Integration of pluripotency pathways regulates stem cell maintenance in the Arabidopsis shoot meristem

SKI-INTERACTING PROTEIN interacts with SHOOT MERISTEMLESS to regulate shoot apical meristem formation

The shoot apical meristem (SAM), which is formed during embryogenesis, generates leaves, stems, and floral organs during the plant life cycle. SAM development is controlled by SHOOT MERISTEMLESS (STM), a conserved Class I KNOX transcription factor that interacts with another subclass homeodomain protein, BELL, to form a heterodimer, which regulates gene expression at the transcriptional level in Arabidopsis (Arabidopsis thaliana). Meanwhile, SKI-INTERACTING PROTEIN (SKIP), a conserved protein in eukaryotes, works as both a splicing factor and as a transcriptional regulator in plants to control gene expression at the transcriptional and posttranscriptional levels by interacting with distinct partners. Here, we show that, similar to plants with a loss of function of STM, a loss of function of SKIP or the specific knockout of SKIP in the SAM region resulted in failed SAM development and the inability of the mutants to complete their life cycle. In comparison, Arabidopsis mutants that expressed SKIP specifically in the SAM region formed a normal SAM and were able to generate a shoot system, including leaves and floral organs. Further analysis confirmed that SKIP interacts with STM in planta and that SKIP and STM regulate the expression of a similar set of genes by binding to their promoters. In addition, STM also interacts with EARLY FLOWERING 7 (ELF7), a component of Polymerase-Associated Factor 1 complex, and mutation in ELF7 exhibits similar SAM defects to that of STM and SKIP. This work identifies a component of the STM transcriptional complex and reveals the mechanism underlying SKIP-mediated SAM formation in Arabidopsis.

Genome-wide characterization of the TALE homeodomain family and the KNOX-BLH interaction network in tomato

Comprehensive yeast and protoplast two-hybrid analyses illustrated the protein-protein interaction network of the TALE homeodomain protein family, KNOX and BLH proteins, in tomato leaf and fruit development. KNOTTED-like (KNOX, KN) proteins and BELL1-like (BLH) proteins, which belong to the same TALE homeodomain family, act together by forming KNOX-BLH heterodimer modules. These modules play crucial roles in regulating multiple developmental processes in plants, like organ differentiation. However, despite the increasing knowledge about individual KNOX and BLH functions, a comprehensive view of their functional protein-protein interaction (PPI) network remains elusive in most plants, including tomato (Solanum lycopersicum), an important model plant to study fruit and leaf development. Here, we characterized eight tomato KNOX genes (SlKN1 to SlKN8) and fourteen tomato BLH genes (SlBLH1 to SlBLH14) by expression profiling, co-expression analysis, and PPI network analysis using two-hybrid techniques in yeasts (Y2H) and protoplasts (P2H). We identified 75 pairwise KNOX-BLH interactions, including ten novel interactors of SlKN2/TKN2, a primary class I KNOX protein, and nine novel interactors of SlKN5, a primary class II KNOX protein. Based on these data, we classified KNOX-BLH modules into several categories, which made us infer the order and combination of the KNOX-BLH modules involved in differentiation processes in leaf and fruit. Notably, the co-expression and interaction of SlKN5 and fruit preferentially expressing BLH1-clade paralogs (SlBLH5/SlBEL11 and SlBLH7) suggest their important roles in regulating fruit differentiation. Furthermore, in silico modeling of the KNOX-BLH modules, sequence analysis, and P2H assay identified several residues and a linker region potentially influencing the affinity of BLHs to KNOXs within their conserved dimerization domains. Together, these findings provide insights into the regulatory mechanism of KNOX-BLH modules underlying tomato organ differentiation.

Molecular mechanisms underlying leaf development, morphological diversification, and beyond

The basic mechanisms of leaf development have been revealed through a combination of genetics and intense analyses in select model species. The genetic basis for diversity in leaf morphology seen in nature is also being unraveled through recent advances in techniques and technologies related to genomics and transcriptomics, which have had a major impact on these comparative studies. However, this has led to the emergence of new unresolved questions about the mechanisms that generate the diversity of leaf form. Here, we provide a review of the current knowledge of the fundamental molecular genetic mechanisms underlying leaf development with an emphasis on natural variation and conserved gene regulatory networks involved in leaf development. Beyond that, we discuss open questions/enigmas in the area of leaf development, how recent technologies can best be deployed to generate a unified understanding of leaf diversity and its evolution, and what untapped fields lie ahead.

New Insights Into Tissue Culture Plant-Regeneration Mechanisms

Plant regeneration occurs when plants repair or replace damaged structures based on the totipotency and pluripotency of their cells. Tissue culture is one of the most widely used regenerative technologies. Recently, a series of breakthroughs were made in the study of plant regeneration. This review summarizes two regenerative pathways in tissue culture: somatic embryogenesis and de novo organogenesis. Furthermore, we review the environmental factors influencing plant regeneration from explant sources, basal culture medium, plant growth regulators, and light/dark treatment. Additionally, we analyse the molecular mechanisms underlying two pathways. This knowledge will promote an understanding of the fundamental principles of plant regeneration from precursor cells and lay a solid foundation for applying plant micropropagation and genetic modification.

Meristem Initiation and de novo Stem Cell Formation

Plant aerial development relies on meristem activity which ensures main body plant axis development during plant life. While the shoot apical meristem (SAM) formed in the embryo only contributes to the main stem, the branched structure observed in many plants relies on axillary meristems (AMs) formed post-embryonically. These AMs initiate from a few cells of the leaf axil that retain meristematic characteristics, increase in number, and finally organize into a structure similar to the SAM. In this review, we will discuss recent findings on de novo establishment of a stem cell population and its regulatory niche, a key step essential for the indeterminate fate of AMs. We stress that de novo stem cell formation is a progressive process, which starts with a transient regulatory network promoting stem cell formation and that is different from the one acting in functional meristems. This transient stage can be called premeristems and we discuss whether this concept can be extended to the formation of meristems other than AMs.

Polycomb proteins control floral determinacy by H3K27me3-mediated repression of pluripotency genes in Arabidopsis thaliana

Polycomb group (PcG) protein-mediated histone methylation (H3K27me3) controls the correct spatiotemporal expression of numerous developmental regulators in Arabidopsis. Epigenetic silencing of the stem cell factor gene WUSCHEL (WUS) in floral meristems (FMs) depends on H3K27me3 deposition by PcG proteins. However, the role of H3K27me3 in silencing of other meristematic regulator and pluripotency genes during FM determinacy has not yet been studied. To this end, we report the genome-wide dynamics of H3K27me3 levels during FM arrest and the consequences of strongly depleted PcG activity on early flower morphogenesis including enlarged and indeterminate FMs. Strong depletion of H3K27me3 levels results in misexpression of the FM identity gene AGL24, which partially causes floral reversion leading to ap1-like flowers and indeterminate FMs ectopically expressing WUS and SHOOT MERISTEMLESS (STM). Loss of STM can rescue supernumerary floral organs and FM indeterminacy in H3K27me3-deficient flowers, indicating that the hyperactivity of the FMs is at least partially a result of ectopic STM expression. Nonetheless, WUS remained essential for the FM activity. Our results demonstrate that PcG proteins promote FM determinacy at multiple levels of the floral gene regulatory network, silencing initially floral regulators such as AGL24 that promotes FM indeterminacy and, subsequently, meristematic pluripotency genes such as WUS and STM during FM arrest.

OsbZIP47 Is an Integrator for Meristem Regulators During Rice Plant Growth and Development

Stem cell homeostasis by the WUSCHEL-CLAVATA (WUS-CLV) feedback loop is generally conserved across species; however, its links with other meristem regulators can be species-specific, rice being an example. We characterized the role of rice OsbZIP47 in vegetative and reproductive development. The knockdown (KD) transgenics showed meristem size abnormality and defects in developmental progression. The size of the shoot apical meristem (SAM) in 25-day OsbZIP47KD plants was increased as compared to the wild-type (WT). Inflorescence of KD plants showed reduced rachis length, number of primary branches, and spikelets. Florets had defects in the second and third whorl organs and increased organ number. OsbZIP47KD SAM and panicles had abnormal expression for CLAVATA peptide-like signaling genes, such as FON2-LIKE CLE PROTEIN1 (FCP1), FLORAL ORGAN NUMBER 2 (FON2), and hormone pathway genes, such as cytokinin (CK) ISOPENTEYLTRANSFERASE1 (OsIPT1), ISOPENTEYLTRANSFERASE 8 (OsIPT8), auxin biosynthesis OsYUCCA6, OsYUCCA7 and gibberellic acid (GA) biosynthesis genes, such as GRAIN NUMBER PER PANICLE1 (GNP1/OsGA20OX1) and SHORTENED BASAL INTERNODE (SBI/OsGA2ox4). The effects on ABBERANT PANICLE ORGANIZATION1 (APO1), OsMADS16, and DROOPING LEAF (DL) relate to the second and third whorl floret phenotypes in OsbZIP47KD. Protein interaction assays showed OsbZIP47 partnerships with RICE HOMEOBOX1 (OSH1), RICE FLORICULA/LEAFY (RFL), and OsMADS1 transcription factors. The meta-analysis of KD panicle transcriptomes in OsbZIP47KD, OsMADS1KD, and RFLKD transgenics, combined with global OSH1 binding sites divulge potential targets coregulated by OsbZIP47, OsMADS1, OSH1, and RFL. Further, we demonstrate that OsbZIP47 redox status affects its DNA binding affinity to a cis element in FCP1, a target locus. Taken together, we provide insights on OsbZIP47 roles in SAM development, inflorescence branching, and floret development.

Illuminating the molecular mechanisms underlying shoot apical meristem homeostasis in plants

Unlike animals, terrestrial plants are sessile and able to give rise to new organs throughout their lifetime. In the most extreme cases, they can survive for over a thousand years. With such protracted life cycles, plants have evolved sophisticated strategies to adapt to variable environments by coordinating their morphology as well as their growth, and have consequently acquired a high degree of developmental plasticity, which is supported by small groups of long-lived stem cells found in proliferative centers called meristems. Shoot apical meristems (SAMs) contain multipotent stem cells and provide a microenvironment that ensures both a self-renewable reservoir, to produce primordia and sustain growth, and a differentiating population that develops into all of the above-ground organs of land plants. The homeodomain transcription factor WUSCHEL (WUS) is expressed in the organizing center and acts as a master regulator to govern shoot stem cell homeostasis. In this review, I highlight recent advances in our understanding of the molecular mechanisms and signaling networks that underlie SAM maintenance, and discuss how plants utilize WUS to integrate intrinsic and extrinsic cues.

A Preliminary Investigation on the Functional Validation and Interactions of PoWOX Genes in Peony (Paeonia ostii)

As a woody plant, peony (Paeonia suffruticosa) has a long growth cycle and inefficient traditional breeding techniques. There is an urgent need in peony molecular breeding to establish an efficient and stable in vitro regeneration and genetic transformation system, in order to overcome the recalcitrant characteristics of peony regeneration and shorten the breeding cycle. The development of plant somatic embryos is an important way to establish an efficient and stable in vitro regeneration and genetic transformation system. Plant-specific WUSCHEL-related homeobox (WOX) family transcription factors play important roles in plant development, from embryogenesis to lateral organ development. Therefore, in this research, four PoWOX genes of “Fengdan” (Paeonia ostii) were cloned from the peony genome and transcriptome data of preliminary peony somatic embryos. The sequence characteristics and evolutionary relationships of the PoWOX genes were analyzed. It was demonstrated that the four PoWOX genes, named PoWOX1, PoWOX4, PoWOX11, and PoWOX13, belonged to three branches of the WOX gene family. Their expression patterns were analyzed at different stages of development and in different tissues of peony seedlings. The expression localization of the PoWOX genes was determined to be the nucleus via subcellular localization assay. Finally, the interaction protein of the PoWOX genes was identified via yeast two-hybrid assay combined with bimolecular fluorescence complementation assay. It was shown that PoWOX1 and PoWOX13 proteins could form homodimers by themselves, and PoWOX11 interacted with PoWOX1 and PoWOX13 to form heterodimers. Peony stem cell activity may be regulated from PoWOX1 and PoWOX13 by forming dimers and moving to peony stem cells through plasmodesmata. Additionally, PoWOX11–PoWOX1 and PoWOX11–PoWOX13 may play important regulatory functions in promoting the proliferation of stem cells and maintaining the homeostasis of stem cells in the SAM of peony stems. Exploring the critical genes and regulatory factors in the development of the peony somatic embryo is beneficial not only to understand the molecular and regulatory mechanisms of peony somatic embryo development but also to achieve directed breeding and improvements in efficiency through genetic engineering breeding technology to accelerate the fundamental process of molecular breeding in peony.

CFI 25 Subunit of Cleavage Factor I is Important for Maintaining the Diversity of 3' UTR Lengths in Arabidopsis thaliana (L.) Heynh

Cleavage and polyadenylation at the 3' end of the pre-mRNA is essential for mRNA function, by regulating its translatability, stability and translocation to the cytoplasm. Cleavage factor I (CFI) is a multi-subunit component of the pre-mRNA 3' end processing machinery in eukaryotes. Here, we report that plant CFI 25 subunit of CFI plays an important role in maintaining the diversity of the 3' ends of mRNA. The genome of Arabidopsis thaliana (L.) Heynh. contained four genes encoding three putative CFI subunits (AtCFI 25, AtCFI 59 and AtCFI 68), orthologous to the mammalian CFI subunits. There were two CFI 25 paralogs (AtCFI 25a and AtCFI 25b) that shared homology with human CFI 25. Two null alleles of AtCFI 25a displayed smaller rosette leaves, longer stigmatic papilla, smaller anther, earlier flowering and lower fertility compared to wild-type plants. Null alleles of AtCFI 25b, as well as, plants ectopically expressing full-length cDNA of AtCFI 25a, displayed no obvious morphological defects. AtCFI 25a was shown to interact with AtCFI 25b, AtCFI 68 and itself, suggesting various forms of CFI in plants. Furthermore, we show that AtCFI 25a function was essential for maintaining proper diversity of the 3' end lengths of transcripts coding for CFI subunits, suggesting a self-regulation of the CFI machinery in plants. AtCFI 25a was also important to maintain 3' ends for other genes to different extent. Collectively, AtCFI 25a, but not AtCFI 25b, seemed to play important roles during Arabidopsis development by maintaining proper diversity of the 3' UTR lengths.

The molecular and genetic regulation of shoot branching

The architecture of flowering plants exhibits both phenotypic diversity and plasticity, determined, in part, by the number and activity of axillary meristems and, in part, by the growth characteristics of the branches that develop from the axillary buds. The plasticity of shoot branching results from a combination of various intrinsic and genetic elements, such as number and position of nodes and type of growth phase, as well as environmental signals such as nutrient availability, light characteristics, and temperature (Napoli et al., 1998; Bennett and Leyser, 2006; Janssen et al., 2014; Teichmann and Muhr, 2015; Ueda and Yanagisawa, 2019). Axillary meristem initiation and axillary bud outgrowth are controlled by a complex and interconnected regulatory network. Although many of the genes and hormones that modulate branching patterns have been discovered and characterized through genetic and biochemical studies, there are still many gaps in our understanding of the control mechanisms at play. In this review, we will summarize our current knowledge of the control of axillary meristem initiation and outgrowth into a branch.

The leaf meristem enigma: The relationship between the plate meristem and the marginal meristem

Leaf organogenesis is governed by the spatiotemporal activity of the leaf meristem, which has far greater mitotic activity than the shoot apical meristem. The two types of leaf meristems, the plate meristem and the marginal meristem, are distinguished by the location and longevity of their cell proliferative activity. Most leaf lamina outgrowth depends on the plate meristem. The presence of the marginal meristem was a matter of debate in classic anatomy, but recent genetic analyses of leaf growth in Arabidopsis thaliana confirmed its short-lived activity. Several genes key for the regulation of the two meristem types have been identified, and at least superficially, the systems appear to function independently, as they are regulated by different transcription factors and microRNAs. However, many of the details of these regulatory systems, including how the expression of these key factors is spatially regulated, remain unclear. One major unsolved question is the relationship between the plate meristem and the marginal meristem. Here, I present an overview of our current understanding of this topic and discuss questions that remain to be answered.

Post-Embryonic Lateral Organ Development and Adaxial-Abaxial Polarity Are Regulated by the Combined Effect of ENHANCER OF SHOOT REGENERATION 1 and WUSCHEL in Arabidopsis Shoots

The development of above-ground lateral organs is initiated at the peripheral zone of the shoot apical meristem (SAM). The coordination of cell fate determination and the maintenance of stem cells are achieved through a complex regulatory network comprised of transcription factors. Two AP2/ERF transcription factor family genes, ESR1/DRN and ESR2/DRNL/SOB/BOL, regulate cotyledon and flower formation and de novo organogenesis in tissue culture. However, their roles in post-embryonic lateral organ development remain elusive. In this study, we analyzed the genetic interactions among SAM-related genes, WUS and STM, two ESR genes, and one of the HD-ZIP III members, REV, whose protein product interacts with ESR1 in planta. We found that esr1 mutations substantially enhanced the wus and stm phenotypes, which bear a striking resemblance to those of the wus rev and stm rev double mutants, respectively. Aberrant adaxial–abaxial polarity is observed in wus esr1 at relatively low penetrance. On the contrary, the esr2 mutation partially suppressed stm phenotypes in the later vegetative phase. Such complex genetic interactions appear to be attributed to the distinct expression pattern of two ESR genes because the ESR1 promoter-driving ESR2 is capable of rescuing phenotypes caused by the esr1 mutation. Our results pose the unique genetic relevance of ESR1 and the SAM-related gene interactions in the development of rosette leaves.

Genetics of Shoot Meristem and Shoot Regeneration

Plants exhibit remarkable lineage plasticity, allowing them to regenerate organs that differ from their respective origins. Such developmental plasticity is dependent on the activity of pluripotent founder cells or stem cells residing in meristems. At the shoot apical meristem (SAM), the constant flow of cells requires continuing cell specification governed by a complex genetic network, with the WUSCHEL transcription factor and phytohormone cytokinin at its core. In this review, I discuss some intriguing recent discoveries that expose new principles and mechanisms of patterning and cell specification acting both at the SAM and, prior to meristem organogenesis during shoot regeneration. I also highlight unanswered questions and future challenges in the study of SAM and meristem regeneration. Finally, I put forward a model describing stochastic events mediated by epigenetic factors to explain how the gene regulatory network might be initiated at the onset of shoot regeneration. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

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

Floral organs are properly developed on the basis of timed floral meristem (FM) termination in Arabidopsis In this process, two known regulatory pathways are involved. The WUSCHEL (WUS)-CLAVATA3 (CLV3) feedback loop is vital for the spatial establishment and maintenance of the FM, while AGAMOUS (AG)-WUS transcriptional cascades temporally repress FM. At stage 6 of flower development, a C2H2-type zinc finger repressor that is a target of AG, KNUCKLES (KNU), directly represses the stem cell identity gene WUS in the organizing center for FM termination. However, how the robust FM activity is fully quenched within a limited time frame to secure carpel development is not fully understood. Here, we demonstrate that KNU directly binds to the CLV1 locus and the cis-regulatory element on CLV3 promoter and represses their expression during FM determinacy control. Furthermore, KNU physically interacts with WUS, and this interaction inhibits WUS from sustaining CLV3 in the central zone. The KNU-WUS interaction also interrupts the formation of WUS homodimers and WUS-HAIRYMERISTEM 1 heterodimers, both of which are required for FM maintenance. Overall, our findings describe a regulatory framework in which KNU plays a position-specific multifunctional role for the tightly controlled FM determinacy.

Knox homologs shoot meristemless (STM) and KNAT6 are epistatic to CLAVATA3 (CLV3) during shoot meristem development in Arabidopsis thaliana

BACKGROUND

In Arabidopsis, the genes SHOOT MERISTEMLESS (STM) and CLAVATA3 (CLV3) antagonistically regulate shoot meristem development. STM is essential for both development and maintenance of the meristem, as stm mutants fail to develop a shoot meristem. CLV3, on the other hand, negatively regulates meristem proliferation, and clv3 mutants possess an enlarged shoot meristem. Genetic interaction studies revealed that stm and clv3 dominantly suppress each other's phenotypes. STM works in conjunction with its closely related homologue KNOTTED1-LIKE HOMEOBOX GENE 6 (KNAT6) to promote meristem development and organ separation, as stm knat6 double mutants fail to form shoot meristem and produce a fused cotyledon.

RESULTS

In this study, we show that clv3 fails to promote shoot meristem formation in stm-1 background if we also remove KNAT6. stm-1 knat6 clv3 triple mutants result in shoot meristem termination and produce fused cotyledons similar to stm knat6 double mutant. Notably, the stm-1 knat6 and stm-1 knat6 clv3 alleles lack tissue in the presumed region of SAM that is a novel phenotype reported in Arabidopsis mutants. stm-1 knat6 clv3 also showed reduced inflorescence size as compared to clv3 single or stm clv3 double mutants.

CONCLUSION

In contrast to previously published data, these data suggest that STM and KNAT6 are redundantly required for the vegetative SAM, but insufficient for the inflorescence meristem.

The Arabidopsis MATERNAL EFFECT EMBRYO ARREST45 protein modulates maternal auxin biosynthesis and controls seed size by inducing AINTEGUMENTA

Seed size is a major factor determining crop yields that is controlled through the coordinated development of maternal and zygotic tissues. Here, we identified Arabidopsis MATERNAL EFFECT EMBRYO ARREST45 (MEE45) as a B3 transcription factor that controls cell proliferation and maternally regulates seed size through its transcriptional activation of AINTEGUMENTA (ANT) and its downstream control of auxin biosynthesis in the ovule integument. After characterizing reduced seed and organ size phenotypes in mee45 mutants and finding that overexpression of MEE45 causes oversized seeds, we discovered that the MEE45 protein can bind to the promoter region of the ANT locus and positively regulate its transcription. ANT in-turn activates the expression of auxin biosynthetic genes (e.g. YUCCA4) in the ovule integument. Our results thus illustrate mechanisms underlying maternal tissue-mediated regulation of seed size and suggest that MEE45 and its downstream components can be harnessed to develop higher-yielding crop varieties.

Meeting at the DNA: Specifying Cytokinin Responses through Transcription Factor Complex Formation

Cytokinin is a plant hormone regulating numerous biological processes. Its diverse functions are realized through the expression control of specific target genes. The transcription of the immediate early cytokinin target genes is regulated by type-B response regulator proteins (RRBs), which are transcription factors (TFs) of the Myb family. RRB activity is controlled by phosphorylation and protein degradation. Here, we focus on another step of regulation, the interaction of RRBs among each other or with other TFs to form active or repressive TF complexes. Several examples in Arabidopsis thaliana illustrate that RRBs form homodimers or complexes with other TFs to specify the cytokinin response. This increases the variability of the output response and provides opportunities of crosstalk between the cytokinin signaling pathway and other cellular signaling pathways. We propose that a targeted approach is required to uncover the full extent and impact of RRB interaction with other TFs.

Roles of Phytohormones and Their Signaling Pathways in Leaf Development and Stress Responses

Phytohormones participate in various processes over the course of a plant's lifecycle. In addition to the five classical phytohormones (auxins, cytokinins, gibberellins, abscisic acid, and ethylene), phytohormones such as brassinosteroids, jasmonic acid, salicylic acid, strigolactones, and peptides also play important roles in plant growth and stress responses. Given the highly interconnected nature of phytohormones during plant development and stress responses, it is challenging to study the biological function of a single phytohormone in isolation. In the current Review, we describe the combined functions and signaling cascades (especially the shared points and pathways) of various phytohormones in leaf development, in particular, during leaf primordium initiation and the establishment of leaf polarity and leaf morphology as well as leaf development under various stress conditions. We propose a model incorporating the roles of multiple phytohormones in leaf development and stress responses to illustrate the underlying combinatorial signaling pathways. This model provides a reference for breeding stress-resistant crops.

Molecular Insights into Inflorescence Meristem Specification for Yield Potential in Cereal Crops

Flowering plants develop new organs throughout their life cycle. The vegetative shoot apical meristem (SAM) generates leaf whorls, branches and stems, whereas the reproductive SAM, called the inflorescence meristem (IM), forms florets arranged on a stem or an axis. In cereal crops, the inflorescence producing grains from fertilized florets makes the major yield contribution, which is determined by the numbers and structures of branches, spikelets and florets within the inflorescence. The developmental progression largely depends on the activity of IM. The proper regulations of IM size, specification and termination are outcomes of complex interactions between promoting and restricting factors/signals. Here, we focus on recent advances in molecular mechanisms underlying potential pathways of IM identification, maintenance and differentiation in cereal crops, including rice (Oryza sativa), maize (Zea mays), wheat (Triticum aestivum), and barley (Hordeum vulgare), highlighting the researches that have facilitated grain yield by, for example, modifying the number of inflorescence branches. Combinatorial functions of key regulators and crosstalk in IM determinacy and specification are summarized. This review delivers the knowledge to crop breeding applications aiming to the improvements in yield performance and productivity.

Functional analysis of the cotton CLE polypeptide signaling gene family in plant growth and development

The CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION (ESR)–RELATED (CLE) gene family encodes a large number of polypeptide signaling molecules involved in the regulation of shoot apical meristem division and root and vascular bundle development in a variety of plants. CLE family genes encode important short peptide hormones; however, the functions of these signaling polypeptides in cotton remain largely unknown. In the current work, we studied the effects of the CLE family genes on growth and development in cotton. Based on the presence of a conserved CLE motif of 13 amino acids, 93 genes were characterized as GhCLE gene family members, and these were subcategorized into 7 groups. A preliminary analysis of the cotton CLE gene family indicated that the activity of its members tends to be conserved in terms of both the 13-residue conserved domain at the C-terminus and their subcellular localization pattern. Among the 14 tested genes, the ectopic overexpression of GhCLE5::GFP partially mimicked the phenotype of the clv3 mutant in Arabidopsis. GhCLE5 could affect the endogenous CLV3 in binding to the receptor complex, comprised of CLV1, CLV2, and CRN, in the yeast two-hybrid assay and split-luciferase assay. Silencing GhCLE5 in cotton caused a short seedling phenotype. Therefore, we concluded that the cotton GhCLE gene family is functionally conserved in apical shoot development regulation. These results indicate that CLE also plays roles in cotton development as a short peptide hormone.

WUSCHEL in the shoot apical meristem: old player, new tricks

The maintenance of the stem cell niche in the shoot apical meristem, the structure that generates all of the aerial organs of the plant, relies on a canonical feedback loop between WUSCHEL (WUS) and CLAVATA3 (CLV3). WUS is a homeodomain transcription factor expressed in the organizing centre that moves to the central zone to promote stem cell fate. CLV3 is a peptide whose expression is induced by WUS in the central zone and that can move back to the organizing centre to inhibit WUS expression. Within the past 20 years since the initial formulation of the CLV-WUS feedback loop, the mechanisms of stem cell maintenance have been intensively studied and the function of WUS has been redefined. In this review, we highlight the most recent advances in our comprehension of the molecular mechanisms of WUS function, of its interaction with other transcription factors and hormonal signals, and of its connection to environmental signals. Through this, we will show how WUS can integrate both internal and external cues to adapt meristem function to the plant environment.