Robust control of floral meristem determinacy by position-specific multifunctions of KNUCKLES

Chromosomal inversion at the DG1 promoter drives double-grain spikelets and enhances grain yield in sorghum

The phenomenon of multiple-grain spikelets is frequently observed in gramineous crops. In the case of dual-floret spikelets, the upper fertile floret develops normally to form a single grain, while the lower sterile floret undergoes abortion. Here we elucidate the role of Double-Grain 1 (DG1), a gene encoding a homeobox-domain-containing protein, in regulating the lower floret meristem activity and double-grain spikelet trait in sorghum. A 35.7-kb paracentric inversion in the DG1 promoter region leads to increased DG1 expression, probably by reducing repressive histone modifications. This increase in DG1 expression transforms the degenerated lower floret into a fertile one. The use of the superior DG1 allele results in an increase of approximately 40.7% to 46.1% in grain number per panicle and a 10.1% to 14.3% increase in overall grain yield. Our findings shed light on the sorghum double-grain spikelet characteristic, offering valuable insights for high-yield breeding designs in cereals.

ZINC FINGER PROTEIN2 suppresses funiculus lignification to ensure seed loading efficiency in Arabidopsis

The plant funiculus anchors the developing seed to the placenta within the inner dorsal pod strands of the silique wall and directly transports nutrients to the seeds. The lignified vasculature critically supports nutrient transport through the funiculus. However, molecular mechanisms underlying lignified secondary cell wall (SCW) biosynthesis in the funiculus remain elusive. Here, we show that the transcription factor ZINC FINGER PROTEIN2 (ZFP2) represses SCW formation in the cortex cells that surround the vasculature. This function is essential for efficient nutrient loading into the seeds. Notably, ZFP2 directly acts on the SCW transcription factor NAC SECONDARY WALL THICKENING PROMOTING FACTOR1 (NST1) to repress cortex cell lignification, providing a mechanism of how SCW biosynthesis is restricted to the vasculature of the funiculus to ensure proper seed loading in Arabidopsis.

KNUCKLES regulates floral meristem termination by controlling auxin distribution and cytokinin activity

The termination of floral meristem (FM) activity is essential for the normal development of reproductive floral organs. During this process, KNUCKLES (KNU), a C2H2-type zinc finger protein, crucially regulates FM termination by directly repressing the expression of both the stem cell identity gene WUSCHEL (WUS) and the stem cell marker gene CLAVATA3 (CLV3) to abolish the WUS-CLV3 feedback loop required for FM maintenance. In addition, phytohormones auxin and cytokinin are involved in FM regulation. However, whether KNU modulates auxin and cytokinin activities for FM determinacy control remains unclear. Here, we show that the auxin distribution and the cytokinin activity mediated by KNU in Arabidopsis (Arabidopsis thaliana) promote the termination of FM during stage 6 of flower development. Mutation of KNU leads to altered distribution of auxin and cytokinin in the FM of a stage 6 floral bud. Moreover, KNU directly represses the auxin transporter gene PIN-FORMED1 (PIN1) and the cytokinin biosynthesis gene ISOPENTENYLTRANSFERASE7 (IPT7) via mediating H3K27me3 deposition on these 2 loci to regulate auxin and cytokinin activities. Our study presents a molecular regulatory network that elucidates how the transcriptional repressor KNU integrates and modulates the activities of auxin and cytokinin, thus securing the timed FM termination.

DRN facilitates WUS transcriptional regulatory activity by chromatin remodeling to regulate shoot stem cell homeostasis in Arabidopsis

Shoot stem cells, harbored in the shoot apical meristem (SAM), play key roles during post-embryonic development of Arabidopsis and function as the origin of plant aerial tissues. Multiple transcription factors are involved in the sophisticated transcriptional regulation of stem cell homeostasis, with the WUSCHEL (WUS)/CLAVATA3 (CLV3) negative feedback loop playing a central role. WUS acts as a master regulator in maintaining stem cells through its transcriptional regulatory activity including repressive and activating abilities. Although the interaction between WUS and TOPLESS confers the repressive activity of WUS in transcriptional control, the mechanism by which WUS activates gene expression remains elusive. Here, we showed that DORNRÖSCHEN competitively interacts with WUS and disturbs the WUS homodimer, which recruits BRAHMA to activate CLV3 expression via nucleosome depletion for maintaining the stem cell pool.

Brassinosteroid signaling represses ZINC FINGER PROTEIN11 to regulate ovule development in Arabidopsis

Brassinosteroid (BR) signaling and the C-class MADS-box gene AGAMOUS (AG) play important roles in ovule development in Arabidopsis (Arabidopsis thaliana). However, how BR signaling integrates with AG functions to control the female reproductive process remains elusive. Here, we showed that the regulatory role of BR signaling in proper ovule development is mediated by the transcriptional repressor gene ZINC FINGER PROTEIN 11 (ZFP11), which is a direct target of AG. ZFP11 expression initiates from the placenta upon AG induction and becomes prominent in the funiculus of ovule primordia. Plants harboring zfp11 mutations showed reduced placental length with decreased ovule numbers and some aborted ovules. During ovule development, the transcription factor BRASSINAZOLE-RESISTANT 1 (BZR1), which functions downstream of BR signaling, inhibits ZFP11 expression in the chalaza and nucellus. Weakened BR signaling leads to stunted integuments in ovules, resulting from the direct repression of INNER NO OUTER (INO) and WUSCHEL (WUS) by extended ZFP11 expression in the chalaza and nucellus, respectively. In addition, the zfp11 mutant shows reduced sensitivity to BR biosynthesis inhibitors and can rescue outer integument defects in brassinosteroid insensitive 1 (bri1) mutants. Thus, the precise spatial regulation of ZFP11, which is activated by AG in the placenta and suppressed by BR signaling in the central and distal regions of ovules, is essential for ensuring sufficient ovule numbers and proper ovule formation.

MAC3A and MAC3B mediate degradation of the transcription factor ERF13 and thus promote lateral root emergence

Lateral roots (LRs) increase root surface area and allow plants greater access to soil water and nutrients. LR formation is tightly regulated by the phytohormone auxin. Whereas the transcription factor ETHYLENE-RESPONSIVE ELEMENT BINDING FACTOR13 (ERF13) prevents LR emergence in Arabidopsis (Arabidopsis thaliana), auxin activates MITOGEN-ACTIVATED PROTEIN KINASE14 (MPK14), which leads to ERF13 degradation and ultimately promotes LR emergence. In this study, we discovered interactions between ERF13 and the E3 ubiquitin ligases MOS4-ASSOCIATED COMPLEX 3A (MAC3A) and MAC3B. As MAC3A and MAC3B gradually accumulate in the LR primordium, ERF13 levels gradually decrease. We demonstrate that MAC3A and MAC3B ubiquitinate ERF13, leading to its degradation and accelerating the transition of LR primordia from stages IV to V. Auxin enhances the MAC3A and MAC3B interaction with ERF13 by facilitating MPK14-mediated ERF13 phosphorylation. In summary, this study reveals the molecular mechanism by which auxin eliminates the inhibitory factor ERF13 through the MPK14-MAC3A and MAC3B signaling module, thus promoting LR emergence.

Intercellular Communication in Shoot Meristems

The shoot meristem of land plants maintains the capacity for organ generation throughout its lifespan due to a group of undifferentiated stem cells. Most meristems are shaped like a dome with a precise spatial arrangement of functional domains, and, within and between these domains, cells interact through a network of interconnected signaling pathways. Intercellular communication in meristems is mediated by mobile transcription factors, small RNAs, hormones, and secreted peptides that are perceived by membrane-localized receptors. In recent years, we have gained deeper insight into the underlying molecular processes of the shoot meristem, and we discuss here how plants integrate internal and external inputs to control shoot meristem activities.

Heat stress impairs floral meristem termination and fruit development by affecting the BR-SlCRCa cascade in tomato

Floral meristem termination is a key step leading to carpel initiation and fruit development. The frequent occurrence of heat stress due to global warming often disrupts floral determinacy, resulting in defective fruit formation. However, the detailed mechanism behind this phenomenon is largely unknown. Here, we identify CRABS CLAW a (SlCRCa) as a key regulator of floral meristem termination in tomato. SlCRCa functions as an indispensable floral meristem terminator by suppressing SlWUS activity through the TOMATO AGAMOUS 1 (TAG1)-KNUCKLES (SlKNU)-INHIBITOR OF MERISTEM ACTIVITY (SlIMA) network. A direct binding assay revealed that SlCRCa specifically binds to the promoter and second intron of WUSCHEL (SlWUS). We also demonstrate that SlCRCa expression depends on brassinosteroid homeostasis in the floral meristem, which is repressed by heat stress via the circadian factor EARLY FLOWERING 3 (SlELF3). These results provide new insights into floral meristem termination and the heat stress response in flowers and fruits of tomato and suggest that SlCRCa provides a platform for multiple protein interactions that may epigenetically abrogate stem cell activity at the transition from floral meristem to carpel initiation.

Developmental timing in plants

Plants exhibit reproducible timing of developmental events at multiple scales, from switches in cell identity to maturation of the whole plant. Control of developmental timing likely evolved for similar reasons that humans invented clocks: to coordinate events. However, whereas clocks are designed to run independently of conditions, plant developmental timing is strongly dependent on growth and environment. Using simplified models to convey key concepts, we review how growth-dependent and inherent timing mechanisms interact with the environment to control cyclical and progressive developmental transitions in plants.

The emerging functions of mini zinc finger (MIF) microproteins in seed plants: A minireview

Mini zinc fingers constitute a class of microproteins that appeared early in evolution and expanded in seeds plants. In this review, the phylogenetic history, the functions and the mode of action of Mini zinc fingers in plants are reported and discussed. It appears that mini zinc fingers play an important role in the control of plant development. They are involved in the control of cell division and expansion, in the switch between the determinate/indeterminate state of the meristems and in the regulation of vegetative growth and floral organ development. Their biochemical mode of action seems to be diverse. In some studies, it has been reported that mini zinc fingers can directly bind to DNA and activate target gene expression, whereas other studies have shown that they can interact with and inhibit the activity of specific zinc finger homeodomain transcription factors or act as adaptor proteins necessary to aggregate polymeric protein complexes corresponding to chromatin remodelling factors negatively regulating the expression of specific genes. The diversity of mode of action for mini zinc finger microproteins suggests a wider range of biological functions than what has been that described in the literature thus far, and their involvement in the response to biotic and abiotic stresses should be further investigated in future studies.

Comprehensive analysis of WOX transcription factors provide insight into genes related to the regulation of unisexual flowers development in Akebia trifoliata

Akebia trifoliata is a fascinating economic and medicinal plant that produces functionally unisexual flowers due to stamen/pistil abortion during flower development, and the genetic regulation pathway of this process remain completely unknown. Here, 10 AktWOXs were identified for the first time, all contained a highly conserved homeodomain. AktWOXs were divided into three clades, each with the same or similar intron, exon, and motifs distribution. Many cis-elements related to stress response, growth and development, and hormone response were found in the AktWOXs promoter region. In addition, four candidate genes AktWOX8, AktWOX11, AktWOX13.2 and AktWUS that might be involved in unisexual flowers development were screened, all of which were located in the nucleus and showed transcriptional activation activity. Yeast one-hybrid showed that both AktKNU and AktAG1, the potential core transcription factors in the activity termination pathway of flower meristem stem cells, could bind to the promoter region of AktWUS. Dual-luciferase assay further confirmed that only AktKNU inhibited the expression of AktWUS. Collectively, this study revealed the mechanism of AktWUS that might affect the formation of unisexual flowers by regulating the timely termination of flower meristem in A. trifoliata.

Time-Course Transcriptomic Analysis Reveals Molecular Insights into the Inflorescence and Flower Development of Cardiocrinum giganteum

Cardiocrinum giganteum is an endemic species of east Asia which is famous for its showy inflorescence and medicinal bulbs. Its inflorescence is a determinate raceme and the flowers bloom synchronously. Morphological observation and time-course transcriptomic analysis were combined to study the process of inflorescence and flower development of C. giganteum. The results show that the autonomic pathway, GA pathway, and the vernalization pathway are involved in the flower formation pathway of C. giganteum. A varied ABCDE flowering model was deduced from the main development process. Moreover, it was found that the flowers in different parts of the raceme in C. giganteum gradually synchronized during development, which is highly important for both evolution and ecology. The results obtained in this work improve our understanding of the process and mechanism of inflorescence and flower development and could be useful for the flowering period regulation and breeding of C. giganteum.

An update on evolutionary, structural, and functional studies of receptor-like kinases in plants

All living organisms must develop mechanisms to cope with and adapt to new environments. The transition of plants from aquatic to terrestrial environment provided new opportunities for them to exploit additional resources but made them vulnerable to harsh and ever-changing conditions. As such, the transmembrane receptor-like kinases (RLKs) have been extensively duplicated and expanded in land plants, increasing the number of RLKs in the advanced angiosperms, thus becoming one of the largest protein families in eukaryotes. The basic structure of the RLKs consists of a variable extracellular domain (ECD), a transmembrane domain (TM), and a conserved kinase domain (KD). Their variable ECDs can perceive various kinds of ligands that activate the conserved KD through a series of auto- and trans-phosphorylation events, allowing the KDs to keep the conserved kinase activities as a molecular switch that stabilizes their intracellular signaling cascades, possibly maintaining cellular homeostasis as their advantages in different environmental conditions. The RLK signaling mechanisms may require a coreceptor and other interactors, which ultimately leads to the control of various functions of growth and development, fertilization, and immunity. Therefore, the identification of new signaling mechanisms might offer a unique insight into the regulatory mechanism of RLKs in plant development and adaptations. Here, we give an overview update of recent advances in RLKs and their signaling mechanisms.

Old school, new rules: floral meristem development revealed by 3D gene expression atlases and high-resolution transcription factor-chromatin dynamics

The intricate morphology of the flower is primarily established within floral meristems in which floral organs will be defined and from where the developing flower will emerge. Floral meristem development involves multiscale-level regulation, including lineage and positional mechanisms for establishing cell-type identity, and transcriptional regulation mediated by changes in the chromatin environment. However, many key aspects of floral meristem development remain to be determined, such as: 1) the exact role of cellular location in connecting transcriptional inputs to morphological outcomes, and 2) the precise interactions between transcription factors and chromatin regulators underlying the transcriptional networks that regulate the transition from cell proliferation to differentiation during floral meristem development. Here, we highlight recent studies addressing these points through newly developed spatial reconstruction techniques and high-resolution transcription factor-chromatin environment interactions in the model plant Arabidopsis thaliana. Specifically, we feature studies that reconstructed 3D gene expression atlases of the floral meristem. We also discuss how the precise timing of floral meristem specification, floral organ patterning, and floral meristem termination is determined through temporally defined epigenetic dynamics for fine-tuning of gene expression. These studies offer fresh insights into the well-established principles of floral meristem development and outline the potential for further advances in this field in an age of integrated, powerful, multiscale resolution approaches.

Organogenic events during gynoecium and fruit development in Arabidopsis

Angiosperms are the most successful group of land plants. This success is mainly due to the gynoecium, the innermost whorl of the flower. In Arabidopsis, the gynoecium is a syncarpic structure formed by two congenitally fused carpels. At the fusion edges of the carpels, the carpel margin meristem forms. This quasi-meristem is important for medial-tissue development, including the ovules. After the double fertilization, both the seeds and fruit begin to develop. Due to the importance of seeds and fruits as major food sources worldwide, it has been an important task for the scientific community to study gynoecium development. In this review, we present the most recent advances in Arabidopsis gynoecium patterning, as well as some questions that remain unanswered.

AGAMOUS regulates various target genes via cell cycle-coupled H3K27me3 dilution in floral meristems and stamens

The MADS domain transcription factor AGAMOUS (AG) regulates floral meristem termination by preventing maintenance of the histone modification lysine 27 of histone H3 (H3K27me3) along the KNUCKLES (KNU) coding sequence. At 2 d after AG binding, cell division has diluted the repressive mark H3K27me3, allowing activation of KNU transcription prior to floral meristem termination. However, how many other downstream genes are temporally regulated by this intrinsic epigenetic timer and what their functions are remain unknown. Here, we identify direct AG targets regulated through cell cycle-coupled H3K27me3 dilution in Arabidopsis thaliana. Expression of the targets KNU, AT HOOK MOTIF NUCLEAR LOCALIZED PROTEIN18 (AHL18), and PLATZ10 occurred later in plants with longer H3K27me3-marked regions. We established a mathematical model to predict timing of gene expression and manipulated temporal gene expression using the H3K27me3-marked del region from the KNU coding sequence. Increasing the number of del copies delayed and reduced KNU expression in a polycomb repressive complex 2- and cell cycle-dependent manner. Furthermore, AHL18 was specifically expressed in stamens and caused developmental defects when misexpressed. Finally, AHL18 bound to genes important for stamen growth. Our results suggest that AG controls the timing of expression of various target genes via cell cycle-coupled dilution of H3K27me3 for proper floral meristem termination and stamen development.

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.

Ethylene-responsive SbWRKY50 suppresses leaf senescence by inhibition of chlorophyll degradation in sorghum

The onset of leaf de-greening and senescence is governed by a complex regulatory network including environmental cues and internal factors such as transcription factors (TFs) and phytohormones, in which ethylene (ET) is one key inducer. However, the detailed mechanism of ET signalling for senescence regulation is still largely unknown. Here, we found that the WRKY TF SbWRKY50 from Sorghum bicolor L., a direct target of the key component ETHYLENE INSENSITIVE 3 in ET signalling, functioned for leaf senescence repression. The clustered regularly interspaced short palindromic repeats/CRISPR-associated protein9-edited SbWRKY50 mutant (SbWRKY5O-KO) of sorghum displayed precocious senescent phenotypes, while SbWRKY50 overexpression delayed age-dependent and dark-induced senescence in sorghum. SbWRKY50 negatively regulated chlorophyll degradation through direct binding to the promoters of several chlorophyll catabolic genes. In addition, SbWRKY50 recruited the Polycomb repressive complex 1 through direct interaction with SbBMI1A, to induce histone 2A mono-ubiquitination accumulation on the chlorophyll catabolic genes for epigenetic silencing and thus delayed leaf senescence. Especially, SbWRKY50 can suppress early steps of chlorophyll catabolic pathway via directly repressing SbNYC1 (NON-YELLOW COLORING 1). Other senescence-related hormones could also influence leaf senescence through repression of SbWRKY50. Hence, our work shows that SbWRKY50 is an essential regulator downstream of ET and SbWRKY50 also responds to other phytohormones for senescence regulation in sorghum.

All's well that ends well: the timing of floral meristem termination

Floral meristem termination (FMT) represents one of the defining features of a floral meristem relative to a vegetative meristem. Timing of FMT is a major determinant of the total number of organs in a flower, and canalization toward relatively rapid FMT is considered to have been a major force in shaping angiosperm evolution. For decades, investigation of FMT has been focused on model systems that only produce four whorls of organs in a flower, while little is known about the molecular basis that underlies nature variation in the timing of FMT. Here, we hypothesize on how known pathways could have been modified to generate variation in FMT and explain how developing new model systems will help to deepen our understanding of the genetic control and evolution of FMT.

A novel CLAVATA1 mutation causes multilocularity in Brassica rapa

Locules are the seed-bearing structure of fruits. Multiple locules are associated with increased fruit size and seed set, and therefore, control of locule number is an important agronomic trait. Locule number is controlled in part by the CLAVATA-WUSCHEL pathway. Disruption of either the CLAVATA1 receptor-like kinase or its ligand CLAVATA3 can cause larger floral meristems and an increased number of locules. In an EMS mutagenized population of Brassica rapa, we identified a mutant allele that raises the number of locules from four to a range of from six to eight. Linkage mapping and genetic analysis support that the mutant phenotype is due to a missense mutation in a CLAVATA 1 (CLV1) homolog. In addition to increased locule number, additional internal gynoecia are formed in brclv1 individuals, suggesting a failure to terminate floral meristem development, which results in decreased seed production.

De novo stem cell establishment in meristems requires repression of organ boundary cell fate

Stem cells play important roles in animal and plant biology, as they sustain morphogenesis and tissue replenishment following aging or injury. In plants, stem cells are embedded in multicellular structures called meristems. The formation of new meristems is essential for the plastic expansion of the highly branched shoot and root systems. In particular, axillary meristems (AMs) that produce lateral shoots arise from the division of boundary domain cells at the leaf base. The CUP-SHAPED COTYLEDON (CUC) genes are major determinants of the boundary domain and are required for AM initiation. However, how AMs get structured and how stem cells become established de novo remain elusive. Here, we show that two NGATHA-LIKE (NGAL) transcription factors, DEVELOPMENT-RELATED PcG TARGET IN THE APEX4 (DPA4)/NGAL3 and SUPPRESSOR OF DA1-1 7 (SOD7)/NGAL2, redundantly repress CUC expression in initiating AMs of Arabidopsis thaliana. Ectopic boundary fate leads to abnormal growth and organization of the AM and prevents de novo stem cell establishment. Floral meristems of the dpa4 sod7 double mutant show a similar delay in de novo stem cell establishment. Altogether, while boundary fate is required for the initiation of AMs, our work reveals how it is later repressed to allow proper meristem establishment and de novo stem cell niche formation.

The plant stem-cell niche and pluripotency: 15 years of an epigenetic perspective

Pluripotent stem-cells are slowly dividing cells giving rise to daughter cells that can either differentiate to new tissues and organs, or remain stem-cells. In plants, stem-cells are located in specific niches of the shoot and root apical meristems (SAMs and RAMs). After ablation of stem-cell niches, pluripotent meristematic cells can establish new stem-cells, whereas the removal of the whole meristem destructs the regeneration process. In tissue cultures, after detached plant organs are transferred to rooting or callus induction medium (G5 or CIM), vasculature-associated pluripotent cells (VPCs) immediately start proliferation to form adventitious roots or callus, respectively, while other cell types of the organ explants basically play no part in the process. Hence, in contrast to the widely-held assumption that all plant cells have the ability to reproduce a complete organism, only few cell types are pluripotent in practice, raising the question how pluripotent stem-cells differ from differentiated cells. It is now clear that, in addition to gene regulatory networks of pluripotency factors and phytohormone signaling, epigenetics play a crucial role in initiation, maintenance and determination of plant stem-cells. Although, more and more epigenetic regulators have been shown to control plant stem-cell fate, only a few studies demonstrate how they are recruited and how they change the chromatin structure and transcriptional regulation of pluripotency factors. Here, we highlight recent breakthroughs but also revisited classical studies of epigenetic regulation and chromatin dynamics of plant stem-cells and their pluripotent precursor-cells, and point out open questions and future directions.

A CLAVATA3-like Gene Acts as a Gynoecium Suppression Function in White Campion

How do separate sexes originate and evolve? Plants provide many opportunities to address this question as they have diverse mating systems and separate sexes (dioecy) that evolved many times independently. The classic "two-factor" model for evolution of separate sexes proposes that males and females can evolve from hermaphrodites via the spread of male and female sterility mutations that turn hermaphrodites into females and males, respectively. This widely accepted model was inspired by early genetic work in dioecious white campion (Silene latifolia) that revealed the presence of two sex-determining factors on the Y-chromosome, though the actual genes remained unknown. Here, we report identification and functional analysis of the putative sex-determining gene in S. latifolia, corresponding to the gynoecium suppression factor (GSF). We demonstrate that GSF likely corresponds to a Y-linked CLV3-like gene that is specifically expressed in early male flower buds and encodes the protein that suppresses gynoecium development in S. latifolia. Interestingly, GSFY has a dysfunctional X-linked homolog (GSFX) and their synonymous divergence (dS = 17.9%) is consistent with the age of sex chromosomes in this species. We propose that female development in S. latifolia is controlled via the WUSCHEL-CLAVATA feedback loop, with the X-linked WUSCHEL-like and Y-linked CLV3-like genes, respectively. Evolution of dioecy in the S. latifolia ancestor likely involved inclusion of ancestral GSFY into the nonrecombining region on the nascent Y-chromosome and GSFX loss of function, which resulted in disbalance of the WUSCHEL-CLAVATA feedback loop between the sexes and ensured gynoecium suppression in males.

Evaluation of the Possible Contribution of Various Regulatory Genes to Determination of Carpel Number as a Potential Mechanism for Optimal Agricultural Yield

Various regulatory genes encoding transcription factors and miRNAs regulate carpel number. Multicarpelly is normally associated with increased size of the floral meristem, and several genetic factors have been discovered that influence this characteristic. A fundamental understanding of the regulatory genes affecting carpel number can facilitate strategies for agricultural yield improvement, which is crucial, given that the global population is growing rapidly. A multicarpellate plant may provide a significantly higher yield than a plant bearing fewer carpels. Higher yields can be achieved via various means; in this review, we provide an overview of the current knowledge of the various regulatory factors that contribute to multicarpelly and the potential of increasing carpel number to achieve an increased yield.

Zinc Finger-Homeodomain and Mini Zinc Finger proteins are key players in plant growth and responses to environmental stresses

The ZINC FINGER-HOMEODOMAIN (ZHD) protein family is a plant-specific family of transcription factors containing two conserved motifs: a non-canonical C5H3 zinc finger domain (ZF) and a DNA-binding homeodomain (HD). The MINI ZINC FINGER (MIF) proteins belong to this family, but were possibly derived from the ZHDs by losing the HD. Information regarding the function of ZHD and MIF proteins is scarce. However, different studies have shown that ZHD/MIF proteins play important roles not only in plant growth and development, but also in response to environmental stresses, including drought and pathogen attack. Here we review recent advances relative to ZHD/MIF functions in multiple species, to provide new insights into the diverse roles of these transcription factors in plants. Their mechanism of action in relation to their ability to interact with other proteins and DNA is also discussed. We then propose directions for future studies to understand better their important roles and pinpoint strategies for potential applications in crop improvement.

SARS-CoV-2 Infects Human ACE2-Negative Endothelial Cells through an αvβ3 Integrin-Mediated Endocytosis Even in the Presence of Vaccine-Elicited Neutralizing Antibodies

Integrins represent a gateway of entry for many viruses and the Arg-Gly-Asp (RGD) motif is the smallest sequence necessary for proteins to bind integrins. All Severe Acute Respiratory Syndrome Virus type 2 (SARS-CoV-2) lineages own an RGD motif (aa 403–405) in their receptor binding domain (RBD). We recently showed that SARS-CoV-2 gains access into primary human lung microvascular endothelial cells (HL-mECs) lacking Angiotensin-converting enzyme 2 (ACE2) expression through this conserved RGD motif. Following its entry, SARS-CoV-2 remodels cell phenotype and promotes angiogenesis in the absence of productive viral replication. Here, we highlight the αvβ3 integrin as the main molecule responsible for SARS-CoV-2 infection of HL-mECs via a clathrin-dependent endocytosis. Indeed, pretreatment of virus with αvβ3 integrin or pretreatment of cells with a monoclonal antibody against αvβ3 integrin was found to inhibit SARS-CoV-2 entry into HL-mECs. Surprisingly, the anti-Spike antibodies evoked by vaccination were neither able to impair Spike/integrin interaction nor to prevent SARS-CoV-2 entry into HL-mECs. Our data highlight the RGD motif in the Spike protein as a functional constraint aimed to maintain the interaction of the viral envelope with integrins. At the same time, our evidences call for the need of intervention strategies aimed to neutralize the SARS-CoV-2 integrin-mediated infection of ACE2-negative cells in the vaccine era.

The epigenetic mechanisms regulating floral hub genes and their potential for manipulation

Gene regulatory networks formed by transcription factors play essential roles in the regulation of gene expression during plant reproductive development. These networks integrate endogenous, phytohormonal, and environmental cues. Molecular genetic, biochemical, and chemical analyses performed mainly in Arabidopsis have identified network hub genes and revealed the contributions of individual components to these networks. Here, I outline current understanding of key epigenetic regulatory circuits identified by research on plant reproduction, and highlight significant recent examples of genetic engineering and chemical applications to modulate the epigenetic regulation of gene expression. Furthermore, I discuss future prospects for applying basic plant science to engineer useful floral traits in a predictable manner as well as the potential side effects.

Gynoecium and fruit development in Arabidopsis

Flowering plants produce flowers and one of the most complex floral structures is the pistil or the gynoecium. All the floral organs differentiate from the floral meristem. Various reviews exist on molecular mechanisms controlling reproductive development, but most focus on a short time window and there has been no recent review on the complete developmental time frame of gynoecium and fruit formation. Here, we highlight recent discoveries, including the players, interactions and mechanisms that govern gynoecium and fruit development in Arabidopsis. We also present the currently known gene regulatory networks from gynoecium initiation until fruit maturation.

Receptor-like protein kinases in plant reproduction: Current understanding and future perspectives

Reproduction is a crucial process in the life span of flowering plants, and directly affects human basic requirements in agriculture, such as grain yield and quality. Typical receptor-like protein kinases (RLKs) are a large family of membrane proteins sensing extracellular signals to regulate plant growth, development, and stress responses. In Arabidopsis thaliana and other plant species, RLK-mediated signaling pathways play essential roles in regulating the reproductive process by sensing different ligand signals. Molecular understanding of the reproductive process is vital from the perspective of controlling male and female fertility. Here, we summarize the roles of RLKs during plant reproduction at the genetic and molecular levels, including RLK-mediated floral organ development, ovule and anther development, and embryogenesis. In addition, the possible molecular regulatory patterns of those RLKs with unrevealed mechanisms during reproductive development are discussed. We also point out the thought-provoking questions raised by the research on these plant RLKs during reproduction for future investigation.