Microtubule-based perception of mechanical conflicts controls plant organ morphogenesis

ABSTRACTPrecise coordination between cells and tissues is essential for differential growth in plants. During lateral root formation in Arabidopsis thaliana, the endodermis is actively remodeled to allow outgrowth of the new organ. Here, we show that microtubule arrays facing lateral root founder cells display a higher order compared to arrays on the opposite wall of the same cell, and this asymmetry is required for endodermal remodeling and lateral root initiation. We identify that MICROTUBULE ASSOCIATED PROTEIN 70-5 is necessary for the establishment of this spatially defined microtubule organization and endodermis remodeling, and thus contributes to lateral root morphogenesis. We propose that MAP70-5 and cortical microtubule arrays in the endodermis integrate the mechanical signals generated by lateral root outgrowth, facilitating the channeling of organogenesis.

Lateral Root Initiation and the Analysis of Gene Function Using Genome Editing with CRISPR in Arabidopsis

Lateral root initiation is a post-embryonic process that requires the specification of a subset of pericycle cells adjacent to the xylem pole in the primary root into lateral root founder cells. The first visible event of lateral root initiation in Arabidopsis is the simultaneous migration of nuclei in neighbouring founder cells. Coinciding cell cycle activation is essential for founder cells in the pericycle to undergo formative divisions, resulting in the development of a lateral root primordium (LRP). The plant signalling molecule, auxin, is a major regulator of lateral root development; the understanding of the molecular mechanisms controlling lateral root initiation has progressed tremendously by the use of the Arabidopsis model and a continual improvement of molecular methodologies. Here, we provide an overview of the visible events, cell cycle regulators, and auxin signalling cascades related to the initiation of a new LRP. Furthermore, we highlight the potential of genome editing technology to analyse gene function in lateral root initiation, which provides an excellent model to answer fundamental developmental questions such as coordinated cell division, growth axis establishment as well as the specification of cell fate and cell polarity.

B1L regulates lateral root development by exocytic vesicular trafficking-mediated polar auxin transport

Auxin and auxin-mediated signaling pathways involved in the regulation of lateral root development are well documented. Although exocytic vesicle trafficking plays an important role in PIN-auxin-efflux carrier recycling, and polar auxin transport during lateral root formation, however, the mechanistic details of these processes are not well understood. Here, we demonstrate an essential regulatory mechanism of B1L that interacts with the exocyst to regulate PIN-mediated polar auxin transport and lateral root initiation. B1L is highly expressed in Arabidopsis roots, and genetic and cellular analyses have revealed that B1L is mainly involved in lateral root primordia initiation. Furthermore, DR5::GUS expression analyses revealed that auxin levels were higher in lateral root primordia of the b1l mutant than in the wild-type. Exogenous auxin treatment confirmed that the lateral root phenotype correlated closely with auxin levels. Additionally, auxin transport-inhibitory treatment indicated that B1L regulates auxin efflux. Consistently, b1l mutants exhibited higher levels of auxin efflux carriers PIN1-GFP and PIN3-GFP in lateral root primordia. Moreover, B1L interacts with the exocyst and functions in recycling PIN2-GFP. Finally, the b1l-1/exo70b1-1 double-mutant exhibited a significant increase in the number of lateral roots compared to the wildtype, b1l-1, and exo70b1-1. Collectively, this study improves our understanding of the highly sophisticated processes involved in exocytic vesicular trafficking-mediated polar auxin transport and lateral root initiation in plants.