonistic effects on shoot elongation, but little is known about how these effects are translated into changes of shape. Here we provide genetic evidence that the light/gibberellin-signaling pathway affects the properties of microtubules required to reorient growth. To follow microtubule dynamics for hours without triggering photomorphogenic inhibition of growth, we used Arabidopsis thaliana light mutants in the gibberellic acid/DELLA pathway. Particle velocimetry was used to map the mass movement of microtubule plus ends, providing new insight into the way that microtubules switch between orthogonal axes upon the onset of growth. Longitudinal microtubules are known to signal growth cessation, but we observed that cells also self-organize a strikingly bipolarized longitudinal array before bursts of growth. This gives way to a radial microtubule star that, far from being a random array, seems to be a key transitional step to the transverse array, forecasting the faster elongation that follows. Computational modeling provides mechanistic insight into these transitions. In the faster-growing mutants, the microtubules were found to have faster polymerization rates and to undergo faster reorientations. This suggests a mechanism in which the light-signaling pathway modifies the dynamics of microtubules and their ability to switch between orthogonal axes.