An Auxin-Mediated Shift toward Growth Isotropy Promotes Organ Formation at the Shoot Meristem in Arabidopsis

To control morphogenesis, molecular regulatory networks have to interfere with the mechanical properties of the individual cells of developing organs and tissues, but how this is achieved is not well known. We study this issue here in the shoot meristem of higher plants, a group of undifferentiated cells where complex changes in growth rates and directions lead to the continuous formation of new organs [1, 2]. Here, we show that the plant hormone auxin plays an important role in this process via a dual, local effect on the extracellular matrix, the cell wall, which determines cell shape. Our study reveals that auxin not only causes a limited reduction in wall stiffness but also directly interferes with wall anisotropy via the regulation of cortical microtubule dynamics. We further show that to induce growth isotropy and organ outgrowth, auxin somehow interferes with the cortical microtubule-ordering activity of a network of proteins, including AUXIN BINDING PROTEIN 1 and KATANIN 1. Numerical simulations further indicate that the induced isotropy is sufficient to amplify the effects of the relatively minor changes in wall stiffness to promote organogenesis and the establishment of new growth axes in a robust manner. [Display omitted] •Localized disorganization of CMT precedes and promotes organ initiation at the SAM•Auxin interferes with ABP1 and KTN1 CMT-ordering activities at the SAM•CMT disorganization amplifies the effect of minor reductions in the outer wall rigidity Sassi et al. investigate the role of anisotropy in the regulation of organogenesis in the Arabidopsis shoot apex. They show that auxin alters the organization of cortical microtubules in the shoot meristem cells. This triggers a shift toward isotropic growth that promotes the formation of lateral organs in synergy with limited cell wall loosening.