The Tomato BLADE ON PETIOLE and TERMINATING FLOWER Regulate Leaf Axil Patterning Along the Proximal-Distal Axes

New Advances in the Study of Regulation of Tomato Flowering-Related Genes Using Biotechnological Approaches

The tomato is a convenient object for studying reproductive processes, which has become a classic. Such complex processes as flowering and fruit setting require an understanding of the fundamental principles of molecular interaction, the structures of genes and proteins, the construction of signaling pathways for transcription regulation, including the synchronous actions of cis-regulatory elements (promoter and enhancer), trans-regulatory elements (transcription factors and regulatory RNAs), and transposable elements and epigenetic regulators (DNA methylation and acetylation, chromatin structure). Here, we discuss the current state of research on tomatoes (2017-2023) devoted to studying the function of genes that regulate flowering and signal regulation systems using genome-editing technologies, RNA interference gene silencing, and gene overexpression, including heterologous expression. Although the central candidate genes for these regulatory components have been identified, a complete picture of their relationship has yet to be formed. Therefore, this review summarizes the latest achievements related to studying the processes of flowering and fruit set. This work attempts to display the gene interaction scheme to better understand the events under consideration.

Regulation of secondary growth by poplar BLADE-ON-PETIOLE genes in Arabidopsis

BLADE-ON-PETIOLE (BOP) genes are essential regulators of vegetative and reproductive development in land plants. First characterized in Arabidopsis thaliana (Arabidopsis), members of this clade function as transcriptional co-activators by recruiting TGACG-motif binding (TGA) basic leucine zipper (bZIP) transcription factors. Highly expressed at organ boundaries, these genes are also expressed in vascular tissue and contribute to lignin biosynthesis during secondary growth. How these genes function in trees, which undergo extensive secondary growth to produce wood, remains unclear. Here, we investigate the functional conservation of BOP orthologs in Populus trichocarpa (poplar), a widely-used model for tree development. Within the poplar genome, we identified two BOP-like genes, PtrBPL1 and PtrBPL2, with abundant transcripts in stems. To assess their functions, we used heterologous assays in Arabidopsis plants. The promoters of PtrBPL1 and PtrBPL2, fused with a β-glucuronidase (GUS) reporter gene showed activity at organ boundaries and in secondary xylem and phloem. When introduced into Arabidopsis plants, PtrBPL1 and PtrBPL2 complemented leaf and flower patterning defects in bop1 bop2 mutants. Notably, Arabidopsis plants overexpressing PtrBPL1 and PtrBPL2 showed defects in stem elongation and the lignification of secondary tissues in the hypocotyl and stem. Finally, PtrBPL1 and PtrBPL2 formed complexes with TGA bZIP proteins in yeast. Collectively, our findings suggest that PtrBPL1 and PtrBPL2 are orthologs of Arabidopsis BOP1 and BOP2, potentially contributing to secondary growth regulation in poplar trees. This work provides a foundation for functional studies in trees.

A BLADE-ON-PETIOLE orthologue regulates corolla differentiation in the proximal region in Torenia fournieri

The three-dimensional shape of a flower is integrated by morphogenesis along different axes. Differentiation along the petal proximodistal axis is tightly linked to the specification of pollinators; however, it is still unclear how a petal patterns this axis. The corolla of Torenia fournieri exhibits strong differentiation along the proximodistal axis, and we previously found a proximal regulator, TfALOG3, controlling corolla neck differentiation. Here, we report another gene, TfBOP2, which is predominantly expressed in the proximal region of the corolla. TfBOP2 mutants have shorter proximal corolla tubes and longer distal lobe, demonstrating its function as a proximal regulator. Arabidopsis BOPs mutant shows similar defects, favouring a shared role of BOPs homologues. Genetic analysis demonstrates the interaction between TfBOP2 and TfALOG3, and we further found that TfALOG3 physically interacts with TfBOP2 and can recruit TfBOP2 to the nuclear region. Our study favours a hypothetical shared BOP-ALOG complex that is recruited to regulate corolla differentiation in the proximal region axis of T. fournieri.

The Secrets of Meristems Initiation: Axillary Meristem Initiation and Floral Meristem Initiation

The branching phenotype is an extremely important agronomic trait of plants, especially for horticultural crops. It is not only an important yield character of fruit trees, but also an exquisite ornamental trait of landscape trees and flowers. The branching characteristics of plants are determined by the periodic initiation and later development of meristems, especially the axillary meristem (AM) in the vegetative stage and the floral meristem (FM) in the reproductive stage, which jointly determine the above-ground plant architecture. The regulation of meristem initiation has made great progress in model plants in recent years. Meristem initiation is comprehensively regulated by a complex regulatory network composed of plant hormones and transcription factors. However, as it is an important trait, studies on meristem initiation in horticultural plants are very limited, and the mechanism of meristem initiation regulation in horticultural plants is largely unknown. This review summarizes recent research advances in axillary meristem regulation and mainly reviews the regulatory networks and mechanisms of AM and FM initiation regulated by transcription factors and hormones. Finally, considering the existing problems in meristem initiation studies and the need for branching trait improvement in horticulture plants, we prospect future studies to accelerate the genetic improvement of the branching trait in horticulture plants.

Quantitative Image Processing for Three-Dimensional Episcopic Images of Biological Structures: Current State and Future Directions

Episcopic imaging using techniques such as High Resolution Episcopic Microscopy (HREM) and its variants, allows biological samples to be visualized in three dimensions over a large field of view. Quantitative analysis of episcopic image data is undertaken using a range of methods. In this systematic review, we look at trends in quantitative analysis of episcopic images and discuss avenues for further research. Papers published between 2011 and 2022 were analyzed for details about quantitative analysis approaches, methods of image annotation and choice of image processing software. It is shown that quantitative processing is becoming more common in episcopic microscopy and that manual annotation is the predominant method of image analysis. Our meta-analysis highlights where tools and methods require further development in this field, and we discuss what this means for the future of quantitative episcopic imaging, as well as how annotation and quantification may be automated and standardized across the field.

The transcriptional co-regulators NBCL1 and NBCL2 redundantly coordinate aerial organ development and root nodule identity in legumes

Medicago truncatula NODULE ROOT1 (MtNOOT1) and Pisum sativum COCHLEATA1 (PsCOCH1) are orthologous genes belonging to the NOOT-BOP-COCH-LIKE (NBCL) gene family which encodes key transcriptional co-regulators of plant development. In Mtnoot1 and Pscoch1 mutants, the development of stipules, flowers, and symbiotic nodules is altered. MtNOOT2 and PsCOCH2 represent the single paralogues of MtNOOT1 and PsCOCH1, respectively. In M. truncatula, MtNOOT1 and MtNOOT2 are both required for the establishment and maintenance of symbiotic nodule identity. In legumes, the role of NBCL2 in above-ground development is not known. To better understand the roles of NBCL genes in legumes, we used M. truncatula and P. sativum nbcl mutants, isolated a knockout mutant for the PsCOCH2 locus and generated Pscoch1coch2 double mutants in P. sativum. Our work shows that single Mtnoot2 and Pscoch2 mutants develop wild-type stipules, flowers, and symbiotic nodules. However, the number of flowers was increased and the pods and seeds were smaller compared to the wild type. Furthermore, in comparison to the corresponding nbcl1 single mutants, both the M. truncatula and P. sativum nbcl double mutants show a drastic alteration in stipule, inflorescence, flower, and nodule development. Remarkably, in both M. truncatula and P. sativum nbcl double mutants, stipules are transformed into a range of aberrant leaf-like structures.

Artefacts in Volume Data Generated with High Resolution Episcopic Microscopy (HREM)

High resolution episcopic microscopy (HREM) produces digital volume data by physically sectioning histologically processed specimens, while capturing images of the subsequently exposed block faces. Our study aims to systematically define the spectrum of typical artefacts inherent to HREM data and to research their effect on the interpretation of the phenotype of wildtype and mutant mouse embryos. A total of 607 (198 wildtypes, 409 mutants) HREM data sets of mouse embryos harvested at embryonic day (E) 14.5 were systematically and comprehensively examined. The specimens had been processed according to essentially identical protocols. Each data set comprised 2000 to 4000 single digital images. Voxel dimensions were 3 × 3 × 3 µm³. Using 3D volume models and virtual resections, we identified a number of characteristic artefacts and grouped them according to their most likely causality. Furthermore, we highlight those that affect the interpretation of embryo data and provide examples for artefacts mimicking tissue defects and structural pathologies. Our results aid in optimizing specimen preparation and data generation, are vital for the correct interpretation of HREM data and allow distinguishing tissue defects and pathologies from harmless artificial alterations. In particular, they enable correct diagnosis of pathologies in mouse embryos serving as models for deciphering the mechanisms of developmental disorders.

Spatial transcriptional signatures define margin morphogenesis along the proximal-distal and medio-lateral axes in tomato (Solanum lycopersicum) leaves

Leaf morphogenesis involves cell division, expansion, and differentiation in the developing leaf, which take place at different rates and at different positions along the medio-lateral and proximal-distal leaf axes. The gene expression changes that control cell fate along these axes remain elusive due to difficulties in precisely isolating tissues. Here, we combined rigorous early leaf characterization, laser capture microdissection, and transcriptomic sequencing to ask how gene expression patterns regulate early leaf morphogenesis in wild-type tomato (Solanum lycopersicum) and the leaf morphogenesis mutant trifoliate. We observed transcriptional regulation of cell differentiation along the proximal-distal axis and identified molecular signatures delineating the classically defined marginal meristem/blastozone region during early leaf development. We describe the role of endoreduplication during leaf development, when and where leaf cells first achieve photosynthetic competency, and the regulation of auxin transport and signaling along the leaf axes. Knockout mutants of BLADE-ON-PETIOLE2 exhibited ectopic shoot apical meristem formation on leaves, highlighting the role of this gene in regulating margin tissue identity. We mapped gene expression signatures in specific leaf domains and evaluated the role of each domain in conferring indeterminacy and permitting blade outgrowth. Finally, we generated a global gene expression atlas of the early developing compound leaf.

Patterning a Leaf by Establishing Polarities

Leaves are the major organ for photosynthesis in most land plants, and leaf structure is optimized for the maximum capture of sunlight and gas exchange. Three polarity axes, the adaxial-abaxial axis, the proximal-distal axis, and the medial-lateral axis are established during leaf development to give rise to a flattened lamina with a large area for photosynthesis and blades that are extended on petioles for maximum sunlight. Adaxial cells are elongated, tightly packed cells with many chloroplasts, and their fate is specified by HD-ZIP III and related factors. Abaxial cells are rounder and loosely packed cells and their fate is established and maintained by YABBY family and KANADI family proteins. The activities of adaxial and abaxial regulators are coordinated by ASYMMETRIC LEAVES2 and auxin. Establishment of the proximodistal axis involves the BTB/POZ domain proteins BLADE-ON-PETIOLE1 and 2, whereas homeobox genes PRESSED FLOWER and WUSCHEL-RELATED HOMEOBOX1 mediate leaf development along the mediolateral axis. This review summarizes recent advances in leaf polarity establishment with a focus on the regulatory networks involved.

High-Resolution Episcopic Imaging for Visualization of Dermal Arteries and Nerves of the Auricular Cymba Conchae in Humans

Therapeutic applications of auricular vagus nerve stimulation (VNS) have drawn recent attention. Since the targeted stimulation process and parameters depend on the electrode–tissue interaction, the lack of structural anatomical information on innervation and vascularization of the auricle restrain the current optimization of stimulation paradigms. For the first time, we employed high-resolution episcopic imaging (HREM) to generate histologic volume data from donated human cadaver ears. Optimal parameters for specimen preparation were evaluated. Anatomical 3D vascular and nerve structures were reconstructed in one sample of an auricular cymba conchae (CC). The feasibility of HREM to visualize anatomical structures was assessed in that diameters, occupied areas, volumes, and mutual distances between auricular arteries, nerves, and veins were registered. The selected region of CC (3 × 5.5 mm) showed in its cross-sections 21.7 ± 2.7 (mean ± standard deviation) arteries and 14.66 ± 2.74 nerve fibers. Identified nerve diameters were 33.66 ± 21.71 μm, and arteries had diameters in the range of 71.58 ± 80.70 μm. The respective occupied area showed a share of, on average, 2.71% and 0.3% for arteries and nerves, respectively, and similar volume occupancy for arteries and nerves. Inter-centroid minimum distance between arteries and nerves was 274 ± 222 μm. The density of vessels and nerves around a point within CC on a given grid was assessed, showing that 50% of all vessels and nerves were found in a radial distance of 1.6–1.8 mm from any of these points, which is strategically relevant when using stimulation needles in the auricle for excitation of nerves. HREM seems suitable for anatomical studies of the human ear. A 3D model of CC was established in the micrometer scale, which forms the basis for future optimization of the auricular VNS. Obviously, the presented single cadaver study needs to be validated by additional anatomical data on the innervation and vascularization of the auricle.

High-Resolution Episcopic Microscopy (HREM): Looking Back on 13 Years of Successful Generation of Digital Volume Data of Organic Material for 3D Visualisation and 3D Display

High-resolution episcopic microscopy (HREM) is an imaging technique that permits the simple and rapid generation of three-dimensional (3D) digital volume data of histologically embedded and physically sectioned specimens. The data can be immediately used for high-detail 3D analysis of a broad variety of organic materials with all modern methods of 3D visualisation and display. Since its first description in 2006, HREM has been adopted as a method for exploring organic specimens in many fields of science, and it has recruited a slowly but steadily growing user community. This review aims to briefly introduce the basic principles of HREM data generation and to provide an overview of scientific publications that have been published in the last 13 years involving HREM imaging. The studies to which we refer describe technical details and specimen-specific protocols, and provide examples of the successful use of HREM in biological, biomedical and medical research. Finally, the limitations, potentials and anticipated further improvements are briefly outlined.

BLADE-ON-PETIOLE genes are not involved in the transition from protonema to gametophore in the moss Physcomitrella patens

The timing of the transition between developmental phases is a critical determinant of plant form. In the moss Physcomitrella patens, the transition from protonema to gametophore is a particularly important step as it results in a change from two-dimensional to three-dimensional growth of the plant body. It is well known that this transition is promoted by cytokinin (CK), however, the underlying mechanisms are poorly understood. Previously, it was reported that P. patens orthologs of BLADE-ON-PETIOLE (BOP) genes (PpBOPs) work downstream of CK to promote the transition to gametophore. To further understand the role of PpBOPs in the control of this transition, we performed functional analyses of PpBOP genes. We simultaneously disrupted the function of all three PpBOP genes in P. patens using CRISPR technology, however, no abnormal phenotypes were observed in the triple mutant during either the gametophytic or the sporophytic growth stages. CK treatment did not alter the phase change in the triple mutant. We conclude that PpBOP genes are unnecessary in the control of P. patens development under normal conditions. We propose that BOP genes are not involved in the control of developmental processes in bryophytes and other basal land plants, but may function in physiological processes such as in the defense response.

Cotton plant defence against a fungal pathogen is enhanced by expanding BLADE-ON-PETIOLE1 expression beyond lateral-organ boundaries

In the plant response to pathogen infection, many genes' expression is temporally induced, while few spatially induced expression genes have been reported. Here, we show that GhBOP1 can autonomously expand expression from restrained tissue when Gossypium hirsutum plants are attacked by Verticillium dahliae, which is considered to be spatially induced expression. Loss- and gain-of-function analyses show that GhBOP1 is a positive regulator in the modulation of plant resistance to V. dahliae. Yeast two-hybrid assays, luciferase complementation imaging and GUS reporting show that GhBOP1 interaction with GhTGA3 promotes its activation activity, regulating the expression of down-stream defence-related genes. Moreover, the induced spatial expression of GhBOP1 is accompanied by GhBP1 repression. Both antagonistically regulate the lignin biosynthesis, conferring cotton plants enhanced resistance to V. dahliae. Taken together, these results demonstrate that GhBOP1 is an economic positive regulator participating in plant defence through both the GhBOP1-GhTGA3 module and lignin accumulation.