A non-canonical BZR/BES transcription factor regulates the development of haploid reproductive organs in Marchantia polymorpha.

Gametogenesis, which is essential to the sexual reproductive system, has drastically changed during plant evolution. Bryophytes, lycophytes and ferns develop reproductive organs called gametangia-antheridia and archegonia for sperm and egg production, respectively. However, the molecular mechanism of early gametangium development remains unclear. Here we identified a 'non-canonical' type of BZR/BES transcription factor, MpBZR3, as a regulator of gametangium development in a model bryophyte, Marchantia polymorpha. Interestingly, overexpression of MpBZR3 induced ectopic gametangia. Genetic analysis revealed that MpBZR3 promotes the early phase of antheridium development in male plants. By contrast, MpBZR3 is required for the late phase of archegonium development in female plants. We demonstrate that MpBZR3 is necessary for the successful development of both antheridia and archegonia but functions in a different manner between the two sexes. Together, the functional specialization of this 'non-canonical' type of BZR/BES member may have contributed to the evolution of reproductive systems.

Sucrose Signaling Contributes to the Maintenance of Vascular Cambium by Inhibiting Cell Differentiation.

Plants produce sugars by photosynthesis and use them for growth and development. Sugars are transported from source-to-sink organs via the phloem in the vasculature. It is well known that vascular development is precisely controlled by plant hormones and peptide hormones. However, the role of sugars in the regulation of vascular development is poorly understood. In this study, we examined the effects of sugars on vascular cell differentiation using a vascular cell induction system named 'Vascular Cell Induction Culture System Using Arabidopsis Leaves' (VISUAL). We found that sucrose has the strongest inhibitory effect on xylem differentiation, among several types of sugars. Transcriptome analysis revealed that sucrose suppresses xylem and phloem differentiation in cambial cells. Physiological and genetic analyses suggested that sucrose might function through the BRI1-EMS-SUPPRESSOR1 transcription factor, which is the central regulator of vascular cell differentiation. Conditional overexpression of cytosolic invertase led to a decrease in the number of cambium layers due to an imbalance between cell division and differentiation. Taken together, our results suggest that sucrose potentially acts as a signal that integrates environmental conditions with the developmental program.

Molecular Mechanisms Underlying the Establishment and Maintenance of Vascular Stem Cells in Arabidopsis thaliana.

The vascular system plays pivotal roles in transporting water and nutrients throughout the plant body. Primary vasculature is established as a continuous strand, which subsequently initiates secondary growth through cell division. Key factors regulating primary and secondary vascular developments have been identified in numerous studies, and the regulatory networks including these factors have been elucidated through omics-based approaches. However, the vascular system is composed of a variety of cells such as xylem and phloem cells, which are commonly generated from vascular stem cells. In addition, the vasculature is located deep inside the plant body, which makes it difficult to investigate the vascular development while distinguishing between vascular stem cells and developing xylem and phloem cells. Recent technical advances in the tissue-clearing method, RNA-seq analysis and tissue culture system overcome these problems by enabling the cell-type-specific analysis during vascular development, especially with a special focus on stem cells. In this review, we summarize the recent findings on the establishment and maintenance of vascular stem cells.

Spatio-temporal imaging of cell fate dynamics in single plant cells using luminescence microscope.

Stem cell fates are spatio-temporally regulated during plant development. Time-lapse imaging of fluorescence reporters is the most widely used method for spatio-temporal analysis of biological processes. However, excitation light for imaging fluorescence reporters causes autofluorescence and photobleaching. Unlike fluorescence reporters, luminescence proteins do not require excitation light, and therefore offer an alternative reporter for long-term and quantitative spatio-temporal analysis. We established an imaging system for luciferase, which enabled monitoring cell fate marker dynamics during vascular development in a vascular cell induction system called VISUAL. Single cells expressing the cambium marker, proAtHB8:ELUC, had sharp luminescence peaks at different time points. Furthermore, dual-color luminescence imaging revealed spatio-temporal relationships between cells that differentiated into xylem or phloem, and cells that transitioned from procambium to cambium. This imaging system enables not only the detection of temporal gene expression, but also facilitates monitoring of spatio-temporal dynamics of cell identity transitions at the single cell level.

Gene co-expression network analysis identifies BEH3 as a stabilizer of secondary vascular development in Arabidopsis.

In plants, vascular stem cells located in the cambium continuously undergo self-renewal and differentiation during secondary growth. Recent advancements in cell sorting techniques have enabled access to the transcriptional regulatory framework of cambial cells. However, mechanisms underlying the robust control of vascular stem cells remain unclear. Here, we identified a new cambium-related regulatory module through co-expression network analysis using multiple transcriptome datasets obtained from an ectopic vascular cell transdifferentiation system using Arabidopsis cotyledons, Vascular cell Induction culture System Using Arabidopsis Leaves (VISUAL). The cambium gene list included a gene encoding the transcription factor BES1/BZR1 Homolog 3 (BEH3), whose homolog BES1 negatively affects vascular stem cell maintenance. Interestingly, null beh3 mutant alleles showed a large variation in their vascular size, indicating that BEH3 functions as a stabilizer of vascular stem cells. Genetic analysis revealed that BEH3 and BES1 perform opposite functions in the regulation of vascular stem cells and the differentiation of vascular cells in the context of the VISUAL system. At the biochemical level, BEH3 showed weak transcriptional repressor activity and functioned antagonistically to other BES/BZR members by competing for binding to the brassinosteroid response element. Furthermore, mathematical modeling suggested that the competitive relationship between BES/BZR homologs leads to the robust regulation of vascular stem cells.

Whole Irradiated Plant Leaves Showed Faster Photosynthetic Induction Than Individually Irradiated Leaves via Improved Stomatal Opening.

Rapid photosynthetic induction is crucial for plants under fluctuating light conditions in a crop canopy as well as in an understory. Most previous studies have focused on photosynthetic induction responses in a single leaf, whereas the systemic responses of the whole plant have not been considered. In a natural environment, however, both single leaves and whole plants are exposed to sunlight, since the light environment is not uniform even within a given plant. In the present study, we examined whether there is any difference between the photosynthetic induction response of a leaf of a whole irradiated plant and an individually irradiated leaf in Arabidopsis thaliana to consider photosynthetic induction as the response of a whole plant. We used two methods, the visualization of photosynthesis and direct measurements of gas-exchange and Chl fluorescence, to demonstrate that whole irradiated plant promoted its photosynthetic induction via improved stomatal opening compared with individually irradiated leaf. Furthermore, using two Arabidopsis knockout mutants of abscisic acid transporter, abcg25 and abcg40, the present study suggests that abscisic acid could be involved in this systemic response for stomatal opening, allowing plants to optimize the use of light energy at minimal cost in plants in a dynamic light environment.