Microtubule-based perception of mechanical conflicts controls plant organ morphogenesis.

Precise 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 side of the same cell, and this asymmetry is required for endodermal remodeling and lateral root initiation. We identify that MICROTUBULE ASSOCIATED PROTEIN 70-5 (MAP70-5) 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.

Arabidopsis pavement cell shape formation involves spatially confined ROPGAP regulators.

In many plant species, pavement cell development relies on the coordinated formation of interdigitating lobes and indentations. Polarity signaling via the activity of antagonistic Rho-related GTPases from plants (ROPs) was implicated in pavement cell development, but the spatiotemporal regulation remained unclear. Here, we report on the role of the PLECKSTRIN HOMOLOGY GTPase ACTIVATING PROTEINS (PHGAPS) during multipolar growth in pavement cell shape establishment. Loss of function in phgap1phgap2 double mutants severely affected the shape of Arabidopsis leaf epidermal pavement cells. Predominantly, PHGAPs interacted with ROP2 and displayed a distinct and microtubule-dependent enrichment along the anticlinal cell face and transfacial boundary of pavement cell indentation regions. This localization was established upon undulation initiation and was maintained throughout the expansion of the cell. Our data suggest that PHGAP1/REN2 and PHGAP2/REN3 are key players in the establishment of ROP2 activity gradients and underscore the importance of locally controlled ROP activity for the orchestrated establishment of multipolarity in epidermal cells.

Plant Biology: Journey to the Center of the Casparian Strip.

Diffusion barriers in roots play an important role in regulating the movement of compounds between the soil environment and the vasculature. A new study provides new mechanistic insights into how a pair of copper-binding proteins facilitate the formation of a lignified nanodomain within Casparian strips.

Cytoskeleton Dynamics Are Necessary for Early Events of Lateral Root Initiation in Arabidopsis.

How plant cells re-establish differential growth to initiate organs is poorly understood. Morphogenesis of lateral roots relies on the asymmetric cell division of initially symmetric founder cells. This division is preceded by the tightly controlled asymmetric radial expansion of these cells. The cellular mechanisms that license and ensure the coordination of these events are unknown. Here, we quantitatively analyze microtubule and F-actin dynamics during lateral root initiation. Using mutants and pharmacological and tissue-specific genetic perturbations, we show that dynamic reorganization of both microtubule and F-actin networks is necessary for the asymmetric expansion of the founder cells. This cytoskeleton remodeling intertwines with auxin signaling in the pericycle and endodermis in order for founder cells to acquire a basic polarity required for initiating lateral root development. Our results reveal the conservation of cell remodeling and polarization strategies between the Arabidopsis zygote and lateral root founder cells. We propose that coordinated, auxin-driven reorganization of the cytoskeleton licenses asymmetric cell growth and divisions during embryonic and post-embryonic organogenesis.

The evening complex coordinates environmental and endogenous signals in Arabidopsis.

Plants maximize their fitness by adjusting their growth and development in response to signals such as light and temperature. The circadian clock provides a mechanism for plants to anticipate events such as sunrise and adjust their transcriptional programmes. However, the underlying mechanisms by which plants coordinate environmental signals with endogenous pathways are not fully understood. Using RNA-sequencing and chromatin immunoprecipitation sequencing experiments, we show that the evening complex (EC) of the circadian clock plays a major role in directly coordinating the expression of hundreds of key regulators of photosynthesis, the circadian clock, phytohormone signalling, growth and response to the environment. We find that the ability of the EC to bind targets genome-wide depends on temperature. In addition, co-occurrence of phytochrome B (phyB) at multiple sites where the EC is bound provides a mechanism for integrating environmental information. Hence, our results show that the EC plays a central role in coordinating endogenous and environmental signals in Arabidopsis.

Putative RopGAPs impact division plane selection and interact with kinesin-12 POK1.

Cell shape is defined by the surrounding cell walls in plants. Thus, spatial control over cell division planes and cell expansion polarity are essential to maintain cell morphology. In eukaryotes, cell polarity and expansion are controlled by Rho GTPase signalling, regulating cytoskeletal reorganization and vesicle trafficking(1). However, until now, Rho signalling was not implicated in mitotic events in plants. Here, we report a pair of putative Rho GTPase activating proteins (RhoGAPs) that interact with the mitosis-specific kinesin-12 POK1, a core component of the cortical division zone/site (CDZ/CDS) that is required for division plane maintenance in Arabidopsis(2-4). The designated pleckstrin homology GAPs (PHGAPs) are cytoplasmic and plasma membrane associated in interphase, but during mitosis they additionally localize to the CDZ/CDS in a POK-dependent manner. In contrast to pok1 pok2 mutants, phgap1 phgap2 double mutants show moderate cell wall positioning defects as a consequence of inaccurate positioning of the cortical division zone marker POK1. We conclude that loss of PHGAP function interferes with division plane selection in proliferative cell divisions.

The Phragmoplast-Orienting Kinesin-12 Class Proteins Translate the Positional Information of the Preprophase Band to Establish the Cortical Division Zone in Arabidopsis thaliana.

The preprophase band (PPB) is a faithful but transient predictor of the division plane in somatic cell divisions. Throughout mitosis the PPBs positional information is preserved by factors that continuously mark the division plane at the cell cortex, the cortical division zone, by their distinct spatio-temporal localization patterns. However, the mechanism maintaining these identity factors at the plasma membrane after PPB disassembly remains obscure. The pair of kinesin-12 class proteins PHRAGMOPLAST ORIENTING KINESIN1 (POK1) and POK2 are key players in division plane maintenance. Here, we show that POK1 is continuously present at the cell cortex, providing a spatial reference for the site formerly occupied by the PPB. Fluorescence recovery after photobleaching analysis combined with microtubule destabilization revealed dynamic microtubule-dependent recruitment of POK1 to the PPB during prophase, while POK1 retention at the cortical division zone in the absence of cortical microtubules appeared static. POK function is strictly required to maintain the division plane identity factor TANGLED (TAN) after PPB disassembly, although POK1 and TAN recruitment to the PPB occur independently during prophase. Together, our data suggest that POKs represent fundamental early anchoring components of the cortical division zone, translating and preserving the positional information of the PPB by maintaining downstream identity markers.