Regulation Of Meristem Activity Research Articles

Genome-Wide Classification and Evolutionary Analysis of the KNOX Gene Family in Plants

The Knotted1-like homeobox (KNOX) gene family plays a pivotal role in regulating meristem activity, organ differentiation, and cell meristematic identity. However, there has been a lack of large-scale, systematic, and comprehensive comparative analyses to explore their expression patterns and evolutionary mechanisms. In this study, a total of 1425 KNOX genes were identified across 118 plant species. The result showed that higher plants exhibited a significantly higher abundance of KNOX genes compared to lower plants. Phylogenetic analysis revealed that all KNOX genes can be divided into two classes (class I and II) and evolved independently after species differentiation. An analysis of gene duplication or loss showed that gene loss was more common than gene duplication in lower plants within the KNOX gene family. These findings suggest that gene loss in the KNOX gene family occurs after events such as whole-genome duplication (WGD) or whole-genome triplication (WGT). In addition, conserved motif analysis was also conducted to uncover the evolutionary trajectories of KNOX genes. We found that three motifs (M1, M2, and M4) were present in nearly all KNOX genes, while four novel motifs (M7–M10) were lost in lower plants but present in higher plants. Moreover, the loss of certain motifs in the KNOX genes was also observed in higher plants, indicating sequence divergence in KNOX genes throughout evolution. To understand the expression patterns of KNOX genes, a gene expression pattern analysis was performed in A. thaliana and O. sativa. The results showed that class I KNOX genes exhibit conserved high expression in stems, suggesting their potential similar biological roles across different plant species and the conservation of their functions during evolution. Additionally, we analyzed the KNOX genes in the Citrus genus and closely related species, and we found that the number of KNOX genes evolved at a slower rate in these species, indicating a relatively conservative nature. In conclusion, this study provides valuable resources for the investigation of KNOX gene family evolution and function in plants.

Ectopic Expression of a Maize Gene ZmDUF1645 in Rice Increases Grain Length and Yield, but Reduces Drought Stress Tolerance.

As the human population grows rapidly, food shortages will become an even greater problem; therefore, increasing crop yield has become a focus of rice breeding programs. The maize gene, ZmDUF1645, encoding a putative member of the DUF1645 protein family with an unknown function, was transformed into rice. Phenotypic analysis showed that enhanced ZmDUF1645 expression significantly altered various traits in transgenic rice plants, including increased grain length, width, weight, and number per panicle, resulting in a significant increase in yield, but a decrease in rice tolerance to drought stress. qRT-PCR results showed that the expression of the related genes regulating meristem activity, such as MPKA, CDKA, a novel crop grain filling gene (GIF1), and GS3, was significantly changed in the ZmDUF1645-overexpression lines. Subcellular colocalization showed that ZmDUF1645 was primarily localized on cell membrane systems. Based on these findings, we speculate that ZmDUF1645, like the OsSGL gene in the same protein family, may regulate grain size and affect yield through the cytokinin signaling pathway. This research provides further knowledge and understanding of the unknown functions of the DUF1645 protein family and may serve as a reference for biological breeding engineering to increase maize crop yield.

The AtERF19 gene regulates meristem activity and flower organ size in plants.

Ethylene-responsive factors (ERFs) have diverse functions in the regulation of various plant developmental processes. Here, we demonstrate the dual role of an Arabidopsis ERF gene, AtERF19, in regulating reproductive meristem activity and flower organ size through the regulation of genes involved in CLAVATA-WUSCHEL (CLV-WUS) and auxin signaling, respectively. We found that AtERF19 stimulated the formation of flower primordia and controlled the number of flowers produced by activating WUS and was negatively regulated by CLV3. 35S::AtERF19 expression resulted in significantly more flowers, whereas 35S::AtERF19 + SRDX dominant-negative mutants produced fewer flowers. In addition, AtERF19 also functioned to control flower organ size by promoting the division/expansion of the cells through activating Small Auxin Up RNA Gene 32 (SAUR32), which positively regulated MYB21/24 in the auxin signaling pathway. 35S::AtERF19 and 35S::SAUR32 resulted in similarly larger flowers, whereas 35S::AtERF19 + SRDX and 35S::SAUR32-RNAi mutants produced smaller flowers than the wild type. The functions of AtERF19 were confirmed by the production of similarly more and larger flowers in 35S::AtERF19 transgenic tobacco (Nicotiana benthamiana) and in transgenic Arabidopsis which ectopically expressed the orchid gene (Nicotiana benthamiana) PaERF19 than in wild-type plants. The finding that AtERF19 regulates genes involved in both CLV-WUS and auxin signaling during flower development significantly expands the current knowledge of the multifunctional evolution of ERF genes in plants. The results presented in this work indicate a dual role for the transcription factor AtERF19 in controlling the number of flowers produced and flower organ size through the regulation of genes involved in CLV-WUS and auxin signaling, respectively. Our findings expand the knowledge of the roles of ERF genes in the regulation of reproductive development.

Cell- and noncell-autonomous AUXIN RESPONSE FACTOR3 controls meristem proliferation and phyllotactic patterns.

In cell-cell communication, noncell-autonomous transcription factors play vital roles in controlling plant stem cell fate. We previously reported that AUXIN RESPONSE FACTOR3 (ARF3), a member of the ARF family with critical roles in floral meristem maintenance and determinacy, has a distinct accumulation pattern that differs from the expression domain of its encoding gene in the shoot apical meristem (SAM). However, the biological meaning of this difference is obscure. Here, we demonstrate that ARF3 expression in Arabidopsis (Arabidopsis thaliana) is mainly activated at the periphery of the SAM by auxin where ARF3 cell autonomously regulates the expression of meristem-organ boundary-specific genes, such as CUP-SHAPED COTYLEDON1-3 (CUC1-3), BLADE ON PETIOLE1-2 (BOP1-2), and TARGETS UNDER ETTIN CONTROL3 (TEC3) to regulate the arrangement of organs in regular pattern, a phenomenon referred to as phyllotaxis. We also show that ARF3 is translocated into the organizing center where it represses cytokinin activity and WUSCHEL expression to regulate meristem activity noncell-autonomously. Therefore, ARF3 acts as a molecular link that mediates the interaction of auxin and cytokinin signaling in the SAM while coordinating the balance between meristem maintenance and organogenesis. Our findings reveal an ARF3-mediated coordination mechanism through cell-cell communication in dynamic SAM maintenance.

Homeobox transcription factor OsZHD2 promotes root meristem activity in rice by inducing ethylene biosynthesis.

Root meristem activity is the most critical process influencing root development. Although several factors that regulate meristem activity have been identified in rice, studies on the enhancement of meristem activity in roots are limited. We identified a T-DNA activation tagging line of a zinc-finger homeobox gene, OsZHD2, which has longer seminal and lateral roots due to increased meristem activity. The phenotypes were confirmed in transgenic plants overexpressing OsZHD2. In addition, the overexpressing plants showed enhanced grain yield under low nutrient and paddy field conditions. OsZHD2 was preferentially expressed in the shoot apical meristem and root tips. Transcriptome analyses and quantitative real-time PCR experiments on roots from the activation tagging line and the wild type showed that genes for ethylene biosynthesis were up-regulated in the activation line. Ethylene levels were higher in the activation lines compared with the wild type. ChIP assay results suggested that OsZHD2 induces ethylene biosynthesis by controlling ACS5 directly. Treatment with ACC (1-aminocyclopropane-1-carboxylic acid), an ethylene precursor, induced the expression of the DR5 reporter at the root tip and stele, whereas treatment with an ethylene biosynthesis inhibitor, AVG (aminoethoxyvinylglycine), decreased that expression in both the wild type and the OsZHD2 overexpression line. These observations suggest that OsZHD2 enhances root meristem activity by influencing ethylene biosynthesis and, in turn, auxin.

Hormonal crosstalk in the regulation of meristem activity and the phyllomorph architecture in Streptocarpus (Gesneriaceae): a review

Hormonal crosstalk in the regulation of meristem activity and the phyllomorph architecture in Streptocarpus (Gesneriaceae): a review

Control of proliferation in the haploid meristem by CLE peptide signaling in Marchantia polymorpha.

The homeostasis of meristems in flowering plants is maintained by cell-to-cell communication via CLE (CLAVATA3/EMBRYO SURROUNDING REGION-related) peptide hormones. In contrast, cell signals that regulate meristem activity remains elusive in bryophytes that maintain apical meristems in the gametophyte (haploid) body and undergo a gametophyte-dominant life cycle. We here show that MpCLE1 confines the proliferative activity of gametophytic meristem and affects the overall size of gametangiophores (reproductive organs) in Marchantia polymorpha, which is in sharp contrast with the meristem-promoting function of its ortholog TDIF/CLE41/CLE44 in Arabidopsis vascular meristems. Expression analysis suggests that MpCLE1 and its receptor gene MpTDR are expressed in distinct patterns across the apical meristem. These data suggest that local CLE peptide signaling may have had a role in regulating cell proliferation in the shoot meristem in the ancestral land plant and acts in both sporophytic and gametophytic meristems of extant plants.

Control of stem cell activity in the carpel margin meristem (CMM) in Arabidopsis.

Overview of the current understanding of the molecular mechanisms that regulate meristem activity in the CMM compared to the SAM. Meristems are undifferentiated cells responsible for post-embryonic plant development. The meristems are able to form new organs continuously by carefully balancing between stem cell proliferation and cell differentiation. The plant stem cell niche in each meristem harbors the stem cells that are important to maintain each meristem. The shoot apical meristem (SAM) produces all above-parts of a plant and the molecular mechanisms active in the SAM are actively studied since many years, and models are available. During the reproductive phase of the plant, the inflorescence meristem gives rise to floral meristems, which give rise to the flowers. During floral development, the gynoecium forms that contains a new meristem inside, called the carpel margin meristem (CMM). In Arabidopsis, the gynoecium consists out of two fused carpels, where the CMM forms along the fused carpel margins. In this review, we focus on the molecular mechanisms taking place in the CMM, and we discuss similarities and differences found in the SAM.

The signaling peptide-encoding genes CLE16, CLE17 and CLE27 are dispensable for Arabidopsis shoot apical meristem activity.

The shoot apical meristem produces all of the leaves, stems and flowers of a flowering plant from a reservoir of stem cells at its growing tip. In Arabidopsis, the small polypeptide signaling molecule CLAVATA3 (CLV3), a member of the CLV3/EMBRYO SURROUNDING REGION-RELATED (CLE) gene family, is a key component of a negative feedback loop that maintains stem cell activity in shoot and floral meristems throughout development. Because in some plant species multiple CLE genes are involved in regulating shoot apical meristem activity, we tested the hypothesis that CLE genes other than CLV3 might function in stem cell homeostasis in Arabidopsis. We identified three Arabidopsis CLE genes expressed in the post-embryonic shoot apical meristem, generated loss-of-function alleles using genome editing, and analyzed the meristem phenotypes of the resulting mutant plants. We found that null mutations in CLE16, CLE17 or CLE27 affected neither vegetative nor reproductive shoot meristem activity under normal growth conditions, although CLE27 appears to slightly prolong vegetative growth. Our results indicate that the CLE16, CLE17 and CLE27 genes have largely redundant roles in the Arabidopsis shoot apical meristem and/or regulate meristem activity only under specific environmental conditions.

The ERECTA, CLAVATA and class III HD-ZIP Pathways Display Synergistic Interactions in Regulating Floral Meristem Activities.

In angiosperms, the production of flowers marks the beginning of the reproductive phase. At the emergence of flower primordia on the flanks of the inflorescence meristem, the WUSCHEL (WUS) gene, which encodes a homeodomain transcription factor starts to be expressed and establishes de novo stem cell population, founder of the floral meristem (FM). Similarly to the shoot apical meristem a precise spatial and temporal expression pattern of WUS is required and maintained through strict regulation by multiple regulatory inputs to maintain stem cell homeostasis. However, following the formation of a genetically determined fixed number of floral organs, this homeostasis is shifted towards organogenesis and the FM is terminated. In here we performed a genetic study to test how a reduction in ERECTA, CLAVATA and class III HD-ZIP pathways affects floral meristem activity and flower development. We revealed strong synergistic phenotypes of extra flower number, supernumerary whorls, total loss of determinacy and extreme enlargement of the meristem as compared to any double mutant combination indicating that the three pathways, CLV3, ER and HD-ZIPIII distinctively regulate meristem activity and that they act in parallel. Our findings yield several new insights into stem cell-driven development. We demonstrate the crucial requirement for coupling floral meristem termination with carpel formation to ensure successful reproduction in plants. We also show how regulation of meristem size and alternation in spatial structure of the meristem serve as a mechanism to determine flower organogenesis. We propose that the loss of FM determinacy due to the reduction in CLV3, ER and HD-ZIPIII activity is genetically separable from the AGAMOUS core mechanism of meristem termination.

Meristematic characteristics of tumors initiated by Agrobacterium tumefaciens in pea plants

It is known that two key groups of plant hormones--auxins and cytokinins--play an important role in plant tumor development. The formation of Agrobacterium-induced tumors results from the horizontal transfer of bacterial oncogenes involved in the biosynthesis of these hormones in the plant genome. The role of transcriptional factors in plant tumor development is poorly investigated. It can be assumed that tumor development associated with abnormal cell proliferation can be controlled by the same set of transcription factors that control normal cell proliferation and, in particular, transcription factors that regulate meristem activity. In the present study, we analyzed the histological organization and distribution of proliferating cells in tumors induced by Agrobacterium tumefaciens on pea hypocotyls. In addition, the expression of a set of meristem-specific genes with Agrobacterium tumefaciens-induced tumor development was analyzed. In general, our results indicate that meristematic structures are present in A. tumefaciens-induced tumors and that the development of such tumors is associated with increased expression of a key gene regulating the root apical meristem--the WOX5 gene.

Abnormal development and altered hormone profile and sensitivity in Arabidopsis plants ectopically expressing Brassica shoot apical meristem genes

The previously isolated Brassica genes homologous to the Arabidopsis SHOOT MERISTEMLESS ( STM ), CLAVATA 1 ( CLV1 ), and ZWILLE ( ZLL ) were characterized during embryonic and postembryonic development in vivo. Ectopic expression of the Brassica genes in Arabidopsis caused profound phenotypic deviations from the WT. While the over-expression of BnCLV1 produced smaller embryonic shoot apical meristems (SAMs) with delayed activation at germination, the introduction of Brassica STM enhanced the structure of the SAM and accelerated meristem reactivation. These opposite behaviors were related to differential levels of endogenous cytokinins and abscisic acid (ABA), as well as the expression of genes regulating meristem activity. Low levels of ABA and increased accumulation of the cytokinins trans-zeatin-O-glucoside (t-ZOG), cis-zeatin-O-glucoside (c-ZOG), trans-zeatin riboside (t-ZR), and isopentenyladenosine (iPA) were measured in seedlings of Arabidopsis plants over-expressing the Brassica STM . This was in contrast to BnCLV1 over-expressors which had very low levels of cytokinins. During the early phases of meristem reactivation the expression of the Arabidopsis AtKNAT6 , AtWUSCHEL , and AtCUPSHAPED COTYLEDON - 1 was induced by the introduction of the Brassica STM whereas that of AtCLAVATA 3 was inhibited. An opposite expression profile was measured in lines ectopically expressing BnCLV1 . Other phenotypic abnormalities observed in Arabidopsis plants over-expressing the Brassica STM included lobed leaves, ectopic meristems, and increased number of reproductive organs, i.e. flowers and siliques. The introduction of BnZLL - 1 and - 2 did not cause major developmental abnormalities.

CLE genes may act in a variety of tissues/cells and involve other signaling cascades in addition to CLV3-WUS-like pathways

CLE, which is the term for the CLV3/ESR-related gene family, is thought to participate in CLAVATA3-WUSCHEL (CLV3-WUS) and CLV3-WUS-like signaling pathways to regulate meristem activity in plant. Although some CLE genes are expressed in meristems, many CLE genes appear to express in a variety of tissues/cells. Here we report that CLE14 and CLE20 express in various specific tissues/cells outside the shoot/root apical meristem (SAM/RAM), including in highly differentiated cells, and at different developmental stages. Over-expressing CLE14 or CLE20 also causes multiple phenotypes, which is consistent with its expression pattern in Arabidopsis. These results suggest that CLE genes may play multiple roles and involve other signaling cascades in addition to the CLV3-WUS and CLV3-WUS-like pathways.

Interaction ofKNAT6andKNAT2withBREVIPEDICELLUSandPENNYWISEinArabidopsisInflorescences

The three amino acid loop extension (TALE) homeodomain superfamily, which comprises the KNOTTED-like and BEL1-like families, plays a critical role in regulating meristem activity. We previously demonstrated a function for KNAT6 (for KNOTTED-like from Arabidopsis thaliana 6) in shoot apical meristem and boundary maintenance during embryogenesis. KNAT2, the gene most closely related to KNAT6, does not play such a role. To investigate the contribution of KNAT6 and KNAT2 to inflorescence development, we examined their interactions with two TALE genes that regulate internode patterning, BREVIPEDICELLUS (BP) and PENNYWISE (PNY). Our data revealed distinct and overlapping interactions of KNAT6 and KNAT2 during inflorescence development. Removal of KNAT6 activity suppressed the pny phenotype and partially rescued the bp phenotype. Removal of KNAT2 activity had an effect only in the absence of both BP and KNAT6 or in the absence of both BP and PNY. Consistent with this, KNAT6 and KNAT2 expression patterns were enlarged in both bp and pny mutants. Thus, the defects seen in pny and bp are attributable mainly to the misexpression of KNAT6 and to a lesser extent of KNAT2. Hence, our data showed that BP and PNY restrict KNAT6 and KNAT2 expression to promote correct inflorescence development. This interaction was also revealed in the carpel.

Direct control of shoot meristem activity by a cytokinin-activating enzyme

The growth of plants depends on continuous function of the meristems. Shoot meristems are responsible for all the post-embryonic aerial organs, such as leaves, stems and flowers. It has been assumed that the phytohormone cytokinin has a positive role in shoot meristem function. A severe reduction in the size of meristems in a mutant that is defective in all of its cytokinin receptors has provided compelling evidence that cytokinin is required for meristem activity. Here, we report a novel regulation of meristem activity, which is executed by the meristem-specific activation of cytokinins. The LONELY GUY (LOG) gene of rice is required to maintain meristem activity and its loss of function causes premature termination of the shoot meristem. LOG encodes a novel cytokinin-activating enzyme that works in the final step of bioactive cytokinin synthesis. Revising the long-held idea of multistep reactions, LOG directly converts inactive cytokinin nucleotides to the free-base forms, which are biologically active, by its cytokinin-specific phosphoribohydrolase activity. LOG messenger RNA is specifically localized in shoot meristem tips, indicating the activation of cytokinins in a specific developmental domain. We propose the fine-tuning of concentrations and the spatial distribution of bioactive cytokinins by a cytokinin-activating enzyme as a mechanism that regulates meristem activity.

SERRATE coordinates shoot meristem function and leaf axial patterning in Arabidopsis

Leaves of flowering plants are determinate organs produced by pluripotent structures termed shoot apical meristems. Once specified, leaves differentiate an adaxial (upper) side specialized for light capture, and an abaxial (lower) side specialized for gas exchange. A functional relationship between meristem activity and the differentiation of adaxial leaf fate has been recognized for over fifty years, but the molecular basis of this interaction is unclear. In Arabidopsis thaliana, activity of the class I KNOX (KNOTTED1-like homeobox) genes SHOOTMERISTEMLESS (STM) and BREVIPEDICELLUS (BP) is required for meristem function but excluded from leaves, whereas members of the HD-Zip III (class III homeodomain leucine zipper) protein family function to promote both meristem activity and adaxial leaf fate. Here we show that the zinc-finger protein SERRATE acts in a microRNA (miRNA) gene-silencing pathway to regulate expression of the HD-Zip III gene PHABULOSA (PHB) while also limiting the competence of shoot tissue to respond to KNOX expression. Thus, SERRATE acts to coordinately regulate meristem activity and leaf axial patterning.

Plant hormones and homeoboxes: bridging the gap?

Plant hormones are signalling molecules that control growth and development. Growth of the aerial parts of higher plants requires the continuous activity of the shoot apical meristem, a small mound of cells at the apex of a plant. KNOTTED1-like HOMEOBOX (KNOX) genes are involved in regulating meristem activity, however, little is known about how this regulation is mediated. Recent evidence suggests that KNOX transcription factors may control meristem development by regulating the balance of activities of multiple hormones.

The Arabidopsis det3 mutant reveals a central role for the vacuolar H(+)-ATPase in plant growth and development.

In all multicellular organisms growth and morphogenesis must be coordinated, but for higher plants, this is of particular importance because the timing of organogenesis is not fixed but occurs in response to environmental constraints. One particularly dramatic developmental juncture is the response of dicotyledonous seedlings to light. The det3 mutant of Arabidopsis develops morphologically as a light-grown plant even when it is grown in the dark. In addition, it shows organ-specific defects in cell elongation and has a reduced response to brassinosteroids (BRs). We have isolated the DET3 gene by positional cloning and provide functional and biochemical evidence that it encodes subunit C of the vacuolar H(+)-ATPase (V-ATPase). We show that the hypocotyl elongation defect in the det3 mutant is conditional and provide evidence that this is due to an alternative mechanism of V-ATPase assembly. Together with the expression pattern of the DET3 gene revealed by GFP fluorescence, our data provide in vivo evidence for a role for the V-ATPase in the control of cell elongation and in the regulation of meristem activity.

The TERMINAL FLOWER2 (TFL2) gene controls the reproductive transition and meristem identity in Arabidopsis thaliana.

A new mutant of Arabidopsis thaliana that initiates flowering early and terminates the inflorescence with floral structures has been identified and named terminal flower2 (tfl2). While these phenotypes are similar to that of the terminal flower1 (tfl1) mutant, tfl2 mutant plants are also dwarfed in appearance, have reduced photoperiod sensitivity and have a more variable terminal flower structure. Under long-day and short-day growth conditions tfl1 tfl2 double mutants terminate the inflorescence without development of lateral flowers; thus, unlike tfl1 single mutants the double mutant inflorescence morphology is not affected by day length. The enhanced phenotype of the double mutant suggests that TFL2 acts in a developmental pathway distinct from TFL1. The complex nature of the tfl2 single mutant phenotype suggests that TFL2 has a regulatory role more global than that of TFL1. Double mutant analysis of tfl2 in combination with mutant alleles of the floral meristem identity genes LEAFY and APETALA1 demonstrates that TFL2 function influences developmental processes controlled by APETALA1, but not those regulated by LEAFY. Thus, the TFL2 gene product appears to have a dual role in regulating meristem activity, one being to regulate the meristem response to light signals affecting the development of the plant and the other being the maintenance of inflorescence meristem identity.

The CLAVATA and SHOOT MERISTEMLESS loci competitively regulate meristem activity in Arabidopsis

The CLAVATA (CLV1 and CLV3) and SHOOT MERISTEMLESS (STM) genes specifically regulate shoot meristem development in Arabidopsis. CLV and STH appear to have opposite functions: c1v1 and Clv3 mutants accumulate excess undifferentiated cells in the shoot and floral meristem, while stm mutants fail to form the undifferentiated cells of the shoot meristem during embryonic development. We have identified a weak allele of stm (stm-2) that reveals STM is not only required for the establish- ment of the shoot meristem, but is also required for the continued maintenance of undifferentiated cells in the shoot meristem and for proper proliferation of cells in the floral meristem. We have found evidence of genetic interactions between the CLV and STM loci. clv1 and c1v3 mutations partially suppressed the stm-1 and stm-2 phenotypes, and were capable of suppression in a dominant fashion. clv stm double mutants and plants homozygous for stm but heterozygous for clv, while still lacking an embryonic shoot meristem, exhibited greatly enhanced postembryonic shoot and floral meristem development. Although stm phenotypes are recessive, stm mutations dominantly suppressed clv homozygous and heterozygous phenotypes. These results indicate that the stm phenotype is sensitive to the levels of CLV activity, while the clv phenotype is sensitive to the level of STM activity. We propose that these genes play related but opposing roles in the regulation of cell division and/or cell differentiation in shoot and floral meristems.