Divided by fate: The interplay between division orientation and cell shape underlying lateral root initiation in Arabidopsis.

The development of lateral roots starts with a round of anticlinal, asymmetric cell divisions in lateral root founder cells in the pericycle, deep within the root. The reorientation of the cell division plane occurs in parallel with changes in cell shape and needs to be coordinated with its direct neighbor, the endodermis. This accommodation response requires the integration of biochemical and mechanical signals in both cell types. Recently, it was reported that dynamic changes in the cytoskeleton and possibly the cell wall are part of the molecular mechanism required to correctly orient and position the cell division plane. Here we discuss the latest progress made towards our understanding of the regulation of cell shape and division plane orientation underlying lateral root initiation in Arabidopsis.

CROWN ROOTLESS1 binds DNA with a relaxed specificity and activates OsROP and OsbHLH044 genes involved in crown root formation in rice.

In cereals, the root system is mainly composed of post-embryonic shoot-borne roots, named crown roots. The CROWN ROOTLESS1 (CRL1) transcription factor, belonging to the ASYMMETRIC LEAVES2-LIKE/LATERAL ORGAN BOUNDARIES DOMAIN (ASL/LBD) family, is a key regulator of crown root initiation in rice (Oryza sativa). Here, we show that CRL1 can bind, both in vitro and in vivo, not only the LBD-box, a DNA sequence recognized by several ASL/LBD transcription factors, but also another not previously identified DNA motif that was named CRL1-box. Using rice protoplast transient transactivation assays and a set of previously identified CRL1-regulated genes, we confirm that CRL1 transactivates these genes if they possess at least a CRL1-box or an LBD-box in their promoters. In planta, ChIP-qPCR experiments targeting two of these genes that include both a CRL1- and an LBD-box in their promoter show that CRL1 binds preferentially to the LBD-box in these promoter contexts. CRISPR/Cas9-targeted mutation of these two CRL1-regulated genes, which encode a plant Rho GTPase (OsROP) and a basic helix-loop-helix transcription factor (OsbHLH044), show that both promote crown root development. Finally, we show that OsbHLH044 represses a regulatory module, uncovering how CRL1 regulates specific processes during crown root formation.

PUCHI represses early meristem formation in developing lateral roots of Arabidopsis thaliana.

Lateral root organogenesis is a key process in the development of a plant's root system and its adaptation to the environment. During lateral root formation, an early phase of cell proliferation first produces a four-cell-layered primordium, and only from this stage onwards is a root meristem-like structure, expressing root stem cell niche marker genes, being established in the developing organ. Previous studies reported that the gene regulatory network controlling lateral root formation is organized into two subnetworks whose mutual inhibition may contribute to organ patterning. PUCHI encodes an AP2/ERF transcription factor expressed early during lateral root primordium development and required for correct lateral root formation. To dissect the molecular events occurring during this early phase, we generated time-series transcriptomic datasets profiling lateral root development in puchi-1 mutants and wild types. Transcriptomic and reporter analyses revealed that meristem-related genes were expressed ectopically at early stages of lateral root formation in puchi-1 mutants. We conclude that, consistent with the inhibition of genetic modules contributing to lateral root development, PUCHI represses ectopic establishment of meristematic cell identities at early stages of organ development. These findings shed light on gene network properties that orchestrate correct timing and patterning during lateral root formation.

Recombinant N-glycosylation isoforms of Legume lectins: Production and purification from Nicotiana benthamiana leaves following RuBisCO depletion.

An efficient purification of recombinant proteins often requires a high ratio of recombinant to host proteins. In plants, Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is the most abundant leaf protein, thus strongly impacting purification yield. Here, we describe a simple and robust purification procedure for recombinant proteins based on a differential precipitation of RuBisCO. In this context, four Legume lectin domains of Arabidopsis thaliana which belong to receptor-like kinases and cell wall proteins were produced from Nicotiana benthamiana leaves. The recombinant proteins exhibit a unique lectin domain consisting of around 250 amino acid residues with several predicted N-glycosylation sites and a six His-tag at the N-terminus. After ammonium sulphate precipitation of total soluble proteins, depletion of RuBisCO was obtained using citrate and succinate buffers during the salting-in step: this depletion was pH-dependent and the presence of di- or tri-carboxylic acids was required. The depleted protein extracts were then subjected to two chromatographic steps which were used in the negative mode to submit a protein fraction enriched as much as possible in recombinant lectin domains to a third chromatographic step (immobilized metal-ion chromatography). Three of the Legume lectin domains were purified near to homogeneity and revealed multiple N-glycosylation isoforms, particularly those from receptor-like kinases, which were characterised using specific lectins and deglycosylation enzymes. The production and purification of recombinant lectin domains will facilitate their biochemical characterisation in the context of cell-to-cell signalling and cell wall organisation.

Lateral Root Formation in Arabidopsis: A Well-Ordered LRexit.

Lateral roots (LRs) are crucial for increasing the surface area of root systems to explore heterogeneous soil environments. Major advances have recently been made in the model plant arabidopsis (Arabidopsis thaliana) to elucidate the cellular basis of LR development and the underlying gene regulatory networks (GRNs) that control the morphogenesis of the new root organ. This has provided a foundation for understanding the sophisticated adaptive mechanisms that regulate how plants pattern their root branching to match the spatial availability of resources such as water and nutrients in their external environment. We review new insights into the molecular, cellular, and environmental regulation of LR development in arabidopsis.

Plant Lectins and Lectin Receptor-Like Kinases: How Do They Sense the Outside?

Lectins are fundamental to plant life and have important roles in cell-to-cell communication; development and defence strategies. At the cell surface; lectins are present both as soluble proteins (LecPs) and as chimeric proteins: lectins are then the extracellular domains of receptor-like kinases (LecRLKs) and receptor-like proteins (LecRLPs). In this review; we first describe the domain architectures of proteins harbouring G-type; L-type; LysM and malectin carbohydrate-binding domains. We then focus on the functions of LecPs; LecRLKs and LecRLPs referring to the biological processes they are involved in and to the ligands they recognize. Together; LecPs; LecRLKs and LecRLPs constitute versatile recognition systems at the cell surface contributing to the detection of symbionts and pathogens; and/or involved in monitoring of the cell wall structure and cell growth.