The tarani mutation alters surface curvature in Arabidopsis leaves by perturbing the patterns of surface expansion and cell division

The 'Candidatus Phytoplasma ziziphi' effectors SJP1/2 negatively control leaf size by stabilizing the transcription factor ZjTCP2 in jujube

Phytoplasmas manipulate host plant development to benefit insect vector colonization and their own invasion. However, the virulence factors and mechanisms underlying small-leaf formation caused by jujube witches' broom (JWB) phytoplasmas remain largely unknown. Here, effectors SJP1 and SJP2 from JWB phytoplasmas were identified to induce small-leaf formation in jujube (Ziziphus jujuba). In vivo interaction and expression assays showed that SJP1 and SJP2 interacted with and stabilized the transcription factor ZjTCP2. Overexpression of SJP1 and SJP2 in jujube induced ZjTCP2 accumulation. In addition, the abundance of miRNA319f_1 was significantly reduced in leaves of SJP1 and SJP2 transgenic jujube plants and showed the opposite pattern to the expression of its target, ZjTCP2, which was consistent with the pattern in diseased leaves. Overexpression of ZjTCP2 in Arabidopsis promoted ectopic leaves arising from the adaxial side of cotyledons and reduced leaf size. Constitutive expression of the miRNA319f_1 precursor in the 35S::ZjTCP2 background reduced the abundance of ZjTCP2 mRNA and reversed the cotyledon and leaf defects in Arabidopsis. Therefore, these observations suggest that effectors SJP1 and SJP2 induced small-leaf formation, at least partly, by interacting with and activating ZjTCP2 expression both at the transcriptional and the protein level, providing new insights into small-leaf formation caused by phytoplasmas in woody plants.

Up-regulation of LsKN1 promotes cytokinin and suppresses gibberellin biosynthesis to generate wavy leaves in lettuce

Lettuce (Lactuca sativa) is one of the most popular vegetables worldwide, and diverse leaf shapes, including wavy leaves, are important commercial traits. In this study, we examined the genetics of wavy leaves using an F2 segregating population, and identified a major QTL controlling wavy leaves. The candidate region contained LsKN1, which has previously been shown to be indispensable for leafy heads in lettuce. Complementation tests and knockout experiments verified the function of LsKN1 in producing wavy leaves. The LsKN1∇ allele, which has the insertion of a transposon and has previously been shown to control leafy heads, promoted wavy leaves in our population. Transposition of the CACTA transposon from LsKN1 compromised its function for wavy leaves. High expression of LsKN1 up-regulated several key genes associated with cytokinin (CK) to increase the content in the leaves, whereas it down-regulated the expression of genes in the gibberellin (GA) biosynthesis pathway to decrease the content. Application of CK to leaves enhanced the wavy phenotype, while application of GA dramatically flattened the leaves. We conclude that the changes in CK and GA contents that result from high expression of LsKN1 switch determinate cells to indeterminate, and consequently leads to the development of wavy leaves.

Research strategies for single-cell transcriptome analysis in plant leaves

The recent and continuous improvement in single-cell RNA sequencing (scRNA-seq) technology has led to its emergence as an efficient experimental approach in plant research. However, compared with single-cell research in animals and humans, the application of scRNA-seq in plant research is limited by several challenges, including cell separation, cell type annotation, cellular function analysis, and cell-cell communication networks. In addition, the unavailability of corresponding reliable and stable analysis methods and standards has resulted in the relative decentralization of plant single-cell research. Considering these shortcomings, this review summarizes the research progress in plant leaf using scRNA-seq. In addition, it describes the corresponding feasible analytical methods and associated difficulties and problems encountered in the current research. In the end, we provide a speculative overview of the development of plant single-cell transcriptome research in the future.

PEAPOD repressors modulate and coordinate developmental responses to light intensity in Arabidopsis

Higher plants adapt to different light intensities by altering hypocotyl elongation, stomatal density, seed size, and flowering time. Despite the importance of this developmental plasticity, knowledge of the underlying genetic and molecular mechanisms modulating and coordinating responses to light intensity remains incomplete. Here, I report that in Arabidopsis the PEAPOD (PPD) repressors PPD1 and PPD2 prevent exaggerated responses to light intensity. Genetic and transcriptome analyses, of a ppd deletion mutant and a PPD1 overexpression genotype, were used to identify how PPD repressors modulate the light signalling network. A ppd1/ppd2 deletion mutant has elongated hypocotyls, elevated stomatal density, enlarged seed, and delayed flowering, whereas overexpression of PPD1 results in the reverse. Transcription of both PPD1 and PPD2, upregulated in low light and downregulated in higher light, is activated by PHYTOCHROME INTERACTING FACTOR 4. I found PPDs modulate light signalling by negative regulation of SUPPRESSOR OF phyA-105 (SPA1) transcription. Whereas PPDs coordinate many of the responses to light intensity - hypocotyl elongation, flowering time, and stomatal density - by repression/de-repression of SPA1, PPD regulation of seed size occurs independent of SPA1. In conclusion PPD repressors modulate and coordinate developmental responses to light intensity by altering light signal transduction.

Leaf morphogenesis: The multifaceted roles of mechanics

Plants produce a rich diversity of biological forms, and the diversity of leaves is especially notable. Mechanisms of leaf morphogenesis have been studied in the past two decades, with a growing focus on the interactive roles of mechanics in recent years. Growth of plant organs involves feedback by mechanical stress: growth induces stress, and stress affects growth and morphogenesis. Although much attention has been given to potential stress-sensing mechanisms and cellular responses, the mechanical principles guiding morphogenesis have not been well understood. Here we synthesize the overarching roles of mechanics and mechanical stress in multilevel and multiple stages of leaf morphogenesis, encompassing leaf primordium initiation, phyllotaxis and venation patterning, and the establishment of complex mature leaf shapes. Moreover, the roles of mechanics at multiscale levels, from subcellular cytoskeletal molecules to single cells to tissues at the organ scale, are articulated. By highlighting the role of mechanical buckling in the formation of three-dimensional leaf shapes, this review integrates the perspectives of mechanics and biology to provide broader insights into the mechanobiology of leaf morphogenesis.

Evolution and conserved functionality of organ size and shape regulator PEAPOD

Transcriptional regulator PEAPOD (PPD) and its binding partners comprise a complex that is conserved throughout many core eudicot plants with regard to protein domain sequence and the function of controlling organ size and shape. Orthologues of PPD also exist in the basal angiosperm Amborella trichopoda, various gymnosperm species, the lycophyte Selaginella moellendorffii and several monocot genera, although until now it was not known if these are functional sequences. Here we report constitutive expression of orthologues from species representing diverse taxa of plant phylogeny in the Arabidopsis Δppd mutant. PPD orthologues from S. moellendorffii, gymnosperm Picea abies, A. trichopoda, monocot Musa acuminata, and dicot Trifolium repens were able to complement the mutant and return it to the wild-type phenotype, demonstrating the conserved functionality of PPD throughout vascular plants. In addition, analysis of bryophyte genomes revealed potential PPD orthologues in model liverwort and moss species, suggesting a more primitive lineage for this conserved regulator. The Poaceae (grasses) lack the genes for the PPD module and the reason for loss of the complex from this economically significant family is unclear, given that grasses were the last of the flowering plants to evolve. Bioinformatic analyses identified putative PPD orthologues in close relatives of the Poaceae, indicating that the explanation for absence of PPD in the grasses may be more complex than previously considered. Understanding the mechanisms which led to loss of PPD from the grasses will provide insight into evolution of the Poaceae.

Active suppression of leaflet emergence as a mechanism of simple leaf development

Angiosperm leaves show extensive shape diversity and are broadly divided into two forms; simple leaves with intact lamina and compound leaves with lamina dissected into leaflets. The mechanistic basis of margin dissection and leaflet initiation has been inferred primarily by analysing compound-leaf architecture, and thus whether the intact lamina of simple leaves has the potential to initiate leaflets upon endogenous gene inactivation remains unclear. Here, we show that the CINCINNATA-like TEOSINTE BRANCHED1, CYCLOIDEA, PROLIFERATING CELL FACTORS (CIN-TCP) transcription factors activate the class II KNOTTED1-LIKE (KNOX-II) genes and the CIN-TCP and KNOX-II proteins together redundantly suppress leaflet initiation in simple leaves. Simultaneous downregulation of CIN-TCP and KNOX-II in Arabidopsis leads to the reactivation of the stemness genes KNOX-I and CUPSHAPED COTYLEDON (CUC) and triggers ectopic organogenesis, eventually converting the simple lamina to a super-compound form that appears to initiate leaflets indefinitely. Thus, a conserved developmental mechanism promotes simple leaf architecture in which CIN-TCP-KNOX-II forms a strong differentiation module that suppresses the KNOX-I-CUC network and leaflet initiation.

The TARANI/ UBIQUITIN PROTEASE 14 protein is required for lateral root development in Arabidopsis

In our article published in Plant Physiology, we had reported tarani (tni) mutant in Arabidopsis, in which poly-ubiquitin hydrolysis is adversely affected, shows pleiotropic phenotypic defects including fewer lateral roots due to the stabilization of several AUX/IAAs and reduced auxin response. TNI encodes UBIQUITIN-SPECIFIC PROTEASE14 that maintains normal auxin response through ubiquitin recycling. Fewer lateral roots observed in tni could be due to defects in their primordia initiation or subsequent elongation post-initiation. Here we have tested this by marking the lateral root primordia with pCycB1;1::CycB1;1(DB):GUS reporter and counting the number of lateral root at various stages development of as a marker of lateral root primordium. The results suggest that TNI/UBP14 is required for LRP development, and a reduction in TNI activity causes a delay in LRP initiation and consequently shorter lateral roots in the tni seedlings. ABBREVIATIONS: LRP, lateral root primordium; XPP, xylem pole pericycle; LRFC, lateral root founder cells.

The PEAPOD Pathway and Its Potential To Improve Crop Yield

A key strategy to increase plant productivity is to improve intrinsic organ growth. Some of the regulatory networks underlying organ growth and development, as well as the interconnections between these networks, are highly conserved. An example of such a growth-regulatory module with a highly conserved role in final organ size and shape determination in eudicot species is the PEAPOD (PPD)/KINASE-INDUCIBLE DOMAIN INTERACTING (KIX)/STERILE APETALA (SAP) module. We review the proteins constituting the PPD pathway and their roles in different plant developmental processes, and explore options for future research. We also speculate on strategies to exploit knowledge about the PPD pathway for targeted yield improvement to engineer crop traits of agronomic interest, such as leaf, fruit, and seed size.

The Ubiquitin-Specific Protease TNI/UBP14 Functions in Ubiquitin Recycling and Affects Auxin Response

The ubiquitin-mediated proteasomal pathway regulates diverse cellular processes in plants by rapidly degrading target proteins, including the repressors of hormone signaling. Though ubiquitin proteases play a key role in this process by cleaving polyubiquitin chains to monomers, their function has not been studied in detail by mutational analysis. Here, we show that mutation in TARANI/UBIQUITIN-SPECIFIC PROTEASE14 (TNI/UBP14) leads to reduced auxin response and widespread auxin-related phenotypic defects in Arabidopsis (Arabidopsis thaliana). In a tni partial loss-of-function mutant that was originally isolated based on altered leaf shape, activity of the auxin-responsive reporters DR5::GUS, DR5::nYFP, and IAA2::GUS was reduced. Genetic interaction studies suggest that TNI is involved in auxin signaling and acts alongside TIR1, ARF7, and AUX1 Map-based cloning identified TNI as UBP14 Inefficient splicing of the mutant TNI transcript resulted in the formation of an inactive UBP14 protein, which led to accumulation of polyubiquitin chains and excess polyubiquitinated proteins in the mutant. In addition to the reduced auxin response, increased levels of DII:VENUS, IAA18:GUS, and HS::AXR3-NT:GUS were also observed in tni, perhaps due to inefficient polyubiquitin hydrolysis and proteasome-mediated degradation. Together, our study identifies a function for TNI/UBP14 in the auxin response through ubiquitin recycling.

WUSCHEL-related homeobox1 (WOX1) regulates vein patterning and leaf size in Cucumis sativus

In plants, WUSCHEL-related homeobox1 (WOX1) homologs promote lamina mediolateral outgrowth. However, the downstream components linking WOX1 and lamina development remain unclear. In this study, we revealed the roles of WOX1 in palmate leaf expansion in cucumber (Cucumis sativus). A cucumber mango fruit (mf) mutant, resulting from truncation of a WOX1-type protein (CsWOX1), displayed abnormal lamina growth and defects in the development of secondary and smaller veins. CsWOX1 was expressed in the middle mesophyll and leaf margins and rescued defects of the Arabidopsis wox1 prs double mutant. Transcriptomic analysis revealed that genes involved in auxin polar transport and auxin response were highly associated with leaf development. Analysis of the cucumber mf rl (round leaf) double mutant revealed that CsWOX1 functioned in vein development via PINOID (CsPID1)-controlled auxin transport. Overexpression of CsWOX1 in cucumber (CsWOX1-OE) affected vein patterning and produced 'butterfly-shaped' leaves. CsWOX1 physically interacted with CsTCP4a, which may account for the abnormal lamina development in the mf mutant line and the smaller leaves in the CsWOX1-OE plants. Our findings demonstrated that CsWOX1 regulates cucumber leaf vein development by modulating auxin polar transport; moreover, CsWOX1 regulates leaf size by controlling CIN-TCP genes.

Genome-wide identification of microRNAs involved in the regulation of fruit ripening and climacteric stages in melon (Cucumis melo)

Fruit ripening is influenced by multiple plant hormones and the regulation of genes. However, studies on posttranscriptional regulators (e.g., miRNAs) of fruit growth and ripening are limited. We used miRNA sequencing and degradome methods to identify miRNAs and their target genes in melon (Cucumis melo cv. Hetao melon). A total of 61 conserved miRNAs and 36 novel miRNAs were identified from fruit growth, ripening, climacteric, and postclimacteric developmental stage samples, of which 32 conserved miRNAs were differentially expressed between developmental stage samples. Sixty-two target genes of 43 conserved miRNAs and 1 novel miRNA were identified from degradome sequencing. To further investigate miRNA influencing fruit ripening, transgenic melon plants overexpressing pre-cme-miR393 (cme-miR393-OE) were generated and characterized. The results showed that fruit ripening was delayed in cme-miR393-OE transgenic lines compared to nontransgenic fruits. The target of cme-miR393 was also identified, and the expression of CmAFB2 was repressed in transgenic plants. These results provide evidence that miRNA regulates melon fruit ripening and provide potential targets to improve the horticultural traits of melon fruit.

De-ubiquitinases on the move: an emerging field in plant biology

A balance between the synthesis and degradation of active proteins governs diverse cellular processes in plants, spanning from cell-cycle progression and circadian rhythm to the outcome of several hormone signalling pathways. Ubiquitin-mediated post-translational modification determines the degradative fate of the target proteins, thereby altering the output of cellular processes. An equally important, and perhaps under-appreciated, aspect of this pathway is the antagonistic process of de-ubiquitination. De-ubiquitinases (DUBs), a group of processing enzymes, play an important role in maintaining cellular ubiquitin homeostasis by hydrolyzing ubiquitin poly-proteins and free poly-ubiquitin chains into mono-ubiquitin. Further, DUBs rescue the cellular proteins from 26S proteasome-mediated degradation to their active form by cleaving the poly-ubiquitin chain from the target protein. Any perturbation in DUB activity is likely to affect proteostasis and downstream cellular processes. This review illustrates recent findings on the biological significance and mechanisms of action of the DUBs in Arabidopsis thaliana, with an emphasis on ubiquitin-specific proteases (UBPs), the largest family among the DUBs. We focus on the putative roles of various protein-protein interaction interfaces in DUBs and their generalized function in ubiquitin recycling, along with their pre-eminent role in plant development.

Comparative transcriptomics enables the identification of functional orthologous genes involved in early leaf growth

Leaf growth is a complex trait for which many similarities exist in different plant species, suggesting functional conservation of the underlying pathways. However, a global view of orthologous genes involved in leaf growth showing conserved expression in dicots and monocots is currently missing. Here, we present a genome-wide comparative transcriptome analysis between Arabidopsis and maize, identifying conserved biological processes and gene functions active during leaf growth. Despite the orthology complexity between these distantly related plants, 926 orthologous gene groups including 2829 Arabidopsis and 2974 maize genes with similar expression during leaf growth were found, indicating conservation of the underlying molecular networks. We found 65% of these genes to be involved in one-to-one orthology, whereas only 28.7% of the groups with divergent expression had one-to-one orthology. Within the pool of genes with conserved expression, 19 transcription factor families were identified, demonstrating expression conservation of regulators active during leaf growth. Additionally, 25 Arabidopsis and 25 maize putative targets of the TCP transcription factors with conserved expression were determined based on the presence of enriched transcription factor binding sites. Based on large-scale phenotypic data, we observed that genes with conserved expression have a higher probability to be involved in leaf growth and that leaf-related phenotypes are more frequently present for genes having orthologues between dicots and monocots than clade-specific genes. This study shows the power of integrating transcriptomic with orthology data to identify or select candidates for functional studies during leaf development in flowering plants.

The TCP4 Transcription Factor Directly Activates TRICHOMELESS1 and 2 and Suppresses Trichome Initiation

Trichomes are the first line of defense on the outer surface of plants against biotic and abiotic stresses. Because trichomes on leaf surfaces originate from the common epidermal progenitor cells that also give rise to pavement cells and stomata, their density and distribution are under strict genetic control. Regulators of trichome initiation have been identified and incorporated into a biochemical pathway wherein an initiator complex promotes trichome fate in an epidermal progenitor cell, while an inhibitor complex suppresses it in the neighboring cells. However, it is unclear how these regulator proteins, especially the negative regulators, are induced by upstream transcription factors and integrated with leaf morphogenesis. Here, we show that the Arabidopsis (Arabidopsis thaliana) class II TCP proteins activate TRICHOMELESS1 (TCL1) and TCL2, the two established negative regulators of trichome initiation, and reduce trichome density on leaves. Loss-of-function of these TCP proteins increased trichome density whereas TCP4 gain-of-function reduced trichome number. TCP4 binds to the upstream regulatory elements of both TCL1 and TCL 2 and directly promotes their transcription. Further, the TCP-induced trichome suppression is independent of the SQUAMOSA PROMOTER BINDING PROTEIN LIKE family of transcription factors, proteins that also reduce trichome density at later stages of plant development. Our work demonstrates that the class II TCP proteins couple leaf morphogenesis with epidermal cell fate determination.

A photometric stereo-based 3D imaging system using computer vision and deep learning for tracking plant growth

BACKGROUND

Tracking and predicting the growth performance of plants in different environments is critical for predicting the impact of global climate change. Automated approaches for image capture and analysis have allowed for substantial increases in the throughput of quantitative growth trait measurements compared with manual assessments. Recent work has focused on adopting computer vision and machine learning approaches to improve the accuracy of automated plant phenotyping. Here we present PS-Plant, a low-cost and portable 3D plant phenotyping platform based on an imaging technique novel to plant phenotyping called photometric stereo (PS).

RESULTS

We calibrated PS-Plant to track the model plant Arabidopsis thaliana throughout the day-night (diel) cycle and investigated growth architecture under a variety of conditions to illustrate the dramatic effect of the environment on plant phenotype. We developed bespoke computer vision algorithms and assessed available deep neural network architectures to automate the segmentation of rosettes and individual leaves, and extract basic and more advanced traits from PS-derived data, including the tracking of 3D plant growth and diel leaf hyponastic movement. Furthermore, we have produced the first PS training data set, which includes 221 manually annotated Arabidopsis rosettes that were used for training and data analysis (1,768 images in total). A full protocol is provided, including all software components and an additional test data set.

CONCLUSIONS

PS-Plant is a powerful new phenotyping tool for plant research that provides robust data at high temporal and spatial resolutions. The system is well-suited for small- and large-scale research and will help to accelerate bridging of the phenotype-to-genotype gap.

The CIN-TCP transcription factors promote commitment to differentiation in Arabidopsis leaf pavement cells via both auxin-dependent and independent pathways

Cells in organ primordia undergo active proliferation at an early stage to generate sufficient number, before exiting proliferation and entering differentiation. However, how the actively proliferating cells are developmentally reprogrammed to acquire differentiation potential during organ maturation is unclear. Here, we induced a microRNA-resistant form of TCP4 at various developmental stages of Arabidopsis leaf primordium that lacked the activity of TCP4 and its homologues and followed its effect on growth kinematics. By combining this with spatio-temporal gene expression analysis, we show that TCP4 commits leaf cells within the transition zone to exit proliferation and enter differentiation. A 24-hour pulse of TCP4 activity was sufficient to impart irreversible differentiation competence to the actively dividing cells. A combination of biochemical and genetic analyses revealed that TCP4 imparts differentiation competence by promoting auxin response as well as by directly activating HAT2, a HD-ZIP II transcription factor-encoding gene that also acts downstream to auxin response. Our study offers a molecular link between the two major organ maturation factors, CIN-like TCPs and HD-ZIP II transcription factors and explains how TCP activity restricts the cell number and final size in a leaf.

Cells in a young organ primordium proliferate to generate sufficient number, before they exit division cycle and enter differentiation programme at later stages. While factors that drive cell cycle progression have been identified and studied in detail in diverse eukaryotic species, developmental factors that promote exit from division and entry into differentiation are less known, especially in the plant kingdom. Here, we show that the class II TCP proteins, notably TCP4, irreversibly reprogram the mitotic cells to exit division and acquire differentiation competence by auxin response as well as direct activation of HAT2 transcription. Our work offers a molecular link between class II TCP and HD-ZIP II genes during the cell differentiation and leaf maturation.

Ornithine δ-aminotransferase is critical for floret development and seed setting through mediating nitrogen reutilization in rice

Nitrogen is one of the most important nutrient element that is essential for plant growth and development. Many genes have been reported to contribute to nitrogen absorption and transportation. However, genes involved in nitrogen reutilization are seldom reported. Ornithine δ-aminotransferase (δOAT) is the enzyme connecting arginine cycling and proline cycling. Here, we found that OsOAT, the homologue of δOAT in rice, is essential for nitrogen reutilization through mediating arginase activity. In the Osoat mutant, metabolic abnormality induced by nitrogen deficiency in floret causes malformed glumes, incapable glume opening and anther indehiscence. These defects in the mutant affect the pollination process and lead to a low seed setting rate as well as abnormal seed shape. Intriguingly, urea can rescue the phenotypes of the Osoat mutant. Therefore, OsOAT is crucial for nitrogen reutilization and plays a critical role in floret development and seed setting in rice.

Arabidopsis Leaf Flatness Is Regulated by PPD2 and NINJA through Repression of CYCLIN D3 Genes

In Arabidopsis (Arabidopsis thaliana), reduced expression of the transcriptional regulator PEAPOD2 (PPD2) results in propeller-like rosettes with enlarged and dome-shaped leaves. However, the molecular and cellular processes underlying this peculiar phenotype remain elusive. Here, we studied the interaction between PPD2 and NOVEL INTERACTOR OF JAZ (NINJA) and demonstrated that ninja loss-of-function plants produce rosettes with dome-shaped leaves similar to those of ppd mutants but without the increase in size. We showed that ninja mutants have a convex-shaped primary cell cycle arrest front, putatively leading to excessive cell division in the central leaf blade region. Furthermore, ppd and ninja mutants have a similar increase in the expression of CYCLIN D3;2 (CYCD3;2), and ectopic overexpression of CYCD3;2 phenocopies the ppd and ninja rosette and leaf shape phenotypes without affecting the size. Our results reveal a pivotal contribution of NINJA in leaf development, in addition to its well-studied function in jasmonate signaling, and imply a new function for D3-type cyclins in, at least partially, uncoupling the size and shape phenotypes of ppd leaves.

CIN-TCP transcription factors: Transiting cell proliferation in plants

Leaves are the most conspicuous planar organs in plants, designed for efficient capture of sunlight and its conversion to energy that is channeled into sustaining the entire biosphere. How a few founder cells derived from the shoot apical meristem give rise to diverse leaf forms has interested naturalists and developmental biologists alike. At the heart of leaf morphogenesis lie two simple cellular processes, division and expansion, that are spatially and temporally segregated in a developing leaf. In leaves of dicot model species, cell division occurs predominantly at the base, concomitant with the expansion and differentiation of cells at the tip of the lamina that drives increase in leaf surface area. The timing of the transition from one cell fate (division) to the other (expansion) within a growing leaf lamina is a critical determinant of final leaf shape, size, complexity and flatness. The TCP proteins, unique to plant kingdom, are sequence-specific DNA-binding transcription factors that control several developmental and physiological traits. A sub-group of class II TCPs, called CINCINNATA-like TCPs (CIN-TCPs henceforth), are key regulators of the timing of the transition from division to expansion in dicot leaves. The current review highlights recent advances in our understanding of how the pattern of CIN-TCP activity is translated to the dynamic spatio-temporal control of cell-fate transition through the transactivation of cell-cycle regulators, growth-repressing microRNAs, and interactions with the chromatin remodeling machinery to modulate phytohormone responses. Unravelling how environmental inputs influence CIN-TCP-mediated growth control is a challenge for future studies. © 2018 IUBMB Life, 70(8):718-731, 2018.

Heterogeneity and Robustness in Plant Morphogenesis: From Cells to Organs

Development is remarkably reproducible, producing organs with the same size, shape, and function repeatedly from individual to individual. For example, every flower on the Antirrhinum stalk has the same snapping dragon mouth. This reproducibility has allowed taxonomists to classify plants and animals according to their morphology. Yet these reproducible organs are composed of highly variable cells. For example, neighboring cells grow at different rates in Arabidopsis leaves, sepals, and shoot apical meristems. This cellular variability occurs in normal, wild-type organisms, indicating that cellular heterogeneity (or diversity in a characteristic such as growth rate) is either actively maintained or, at a minimum, not entirely suppressed. In fact, cellular heterogeneity can contribute to producing invariant organs. Here, we focus on how plant organs are reproducibly created during development from these highly variable cells.

The TCP4 transcription factor regulates trichome cell differentiation by directly activating GLABROUS INFLORESCENCE STEMS in Arabidopsis thaliana

Trichomes are the first cell type to be differentiated during the morphogenesis of leaf epidermis and serve as an ideal model to study cellular differentiation. Many genes involved in the patterning and differentiation of trichome cells have been studied over the past decades, and the majority of these genes encode transcription factors that specifically regulate epidermal cell development. However, the upstream regulators of these genes that link early leaf morphogenesis with cell type differentiation are less studied. The TCP proteins are the plant-specific transcription factors involved in regulating diverse aspects of plant development including lateral organ morphogenesis by modulating cell proliferation and differentiation. Here, we show that the miR319-regulated class II TCP proteins, notably TCP4, suppress trichome branching in Arabidopsis leaves and inflorescence stem by direct transcriptional activation of GLABROUS INFLORESCENCE STEMS (GIS), a known negative regulator of trichome branching. The trichome branch number is increased in plants with reduced TCP activity and decreased in the gain-of-function lines of TCP4. Biochemical analyses show that TCP4 binds to the upstream regulatory region of GIS and activates its expression. Detailed genetic analyses show that GIS and TCP4 work in same pathway and GIS function is required for TCP4-mediated regulation of trichome differentiation. Taken together, these results identify a role for the class II TCP genes in trichome differentiation, thus providing a connection between organ morphogenesis and cellular differentiation.

Comparative Analysis of Fruit Ripening-Related miRNAs and Their Targets in Blueberry Using Small RNA and Degradome Sequencing

MicroRNAs (miRNAs) play vital roles in the regulation of fruit development and ripening. Blueberry is an important small berry fruit crop with economical and nutritional value. However, nothing is known about the miRNAs and their targets involved in blueberry fruit ripening. In this study, using high-throughput sequencing of small RNAs, 84 known miRNAs belonging to 28 families and 16 novel miRNAs were identified in white fruit (WF) and blue fruit (BF) libraries, which represent fruit ripening onset and in progress, respectively. Among them, 41 miRNAs were shown to be differentially expressed during fruit maturation, and 16 miRNAs representing 16 families were further chosen to validate the sRNA sequencing data by stem-loop qRT-PCR. Meanwhile, 178 targets were identified for 41 known and 7 novel miRNAs in WF and BF libraries using degradome sequencing, and targets of miR160 were validated using RLM-RACE (RNA Ligase-Mediated (RLM)-Rapid Amplification of cDNA Ends) approach. Moreover, the expression patterns of 6 miRNAs and their targets were examined during fruit development and ripening. Finally, integrative analysis of miRNAs and their targets revealed a complex miRNA-mRNA regulatory network involving a wide variety of biological processes. The findings will facilitate future investigations of the miRNA-mediated mechanisms that regulate fruit development and ripening in blueberry.

Variable Cell Growth Yields Reproducible OrganDevelopment through Spatiotemporal Averaging

Organ sizes and shapes are strikingly reproducible, despite the variable growth and division of individual cells within them. To reveal which mechanisms enable this precision, we designed a screen for disrupted sepal size and shape uniformity in Arabidopsis and identified mutations in the mitochondrial i-AAA protease FtsH4. Counterintuitively, through live imaging we observed that variability of neighboring cell growth was reduced in ftsh4 sepals. We found that regular organ shape results from spatiotemporal averaging of the cellular variability in wild-type sepals, which is disrupted in the less-variable cells of ftsh4 mutants. We also found that abnormal, increased accumulation of reactive oxygen species (ROS) in ftsh4 mutants disrupts organ size consistency. In wild-type sepals, ROS accumulate in maturing cells and limit organ growth, suggesting that ROS are endogenous signals promoting termination of growth. Our results demonstrate that spatiotemporal averaging of cellular variability is required for precision in organ size.

Genome-wide identification of the TIFY gene family in three cultivated Gossypium species and the expression of JAZ genes

TIFY proteins are plant-specific proteins containing TIFY, JAZ, PPD and ZML subfamilies. A total of 50, 54 and 28 members of the TIFY gene family in three cultivated cotton species—Gossypium hirsutum, Gossypium barbadense and Gossypium arboretum—were identified, respectively. The results of phylogenetic analysis showed that these TIFY genes were divided into eight clusters. The different clusters of gene family members often have similar gene structures, including the number of exons. The results of quantitative reverse transcription polymerase chain reaction (qRT-PCR) showed that different JAZ genes displayed distinct expression patterns in the leaves of upland cotton under treatment with Gibberellin (GA), methyl jasmonate (MeJA), Jasmonic acid (JA) and abscisic acid (ABA). Different groups of JAZ genes exhibited different expression patterns in cotton leaves infected with Verticillium dahliae. The results of the comparative analysis of TIFY genes in the three cultivated species will be useful for understanding the involvement of these genes in development and stress resistance in cotton.

Activation of YUCCA5 by the Transcription Factor TCP4 Integrates Developmental and Environmental Signals to Promote Hypocotyl Elongation in Arabidopsis

Cell expansion is an essential process in plant morphogenesis and is regulated by the coordinated action of environmental stimuli and endogenous factors, such as the phytohormones auxin and brassinosteroid. Although the biosynthetic pathways that generate these hormones and their downstream signaling mechanisms have been extensively studied, the upstream transcriptional network that modulates their levels and connects their action to cell morphogenesis is less clear. Here, we show that the miR319-regulated TCP (TEOSINTE BRANCHED1, CYCLODEA, PROLIFERATING CELL FACTORS) transcription factors, notably TCP4, directly activate YUCCA5 transcription and integrate the auxin response to a brassinosteroid-dependent molecular circuit that promotes cell elongation in Arabidopsis thaliana hypocotyls. Furthermore, TCP4 modulates the common transcriptional network downstream to auxin-brassinosteroid signaling, which is also triggered by environmental cues, such as light, to promote cell expansion. Our study links TCP function with the hormone response during cell morphogenesis and shows that developmental and environmental signals converge on a common transcriptional network to promote cell elongation.