Meristem Formation Research Articles

Exploring Hidden Connections: Endophytic System and Flower Meristem Development of Pilostyles berteroi (Apodanthaceae) and Interaction with Its Host Adesmia trijuga (Fabaceae).

Pilostyles, an endoparasitic genus within the Apodanthaceae family, grows inside host stems with flowers and fruits being the only external manifestations. Previous studies of P. berteroi growing on Adesmia trijuga provided limited details of the endophyte and omitted the origin of flowers and sinker structure. This study, using classical methods of optical microscopy applied to the analysis with scanning electron microscopy and confocal laser scanning microscopy, expands the understanding of the P. berteroi/A. trijuga complex. We find that P. berteroi develops isophasically with its host, forming endophytic patches between the host's secondary phloem cells. The parasitized Adesmia stem's cambium primarily produces xylem parenchyma, with limited vessel production and halting fiber formation. The radial polarization of endophytic patches led to the formation of floral meristems. Flowers develop endogenously and emerge by the breakthrough of the host stem. Flowers are connected to the host cambium via chimeric sinkers, combining P. berteroi parenchyma and tracheoids with Adesmia vessels. Unlike previous studies that show uniformity among Pilostyles species, our analysis reveals new insights into the structural interaction between P. berteroi and A. trijuga.

Natural alleles of LEAFY and WAPO1 interact to regulate spikelet number per spike in wheat

Key messageSpecific combinations ofLFY andWAPO1 natural alleles maximize spikelet number per spike in wheat. Spikelet number per spike (SNS) is an important yield component in wheat that determines the maximum number of grains that can be formed in a wheat spike. In wheat, loss-of-function mutations in LEAFY (LFY) or its interacting protein WHEAT ORTHOLOG OF APO1 (WAPO1) significantly reduce SNS by reducing the rate of formation of spikelet meristems. In previous studies, we identified a natural amino acid change in WAPO1 (C47F) that significantly increases SNS in hexaploid wheat. In this study, we searched for natural variants in LFY that were associated with differences in SNS and detected significant effects in the LFY-B region in a nested association mapping population. We generated a large mapping population and confirmed that the LFY-B polymorphism R80S is linked with the differences in SNS, suggesting that LFY-B is the likely causal gene. A haplotype analysis revealed two amino acid changes P34L and R80S, which were both enriched during wheat domestication and breeding suggesting positive selection. We also explored the interactions between the LFY and WAPO1 natural variants for SNS using biparental populations and identified significant interaction, in which the positive effect of the 80S and 34L alleles from LFY-B was only detected in the WAPO-A1 47F background but not in the 47C background. Based on these results, we propose that the allele combination WAPO-A1-47F/LFY-B 34L 80S can be used in wheat breeding programs to maximize SNS and increase grain yield potential in wheat.

Mutation of rice SM1 enhances solid leaf midrib formation and increases methane emissions.

The leaf midrib system is essential for plant growth and development, facilitating nutrient transport, providing structural support, enabling gas exchange, and enhancing resilience to environmental stresses. However, the molecular mechanism regulating leaf midrib development is still unclear.In this study, we reported a rice solid midrib 1 (sm1) mutant, exhibiting solid leaf aerenchyma and abaxial rolling leaves due to abnormal development of parenchyma and bulliform cells. Map-based cloning revealed that SM1 encodes a litter zipper protein (ZPR). SM1 was mainly expressed in the sheaths and basal midrib and was associated with the nucleus. Further experiments indicated that SM1 can interact with OSHB1, preventing the formation of OSHB:OSHB dimers and subsequently repressing the expression of OSH1 involved in the regulation and maintenance of apical stem meristem formation. The sm1 mutant reduced long-distance oxygen transport ability from shoot to root. The impaired oxygen transport in the sm1 mutant may have contributed to the increase in methanogens and elevated methane emissions. Collectively, our findings revealed that the SM1-OSHB1-OSH1 modules regulate leaf aerenchyma development in rice. These modules not only enhance our understanding of the molecular mechanism of rice leaf aerenchyma development but also offer insights for reducing methane emissions through genetic modification.

FINE CULM1 Encoding a TEOSINTE BRANCHED1-like TCP Transcription Factor Negatively Regulates Axillary Meristem Formation in Rice.

Shoot branching is a critical determinant of plant architecture and a key factor affecting crop yield. The shoot branching involves two main processes: axillary meristem formation and subsequent bud outgrowth. While considerable progress has been made in elucidating the genetic mechanisms underlying the latter process, our understanding of the former process remains limited. Rice FINE CULM1 (FC1), which is an ortholog of teosinte branched1 in maize (Zea mays) and BRANCHED1/2 in Arabidopsis (Arabidopsis thaliana), is known to act in the latter process by repressing bud outgrowth. In this study, we found that FC1 also plays a role in the former process, i.e. axillary meristem formation, in rice. This study was triggered by our unexpected observation that fc1 mutation suppresses the loss of axillary meristems in the loss-of-function mutant of the rice WUSCHEL gene TILLERS ABSENT1 (TAB1). In tab1 fc1, unlike in tab1, both stem cells and undifferentiated cells were maintained during axillary meristem formation, similar to the wild type. Morphological analysis showed that axillary meristem formation was accelerated in fc1, compared to the wild type. Consistent with this, cell proliferation was more active in the region containing stem cells and undifferentiated cells during axillary meristem formation in fc1 than in the wild type. Taken altogether, these findings suggest that FC1 negatively regulates axillary meristem formation by mildly repressing cell proliferation during this process.

A dynamic WUSCHEL/Layer 1 interplay directs shoot apical meristem formation during regeneration in tobacco.

De novo shoot apical meristem (SAM) organogenesis during regeneration in tissue culture has been investigated for several decades, but the precise mechanisms governing early-stage cell fate specification remain elusive. In contrast to SAM establishment during embryogenesis, in vitro SAM formation occurs without positional cues and is characterized by autonomous initiation of cellular patterning. Here, we report on the initial stages of SAM organogenesis and on the molecular mechanisms that orchestrate gene patterning to establish SAM homeostasis. We found that SAM organogenesis in tobacco calli starts with protuberance formation followed by the formation of an intact L1 layer covering the nascent protuberance. We also exposed a complex interdependent relationship between L1 and WUS expression and revealed that any disruption in this interplay compromises shoot formation. Silencing WUS in nascent protuberances prevented L1 formation and caused the disorganization of the outer cell layers exhibiting both anticlinal and periclinal divisions, suggesting WUS plays a critical role in the proper establishment and organization of L1 during SAM organogenesis. We further discovered that silencing TONNEAU1 prevents the exclusive occurrence of anticlinal divisions in the outermost layer of the protuberances and suppresses the acquisition of L1 cellular identity and L1 formation, ultimately impeding SAM formation and regeneration. This study provides a novel molecular framework for the characterization of a WUS/L1 interplay that mediates SAM formation during regeneration.

Adaxial-abaxial bipolar leaf genes encode a putative cytokinin receptor and HD-Zip III, and control the formation of ectopic shoot meristems in rice.

Shoot apical meristems (SAMs) continuously initiate organ formation and maintain pluripotency through dynamic genetic regulations and cell-to-cell communications. The activity of meristems directly affects the plant's structure by determining the number and arrangement of organs and tissues. We have taken a forward genetic approach to dissect the genetic pathway that controls cell differentiation around the SAM. The rice mutants, adaxial-abaxial bipolar leaf 1 and 2 (abl1 and abl2), produce an ectopic leaf that is fused back-to-back with the fourth leaf, the first leaf produced after embryogenesis. The abaxial-abaxial fusion is associated with the formation of an ectopic shoot meristem at the adaxial base of the fourth leaf primordium. We cloned the ABL1 and ABL2 genes of rice by mapping their chromosomal positions. ABL1 encodes OsHK6, a histidine kinase, and ABL2 encodes a transcription factor, OSHB3 (Class III homeodomain leucine zipper). Expression analyses of these mutant genes as well as OSH1, a rice ortholog of the Arabidopsis STM gene, unveiled a regulatory circuit that controls the formation of an ectopic meristem near the SAM at germination.

Spatiotemporal transcriptome atlas reveals gene regulatory patterns during the organogenesis of the rapid growing bamboo shoots.

Bamboo with its remarkable growth rate and economic significance, offers an ideal system to investigate the molecular basis of organogenesis in rapidly growing plants, particular in monocots, where gene regulatory networks governing the maintenance and differentiation of shoot apical and intercalary meristems remain a subject of controversy. We employed both spatial and single-nucleus transcriptome sequencing on 10× platform to precisely dissect the gene functions in various tissues and early developmental stages of bamboo shoots. Our comprehensive analysis reveals distinct cell trajectories during shoot development, uncovering critical genes and pathways involved in procambium differentiation, intercalary meristem formation, and vascular tissue development. Spatial and temporal expression patterns of key regulatory genes, particularly those related to hormone signaling and lipid metabolism, strongly support the hypothesis that intercalary meristem origin from surrounded parenchyma cells. Specific gene expressions in intercalary meristem exhibit regular and dispersed distribution pattern, offering clues for understanding the intricate molecular mechanisms that drive the rapid growth of bamboo shoots. The single-nucleus and spatial transcriptome analysis reveal a comprehensive landscape of gene activity, enhancing the understanding of the molecular architecture of organogenesis and providing valuable resources for future genomic and genetic studies relying on identities of specific cell types.

LEAFY and WAPO1 jointly regulate spikelet number per spike and floret development in wheat.

In wheat, the transition of the inflorescence meristem to a terminal spikelet (IM→TS) determines the spikelet number per spike (SNS), an important yield component. In this study, we demonstrate that the plant-specific transcription factor LEAFY (LFY) physically and genetically interacts with WHEAT ORTHOLOG OF APO1 (WAPO1) to regulate SNS and floret development. Loss-of-function mutations in either or both genes result in significant and similar reductions in SNS, as a result of a reduction in the rate of spikelet meristem formation per day. SNS is also modulated by significant genetic interactions between LFY and the SQUAMOSA MADS-box genes VRN1 and FUL2, which promote the IM→TS transition. Single-molecule fluorescence in situ hybridization revealed a downregulation of LFY and upregulation of the SQUAMOSA MADS-box genes in the distal part of the developing spike during the IM→TS transition, supporting their opposite roles in the regulation of SNS in wheat. Concurrently, the overlap of LFY and WAPO1 transcription domains in the developing spikelets contributes to normal floret development. Understanding the genetic network regulating SNS is a necessary first step to engineer this important agronomic trait.

Uncovering PheCLE1 and PheCLE10 Promoting Root Development Based on Genome-Wide Analysis.

Moso bamboo (Phyllostachys edulis), renowned for its rapid growth, is attributed to the dynamic changes in its apical meristem. The CLAVATA3/EMBRYO SURROUNDING REGION-RELATED (CLE) family genes are known to play crucial roles in regulating meristem and organ formation in model plants, but their functions in Moso bamboo remain unclear. Here, we conducted a genome-wide identification of the CLE gene family of Moso bamboo and investigated their gene structure, chromosomal localization, evolutionary relationships, and expression patterns. A total of 11 PheCLE genes were identified, all of which contained a conserved CLE peptide core functional motif (Motif 1) at their C-termini. Based on Arabidopsis classification criteria, these genes were predominantly distributed in Groups A-C. Collinearity analysis unveiled significant synteny among CLE genes in Moso bamboo, rice, and maize, implying potential functional conservation during monocot evolution. Transcriptomic analysis showed significant expression of these genes in the apical tissues of Moso bamboo, including root tips, shoot tips, rhizome buds, and flower buds. Particularly, single-cell transcriptomic data and in situ hybridization further corroborated the heightened expression of PheCLE1 and PheCLE10 in the apical tissue of basal roots. Additionally, the overexpression of PheCLE1 and PheCLE10 in rice markedly promoted root growth. PheCLE1 and PheCLE10 were both located on the cell membrane. Furthermore, the upstream transcription factors NAC9 and NAC6 exhibited binding affinity toward the promoters of PheCLE1 and PheCLE10, thereby facilitating their transcriptional activation. In summary, this study not only systematically identified the CLE gene family in Moso bamboo for the first time but also emphasized their central roles in apical tissue development. This provides a valuable theoretical foundation for the further exploration of functional peptides and their signaling regulatory networks in bamboo species.

CDC48A, an interactor of WOX2, is required for embryonic patterning in Arabidopsis thaliana

Key messageInteractor of WOX2, CDC48A, is crucial for early embryo patterning and shoot meristem stem cell initiation, but is not required for WOX2 protein turnover or subcellular localization.During Arabidopsis embryo patterning, the WUSCHEL HOMEOBOX 2 (WOX2) transcription factor is a major regulator of protoderm and shoot stem cell initiation. Loss of WOX2 function results in aberrant protodermal cell divisions and, redundantly with its paralogs WOX1, WOX3, and WOX5, compromised shoot meristem formation. To elucidate the molecular basis for WOX2 function, we searched for protein interactors by IP–MS/MS from WOX2-overexpression roots displaying reprogramming toward shoot-like cell fates. Here, we report that WOX2 directly interacts with the type II AAA ATPase molecular chaperone CELL DIVISION CYCLE 48A (CDC48A). We confirmed this interaction with bimolecular fluorescence complementation and co-immunoprecipitation and found that both proteins co-localize in the nucleus. We show that CDC48A loss of function results in protoderm and shoot meristem stem cell initiation defects similar to WOX2 loss of function. We also provide evidence that CDC48A promotes WOX2 activity independently of proteolysis or the regulation of nuclear localization, common mechanisms of CDC48A function in other processes. Our results point to a new role of CDC48A in potentiating WOX2 function during early embryo patterning.

Progressive meristem and single-cell transcriptomes reveal the regulatory mechanisms underlying maize inflorescence development and sex differentiation

Maize develops separate ear and tassel inflorescences with initially similar morphology but ultimately different architecture and sexuality. The detailed regulatory mechanisms underlying these changes still remain largely unclear. In this study, through analyzing the time-course meristem transcriptomes and floret single-cell transcriptomes of ear and tassel, we revealed the regulatory dynamics and pathways underlying inflorescence development and sex differentiation. We identified 16 diverse gene clusters with differential spatiotemporal expression patterns and revealed biased regulation of redox, programmed cell death, and hormone signals during meristem differentiation between ear and tassel. Notably, based on their dynamic expression patterns, we revealed the roles of two RNA-binding proteins in regulating inflorescence meristem activity and axillary meristem formation. Moreover, using the transcriptional profiles of 53 910 single cells, we uncovered the cellular heterogeneity between ear and tassel florets. We found that multiple signals associated with either enhanced cell death or reduced growth are responsible for tassel pistil suppression, while part of the gibberellic acid signal may act non-cell-autonomously to regulate ear stamen arrest during sex differentiation. We further showed that the pistil-protection gene SILKLESS 1 (SK1) functions antagonistically to the known pistil-suppression genes through regulating common molecular pathways, and constructed a regulatory network for pistil-fate determination. Collectively, our study provides a deep understanding of the regulatory mechanisms underlying inflorescence development and sex differentiation in maize, laying the foundation for identifying new regulators and pathways for maize hybrid breeding and improvement.

Comparative transcriptome and functional analyses provide insights into the key factors regulating shoot regeneration in highbush blueberry.

Establishing an efficient plant regeneration system is a crucial prerequisite for genetic engineering technology in plants. However, the regeneration rate exhibits considerable variability among genotypes, and the key factors underlying shoot regeneration capacity remain largely elusive. Blueberry leaf explants cultured on a medium rich in cytokinins exhibit direct shoot organogenesis without prominent callus formation, which holds promise for expediting genetic transformation while minimizing somatic mutations during culture. The objective of this study is to unravel the molecular and genetic determinants that govern cultivar-specific shoot regeneration potential in highbush blueberry (Vaccinium corymbosum L.). We conducted comparative transcriptome analysis using two highbush blueberry genotypes: 'Blue Muffin' ('BM') displaying a high regeneration rate (>80%) and 'O'Neal' ('ON') exhibiting a low regeneration rate (<10%). The findings revealed differential expression of numerous auxin-related genes; notably, 'BM' exhibited higher expression of auxin signaling genes compared to 'ON'. Among blueberry orthologs of transcription factors involved in meristem formation in Arabidopsis, expression of VcENHANCER OF SHOOT REGENERATION (VcESR), VcWUSCHEL (VcWUS), and VcCUP-SHAPED COTYLEDON 2.1 were significantly higher in 'BM' relative to 'ON'. Exogenous application of auxin promoted regeneration, as well as VcESR and VcWUS expression, whereas inhibition of auxin biosynthesis yielded the opposite effects. Overexpression of VcESR in 'BM' promoted shoot regeneration under phytohormone-free conditions by activating the expression of cytokinin- and auxin-related genes. These findings provide new insights into the molecular mechanisms underlying blueberry regeneration and have practical implications for enhancing plant regeneration and transformation techniques.

HISTONE DEACETYLASE19 Controls Ovule Number Determination and Transmitting Tract Differentiation.

The gynoecium is critical for the reproduction of flowering plants as it contains the ovules and the tissues that foster pollen germination, growth, and guidance. These tissues, known as the reproductive tract (ReT), comprise the stigma, style, and transmitting tract (TT). The ReT and ovules originate from the carpel margin meristem (CMM) within the pistil. SHOOT MERISTEMLESS (STM) is a key transcription factor for meristem formation and maintenance. In all above-ground meristems, including the CMM, local STM downregulation is required for organ formation. However, how this downregulation is achieved in the CMM is unknown. Here, we have studied the role of HISTONE DEACETYLASE 19 (HDA19) in Arabidopsis (Arabidopsis thaliana) during ovule and ReT differentiation based on the observation that the hda19-3 mutant displays a reduced ovule number and fails to differentiate the TT properly. Fluorescence-activated cell sorting coupled with RNA-sequencing revealed that in the CMM of hda19-3 mutants, genes promoting organ development are downregulated while meristematic markers, including STM, are upregulated. HDA19 was essential to downregulate STM in the CMM, thereby allowing ovule formation and TT differentiation. STM is ectopically expressed in hda19-3 at intermediate stages of pistil development, and its downregulation by RNA interference alleviated the hda19-3 phenotype. Chromatin immunoprecipitation assays indicated that STM is a direct target of HDA19 during pistil development and that the transcription factor SEEDSTICK is also required to regulate STM via histone acetylation. Thus, we identified factors required for the downregulation of STM in the CMM, which is necessary for organogenesis and tissue differentiation.

Rice DSP controls stigma, panicle and tiller primordium initiation.

Tiller and seed number are key determinants of rice (Oryza sativa) yield. These traits are mainly affected by tiller, panicle, spikelet and stigma formation, but to date, no single gene involved in the development of all these organs has been identified. Here, we found a rice mutant defective stigma and panicle (dsp) with greatly reduced numbers of tillers and panicle branches, and ovaries lacking stigmas, due to defects in primordium initiation. We cloned DSP using sequencing-based mapping and verified its function with the CRISPR/Cas9 system. DSP encodes a transcription factor containing an APETALA2/ETHYLENE RESPONSE FACTOR (AP2/ERF) domain that recognizes the GCC motif and a transcription-activating domain at the site of 244-314 that contains an angiosperm-related (AR) motif. Mutating the AR motif resulted in the dsp mutant phenotypes, whereas mutating the AP2/ERF domain led to seedling death. DSP directly regulated PINOID (PID) expression to determine the emergence of rice stigmas, and PID overexpression partially rescued the stigma defect in the dsp cr2-8 and dsp mutants. Moreover, DSP indirectly affected LAX PANICLE1 (LAX1) expression to determine tiller primordium formation and synergistically regulated panicle primordium development. Our results indicated that DSP was a key regulator that modulated different genetic pathways to control the initiation of stigma primordia, the axillary meristem formation of tillers and panicle branches, which revealed their molecular mechanisms and cross-networks, laying the vital foundation for rice yield and trait improvement.

LAX1, functioning with MADS-box genes, determines normal palea development in rice

Normal floral organ development in rice is necessary for grain formation. Many MADS-box family genes that belong to ABCDE model have been widely implicated in rice flower development. The LAX1 allele encodes a plant-specific basic helix-loop-helix (bHLH) transcription factor, which is the main regulator of axillary meristem formation in rice. However, the molecular mechanisms of LAX1 allele together with MADS-box family genes underlying palea development have not been reported. We found a short palea mutant plant in a population of indica rice variety 9311 treated with cobalt 60. We report the map-based cloning and characterization of lax1-7, identified as a new mutant allele of the LAX1 locus, and the role of its wild-type allele LAX1 in rice palea development. Through complementary experiments, combined with genetic and molecular biological analyses, the function of the LAX1 allele was determined. We showed that LAX1 allele is expressed specifically in young spikelets and encodes a nucleus-localized protein. In vitro and in vivo experiments revealed that the LAX1 protein physically interacts with OsMADS1, OsMADS6 and OsMADS7. The LAX1 allele is pleiotropic for the maintenance of rice palea identity via cooperation with MADS-box genes and other traits, including axillary meristem initiation, days to heading, plant height, panicle length and spikelet fertility.

Morphological and Transcriptional Analyses of Regeneration Events in Pepper Plants (Capsicum annuum) Expose Patterns of Shoot Apical Meristem Formation

Morphological and Transcriptional Analyses of Regeneration Events in Pepper Plants (Capsicum annuum) Expose Patterns of Shoot Apical Meristem Formation

Changes in the expression pattern of OsWUS negatively regulate plant stature and panicle development in rice.

WUSCHEL (WUS) and WUSCHEL-RELATED HOMEOBOX (WOX) encode transcription factors and play important roles in regulating the formation and maintenance of shoot and floral meristems. OsWUS have distinct functions in meristem development with slightly tuned expression. However, the mechanisms regulating the specific expression of OsWUS need to be further explored. In this study, an abnormal expression mutant of OsWUS, called Dwarf and aberrant panicle 1 (Dap1) was used. In order to identify the causal gene in Dap1, high-efficiency thermal asymmetric interlaced (hiTAIL)-PCR and co-segregation analysis were performed. We surveyed the growth and yield traits in Dap1 and wild type. Changes in gene expression between Dap1 and wild type were determined by RNA-seq. The Dap1 mutant is due to the T-DNA inserted at 3,628-bp upstream of the translation start codon of OsWUS. Plant height, tiller numbers, panicle length, the number of grains per main panicle, and the number of secondary branches was significantly reduced in the Dap1 mutant. The expression of OsWUS was markedly increased in Dap1 mutant plants compared to the wild type, which might be due to a disruption in the genomic sequence integrity. Simultaneously, the expression levels of gibberellic acid-related genes and genes involved in panicle development were significantly changed in the Dap1 mutant. Our results suggest that OsWUS is a precise regulatory element, its specific spatio-temporal expression pattern is critical for its function, and both loss-of-function and gain-of-function mutations lead to abnormal plant growth.

OsbHLH067, OsbHLH068, and OsbHLH069 redundantly regulate inflorescence axillary meristem formation in rice.

Rice axillary meristems (AMs) are essential to the formation of tillers and panicle branches in rice, and therefore play a determining role in rice yield. However, the regulation of inflorescence AM development in rice remains elusive. In this study, we identified no spikelet 1-Dominant (nsp1-D), a sparse spikelet mutant, with obvious reduction of panicle branches and spikelets. Inflorescence AM deficiency in nsp1-D could be ascribed to the overexpression of OsbHLH069. OsbHLH069 functions redundantly with OsbHLH067 and OsbHLH068 in panicle AM formation. The Osbhlh067 Osbhlh068 Osbhlh069 triple mutant had smaller panicles and fewer branches and spikelets. OsbHLH067, OsbHLH068, and OsbHLH069 were preferentially expressed in the developing inflorescence AMs and their proteins could physically interact with LAX1. Both nsp1-D and lax1 showed sparse panicles. Transcriptomic data indicated that OsbHLH067/068/069 may be involved in the metabolic pathway during panicle AM formation. Quantitative RT-PCR results demonstrated that the expression of genes involved in meristem development and starch/sucrose metabolism was down-regulated in the triple mutant. Collectively, our study demonstrates that OsbHLH067, OsbHLH068, and OsbHLH069 have redundant functions in regulating the formation of inflorescence AMs during panicle development in rice.

KNOX Genes Were Involved in Regulating Axillary Bud Formation of Chrysanthemum × morifolium.

Branching is an important agronomic and economic trait in cut chrysanthemums. The axillary meristem (AM) formation of the axillary buds of cut chrysanthemums has a decisive role in its branching characteristics. However, little is known about the regulation mechanism of axillary meristem formation in chrysanthemums at the molecular level. Members of the Homeobox gene family especially genes belonging to the class I KNOX branch play a key role in regulating the axillary bud growth and development processes of plants. In this study, three genes belonging to the class I KNOX branch, CmKNAT1, CmKNAT6, and CmSTM were cloned from chrysanthemums, and their functions in regulating axillary bud formation were examined. The subcellular localization test showed that these three KNOX genes were expressed in the nucleus, so all of them might function as transcription factors. The results of the expression profile analysis showed that these three KNOX genes were highly expressed in the AM formation stage of axillary buds. Overexpression of KNOX genes result in a wrinkled leaf phenotype in tobacco and Arabidopsis, which may be related to the excessive division of leaf cells, resulting in the proliferation of leaf tissue. Furthermore, overexpression of these three KNOX genes enhances the regeneration ability of tobacco leaves, indicating that these three KNOX genes may participate in the regulation of cell meristematic ability, thus promoting the formation of buds. In addition, the results of fluorescence quantitative testing showed that these three KNOX genes may promote the formation of chrysanthemum axillary buds by promoting the cytokinin pathway while inhibiting the auxin and gibberellin pathways. In conclusion, this study demonstrated that CmKNAT1, CmKNAT6, and CmSTM genes were involved in regulating axillary bud formation of Chrysanthemum × morifolium and preliminarily revealed the molecular mechanism of their regulation of AM formation. These findings may provide a theoretical basis and candidate gene resources for genetic engineering breeding of new varieties of cut chrysanthemums without lateral branches.

PIN-FORMED is required for shoot phototropism/gravitropism and facilitates meristem formation in Marchantia polymorpha.

PIN-FORMED auxin efflux transporters, a subclass of which is plasma membrane-localised, mediate a variety of land-plant developmental processes via their polar localisation and subsequent directional auxin transport. We provide the first characterisation of PIN proteins in liverworts using Marchantia polymorpha as a model system. Marchantia polymorpha possesses a single PIN-FORMED gene, whose protein product is predicted to be plasma membrane-localised, MpPIN1. To characterise MpPIN1, we created loss-of-function alleles and produced complementation lines in both M.polymorpha and Arabidopsis. In M.polymorpha, gene expression and protein localisation were tracked using an MpPIN1 transgene encoding a translationally fused fluorescent protein. Overexpression of MpPIN1 can partially complement loss of an orthologous gene, PIN-FORMED1, in Arabidopsis. In M.polymorpha, MpPIN1 influences development in numerous ways throughout its life cycle. Most notably, MpPIN1 is required to establish gemmaling dorsiventral polarity and for orthotropic growth of gametangiophore stalks, where MpPIN1 is basally polarised. PIN activity is largely conserved within land plants, with PIN-mediated auxin flow providing a flexible mechanism to organise growth. Specifically, PIN is fundamentally linked to orthotropism and to the establishment of de novo meristems, the latter potentially involving the formation of both auxin biosynthesis maxima and auxin-signalling minima.