SHORTROOT-Mediated Intercellular Signals Coordinate Phloem Development in Arabidopsis Roots.

Asymmetric cell division (ACD) and positional signals play critical roles in the tissue patterning process. In the Arabidopsis (Arabidopsis thaliana) root meristem, two major phloem cell types arise via ACDs of distinct origins: one for companion cells (CCs) and the other for proto- and metaphloem sieve elements (SEs). The molecular mechanisms underlying each of these processes have been reported; however, how these are coordinated has remained elusive. Here, we report a new phloem development process coordinated via the SHORTROOT (SHR) transcription factor in Arabidopsis. The movement of SHR into the endodermis regulates the ACD for CC formation by activating microRNA165/6, while SHR moving into the phloem regulates the ACD generating the two phloem SEs. In the phloem, SHR sequentially activates NAC-REGULATED SEED MORPHOLOGY 1 (NARS1) and SECONDARY WALL-ASSOCIATED NAC DOMAIN PROTEIN 2 (SND2), and these three together form a positive feedforward loop. Under this regulatory scheme, NARS1, generated in the CCs of the root differentiation zone, establishes a top-down signal that drives the ACD for phloem SEs in the meristem. SND2 appears to function downstream to amplify NARS1 via positive feedback. This new regulatory mechanism expands our understanding of the sophisticated vascular tissue patterning processes occurring during postembryonic root development.plantcell;32/5/1519/FX1F1fx1.

The Danger-Associated Peptide PEP1 Directs Cellular Reprogramming in the Arabidopsis Root Vascular System.

When perceiving microbe-associated molecular patterns (MAMPs) or plant-derived damage-associated molecular patterns (DAMPs), plants alter their root growth and development by displaying a reduction in the root length and the formation of root hairs and lateral roots. The exogenous application of a MAMP peptide, flg22, was shown to affect root growth by suppressing meristem activity. In addition to MAMPs, the DAMP peptide PEP1 suppresses root growth while also promoting root hair formation. However, the question of whether and how these elicitor peptides affect the development of the vascular system in the root has not been explored. The cellular receptors of PEP1, PEPR1 and PEPR2 are highly expressed in the root vascular system, while the receptors of flg22 (FLS2) and elf18 (EFR) are not. Consistent with the expression patterns of PEP1 receptors, we found that exogenously applied PEP1 has a strong impact on the division of stele cells, leading to a reduction of these cells. We also observed the alteration in the number and organization of cells that differentiate into xylem vessels. These PEP1-mediated developmental changes appear to be linked to the blockage of symplastic connections triggered by PEP1. PEP1 dramatically disrupts the symplastic movement of free green fluorescence protein (GFP) from phloem sieve elements to neighboring cells in the root meristem, leading to the deposition of a high level of callose between cells. Taken together, our first survey of PEP1-mediated vascular tissue development provides new insights into the PEP1 function as a regulator of cellular reprogramming in the Arabidopsis root vascular system.

Information on the move: vascular tissue development in space and time during postembryonic root growth.

Cascades of temporal and spatial regulation of gene expression play crucial roles in the vascular development in plant roots. Once vascular cell fates are determined, the timing of their differentiation is tightly controlled in a cell-autonomous manner. In contrast, extensive cell-to-cell communication contributes to the positioning and specifying of vascular cell types in the root meristem. Diverse factors moving short distances in a radial direction were found to be key contributors to these processes. Furthermore, signals from differentiated phloem were found to influence the phloem precursor and determine how the corresponding asymmetric cell division proceeded. These findings highlight the potential importance of underexplored types of intercellular communication in relation to vascular tissue development during postembryonic root growth.

Identification of Conserved Gene-Regulatory Networks that Integrate Environmental Sensing and Growth in the Root Cambium.

Cambium drives the lateral growth of stems and roots, contributing to diverse plant growth forms. The root crop is one of the outstanding examples of the cambium-driven growth. To understand its molecular basis, we used radish to generate a compendium of root-tissue- and stage-specific transcriptomes from two contrasting inbred lines during root growth. Expression patterns of key cambium regulators and hormone signaling components were validated. Clustering and gene ontology (GO) enrichment analyses of radish datasets followed by a comparative analysis against the newly established Arabidopsis early cambium data revealed evolutionary conserved stress-response transcription factors that may intimately control the cambium. Indeed, an in vivo network consisting of selected stress-response and cambium regulators indicated ERF-1 as a potential key checkpoint of cambial activities, explaining how cambium-driven growth is altered in response to environmental changes. The findings here provide valuable information about dynamic gene expression changes during cambium-driven root growth and have implications with regard to future engineering schemes, leading to better crop yields.

Reprogramming of the cambium regulators during adventitious root development upon wounding of storage tap roots in radish (Raphanus sativus L.).

Cambium contains a stem cell population that produces xylem and phloem tissues in a radial direction during the secondary growth stage. The growth of many storage roots, including in the radish, Raphanus sativus L., also depends on cambium. Interestingly, we observed numerous adventitious roots (ARs) emerging from the cambia of cut surfaces when the bases of radish storage tap roots were removed. Previous studies in Arabidopsis showed that the WOX11/12 pathway regulates AR initiation and meristem establishment in an auxin-dependent manner. Here, we provide evidence indicating the evolutionary conservation of the WOX11/12 pathway during the AR development in radishes. Additionally, we found that expression of two cambium regulators, PXY and WOX4, is induced in the cambium regions that are connected to emerging ARs via vascularization. Both AR formation and genes associated with this were induced by exogenous auxin. Our research suggests that some key cambium regulators might be reprogrammed to aid in the AR development in concert with the WOX11/12 pathway.This article has an associated First Person interview with the first author of the paper.