Root Regeneration Triggers an Embryo-like Sequence Guided by Hormonal Interactions.

Plant roots can regenerate after excision of their tip, including the stem cell niche. To determine which developmental program mediates such repair, we applied a combination of lineage tracing, single-cell RNA sequencing, and marker analysis to test different models of tissue reassembly. We show that multiple cell types can reconstitute stem cells, demonstrating the latent potential of untreated plant cells. The transcriptome of regenerating cells prior to stem cell activation resembles that of an embryonic root progenitor. Regeneration defects are more severe in embryonic than in adult root mutants. Furthermore, the signaling domains of the hormones auxin and cytokinin mirror their embryonic dynamics and manipulation of both hormones alters the position of new tissues and stem cell niche markers. Our findings suggest that plant root regeneration follows, on a larger scale, the developmental stages of embryonic patterning and is guided by spatial information provided by complementary hormone domains.

A pan-grass transcriptome reveals patterns of cellular divergence in crops.

Different plant species within the grasses were parallel targets of domestication, giving rise to crops with distinct evolutionary histories and traits1. Key traits that distinguish these species are mediated by specialized cell types2. Here we compare the transcriptomes of root cells in three grass species-Zea mays, Sorghum bicolor and Setaria viridis. We show that single-cell and single-nucleus RNA sequencing provide complementary readouts of cell identity in dicots and monocots, warranting a combined analysis. Cell types were mapped across species to identify robust, orthologous marker genes. The comparative cellular analysis shows that the transcriptomes of some cell types diverged more rapidly than those of others-driven, in part, by recruitment of gene modules from other cell types. The data also show that a recent whole-genome duplication provides a rich source of new, highly localized gene expression domains that favour fast-evolving cell types. Together, the cell-by-cell comparative analysis shows how fine-scale cellular profiling can extract conserved modules from a pan transcriptome and provide insight on the evolution of cells that mediate key functions in crops.

Glutathione accelerates the cell cycle and cellular reprogramming in plant regeneration.

The plasticity of plant cells underlies their wide capacity to regenerate, with increasing evidence in plants and animals implicating cell cycle dynamics in cellular reprogramming. To investigate the cell cycle during cellular reprogramming, we developed a comprehensive set of cell cycle phase markers in the Arabidopsis root. Using single-cell RNA-seq profiles and live imaging during regeneration, we found that a subset of cells near an ablation injury dramatically increases division rate by truncating G1. Cells in G1 undergo a transient nuclear peak of glutathione (GSH) prior to coordinated entry into S phase followed by rapid divisions and cellular reprogramming. A symplastic block of the ground tissue impairs regeneration, which is rescued by exogenous GSH. We propose a model in which GSH from the outer tissues is released upon injury licensing an exit from G1 near the wound to induce rapid cell division and reprogramming.

A rapid and sensitive, multiplex, whole mount RNA fluorescence in situ hybridization and immunohistochemistry protocol.

BACKGROUND: In the past few years, there has been an explosion in single-cell transcriptomics datasets, yet in vivo confirmation of these datasets is hampered in plants due to lack of robust validation methods. Likewise, modeling of plant development is hampered by paucity of spatial gene expression data. RNA fluorescence in situ hybridization (FISH) enables investigation of gene expression in the context of tissue type. Despite development of FISH methods for plants, easy and reliable whole mount FISH protocols have not yet been reported. RESULTS: We adapt a 3-day whole mount RNA-FISH method for plant species based on a combination of prior protocols that employs hybridization chain reaction (HCR), which amplifies the probe signal in an antibody-free manner. Our whole mount HCR RNA-FISH method shows expected spatial signals with low background for gene transcripts with known spatial expression patterns in Arabidopsis inflorescences and monocot roots. It allows simultaneous detection of three transcripts in 3D. We also show that HCR RNA-FISH can be combined with endogenous fluorescent protein detection and with our improved immunohistochemistry (IHC) protocol. CONCLUSIONS: The whole mount HCR RNA-FISH and IHC methods allow easy investigation of 3D spatial gene expression patterns in entire plant tissues.

Plant glutamate receptors mediate a bet-hedging strategy between regeneration and defense.

Wounding is a trigger for both regeneration and defense in plants, but it is not clear whether the two responses are linked by common activation or regulated as trade-offs. Although plant glutamate-receptor-like proteins (GLRs) are known to mediate defense responses, here, we implicate GLRs in regeneration through dynamic changes in chromatin and transcription in reprogramming cells near wound sites. We show that genetic and pharmacological inhibition of GLR activity increases regeneration efficiency in multiple organ repair systems in Arabidopsis and maize. We show that the GLRs work through salicylic acid (SA) signaling in their effects on regeneration, and mutants in the SA receptor NPR1 are hyper-regenerative and partially resistant to GLR perturbation. These findings reveal a conserved mechanism that regulates a trade-off between defense and regeneration, and they also offer a strategy to improve regeneration in agriculture and conservation.

Ground tissue circuitry regulates organ complexity in maize and Setaria.

Most plant roots have multiple cortex layers that make up the bulk of the organ and play key roles in physiology, such as flood tolerance and symbiosis. However, little is known about the formation of cortical layers outside of the highly reduced anatomy of Arabidopsis. Here, we used single-cell RNA sequencing to rapidly generate a cell-resolution map of the maize root, revealing an alternative configuration of the tissue formative transcription factor SHORT-ROOT (SHR) adjacent to an expanded cortex. We show that maize SHR protein is hypermobile, moving at least eight cell layers into the cortex. Higher-order SHR mutants in both maize and Setaria have reduced numbers of cortical layers, showing that the SHR pathway controls expansion of cortical tissue to elaborate anatomical complexity.

Week-long imaging of cell divisions in the Arabidopsis root meristem.

BACKGROUND: Characterizing the behaviors of dynamic systems requires capturing them with high temporal and spatial resolution. Owing to its transparency and genetic tractability, the Arabidopsis thaliana root lends itself well to live imaging when combined with cell and tissue-specific fluorescent reporters. We developed a novel 4D imaging method that utilizes simple confocal microscopy and readily available components to track cell divisions in the root stem cell niche and surrounding region for up to 1 week. RESULTS: Using this method, we performed a direct measurement of cell division intervals within and around the root stem cell niche. The results reveal a short, steep gradient of cell division rates in proximal stem cells, with progressively more rapid cell division rates from quiescent center (QC), to cells in direct contact with the QC (initials), to their immediate daughters, after which division rates appear to become more homogeneous. CONCLUSIONS: These results provide a baseline to study how perturbations in signaling could affect cell division patterns in the root meristem. This new setup further allows us to finely analyze meristematic cell division rates that lead to patterning.

Plant Cell Shape: Trafficking Gets Edgy.

Polyhedral-shaped plant cells have faces, corners, and edges that can have different material properties. As Kirchhelle et al. (2016) now show, RAB-A5c reveals a trafficking compartment that localizes to the edges where two cell walls meet, with a potential role in mediating local wall stiffness.

A Case for Distributed Control of Local Stem Cell Behavior in Plants.

The root meristem has a centrally located group of mitotically quiescent cells, to which current models assign a stem cell organizer function. However, evidence is emerging for decentralized control of stem cell activity, whereby self-renewing behavior emerges from the lack of cell displacement at the border of opposing differentiation gradients. We term this a "stagnation" model due to its reliance on passive mechanics. The position of stem cells is established by two opposing axes that reciprocally control each other's differentiation. Such broad tissue organization programs would allow plants, like some animal systems, to rapidly reconstitute stem cells from non-stem-cell tissues.