ALTERED MERISTEM PROGRAM1 Restricts Shoot Meristem Proliferation and Regeneration by Limiting HD-ZIP III-Mediated Expression of RAP2.6L

ALTERED MERISTEM PROGRAM1 sustains cellular differentiation by limiting HD-ZIP III transcription factor gene expression

Plants show remarkable developmental and regenerative plasticity through the sustained activity of stem cells in meristems. Under certain conditions, pluripotency can even be reestablished in cells that have already entered differentiation. Mutation of the putative carboxypeptidase ALTERED MERISTEM PROGRAM1 (AMP1) in Arabidopsis (Arabidopsis thaliana) causes a set of hypertrophic phenotypes, indicating a defect in the suppression of pluripotency. A role of AMP1 in the miRNA-mediated inhibition of translation has previously been reported; however, how this activity is related to its developmental functions is unclear. Here, we examined the functional interaction between AMP1 and the Class III homeodomain-leucine zipper (HD-ZIP III) transcription factors, which are miRNA-controlled determinants of shoot meristem specification. We found that the HD-ZIP III transcriptional output is enhanced in the amp1 mutant and that plant lines with increased HD-ZIP III activity not only developed amp1 mutant-like phenotypes but also showed a synergistic genetic interaction with the mutant. Conversely, the reduction of HD-ZIP III function suppressed the shoot hypertrophy defects of the amp1 mutant. We further provide evidence that the expression domains of HD-ZIP III family members are expanded in the amp1 mutant and that this misexpression occurs at the transcriptional level and does not involve the function of miRNA165/166. Finally, amp1 mutant-specific phenotypes cannot be mimicked by a general inhibition of miRNA function in the AMP1 expression domain. These findings lead us to a model in which AMP1 restricts cellular pluripotency upstream of HD-ZIP III proteins, and this control appears to be not directly mediated by the canonical miRNA pathway.

Endoplasmic reticulum protein ALTERED MERISTEM PROGRAM 1 negatively regulates senescence in Arabidopsis

Plant senescence is a highly regulated developmental program crucial for nutrient reallocation and stress adaptation in response to developmental and environmental cues. Stress-induced and age-dependent natural senescence share both overlapping and distinct molecular responses and regulatory schemes. Previously, we have utilized a carbon-deprivation (C-deprivation) senescence assay using Arabidopsis (Arabidopsis thaliana) seedlings to investigate senescence regulation. Here we conducted a comprehensive time-resolved transcriptomic analysis of Arabidopsis wild type seedlings subjected to C-deprivation treatment at multiple time points, unveiling substantial temporal changes and distinct gene expression patterns. Moreover, we identified ALTERED MERISTEM PROGRAM 1 (AMP1), encoding an endoplasmic reticulum protein, as a potential regulator of senescence based on its expression profile. By characterizing loss-of-function alleles and overexpression lines of AMP1, we confirmed its role as a negative regulator of plant senescence. Genetic analyses further revealed a synergistic interaction between AMP1 and the autophagy pathway in regulating senescence. Additionally, we discovered a functional association between AMP1 and the endosome-localized ABNORMAL SHOOT3 (ABS3)-mediated senescence pathway and positioned key senescence-promoting transcription factors downstream of AMP1. Overall, our findings shed light on the molecular intricacies of transcriptome reprogramming during C-deprivation-induced senescence and the functional interplay among endomembrane compartments in controlling plant senescence.

GWAS for the identification of introgressed candidate genes of Sinapis alba with increased branching numbers in backcross lines of the allohexaploid Brassica

Plant architecture is a crucial determinant of crop yield. The number of primary (PB) and secondary branches (SB) is particularly significant in shaping the architecture of Indian mustard. In this study, we analyzed a panel of 86 backcross introgression lines (BCILs) derived from the first stable allohexaploid Brassicas with 170 Sinapis alba genome-specific SSR markers to identify associated markers with higher PB and SB through association mapping. The structure analysis revealed three subpopulations, i.e., P1, P2, and P3, in the association panel containing a total of 11, 33, and 42 BCILs, respectively. We identified five novel SSR markers linked to higher PB and SB. Subsequently, we explored the 20 kb up- and downstream regions of these SSR markers to predict candidate genes for improved branching and annotated them through BLASTN. As a result, we predicted 47 complete genes within the 40 kb regions of all trait-linked markers, among which 35 were identified as candidate genes for higher PB and SB numbers in BCILs. These candidate genes were orthologous to ANT, RAMOSUS, RAX, MAX, MP, SEU, REV, etc., branching genes. The remaining 12 genes were annotated for additional roles using BLASTP with protein databases. This study identified five novel S. alba genome-specific SSR markers associated with increased PB and SB, as well as 35 candidate genes contributing to plant architecture through improved branching numbers. To the best of our knowledge, this is the first report of introgressive genes for higher branching numbers in B. juncea from S. alba.

Application of Developmental Regulators for Enhancing Plant Regeneration and Genetic Transformation

Establishing plant regeneration systems and efficient genetic transformation techniques plays a crucial role in plant functional genomics research and the development of new crop varieties. The inefficient methods of transformation and regeneration of recalcitrant species and the genetic dependence of the transformation process remain major obstacles. With the advancement of plant meristematic tissues and somatic embryogenesis research, several key regulatory genes, collectively known as developmental regulators, have been identified. In the field of plant genetic transformation, the application of developmental regulators has recently garnered significant interest. These regulators play important roles in plant growth and development, and when applied in plant genetic transformation, they can effectively enhance the induction and regeneration capabilities of plant meristematic tissues, thus providing important opportunities for improving genetic transformation efficiency. This review focuses on the introduction of several commonly used developmental regulators. By gaining an in-depth understanding of and applying these developmental regulators, it is possible to further enhance the efficiency and success rate of plant genetic transformation, providing strong support for plant breeding and genetic engineering research.

Cytokinins - regulators of de novo shoot organogenesis

Plants, unlike animals, possess a unique developmental plasticity, that allows them to adapt to changing environmental conditions. A fundamental aspect of this plasticity is their ability to undergo postembryonic de novo organogenesis. This requires the presence of regulators that trigger and mediate specific spatiotemporal changes in developmental programs. The phytohormone cytokinin has been known as a principal regulator of plant development for more than six decades. In de novo shoot organogenesis and in vitro shoot regeneration, cytokinins are the prime candidates for the signal that determines shoot identity. Both processes of de novo shoot apical meristem development are accompanied by changes in gene expression, cell fate reprogramming, and the switching-on of the shoot-specific homeodomain regulator, WUSCHEL. Current understanding about the role of cytokinins in the shoot regeneration will be discussed.

Functional characterization of MdERF113 in apple

The AP2/ERF family is an important class of transcription factors involved in plant growth and various biological processes. One of the AP2/ERF transcription factors, RAP2.6L, participates in various stresses responses. However, the function of RAP2.6L is largely unknown in apples (Malus domestica). In this study, an apple gene homologous to Arabidopsis AtRAP2.6L, MdERF113, was analyzed by bioinformatic characterization, gene expression analysis and subcellular localization assessment. MdERF113 was highly expressed in the sarcocarp and was responsive to hormonal signals and abiotic stresses. MdERF113-overexpression apple calli were less sensitive to low temperature, drought, salinity, and abscisic acid than wild-type. Subcellular localization revealed that MdERF113 was a nuclear-localized transcription factor, and yeast experiments confirmed that MdERF113 has no autonomous activation activity. Overall, this study indicated that MdERF113 plays a role in regulating plant growth under abiotic conditions.

MAX2-dependent competence for callus formation and shoot regeneration from Arabidopsis thaliana root explants

Although the division of the pericycle cells initiates both lateral root development and root-derived callus formation, these developmental processes are affected differently in the strigolactone and karrikin/KARRIKIN INSENSITIVE 2 (KAI2) ligand signalling mutant more axillary growth 2 (max2). Whereas max2 produces more lateral roots than the wild type, it is defective in the regeneration of shoots from root explants. We suggest that the decreased shoot regeneration of max2 originates from delayed formation of callus primordium, yielding less callus material to regenerate shoots. Indeed, when incubated on callus-inducing medium, the pericycle cell division was reduced in max2 and the early gene expression varied when compared with the wild type, as determined by a transcriptomics analysis. Furthermore, the expression of the LATERAL ORGAN BOUNDARIES DOMAIN genes and of callus-induction genes was modified in correlation with the max2 phenotype, suggesting a role for MAX2 in the regulation of the interplay between cytokinin, auxin, and light signalling in callus initiation. Additionally, we found that the in vitro shoot regeneration phenotype of max2 might be caused by a defect in KAI2, rather than in DWARF14, signalling. Nevertheless, the shoot regeneration assays revealed that the strigolactone biosynthesis mutants max3 and max4 also play a minor role.

Genome-wide analysis of the homeodomain-leucine zipper family in Lotus japonicus and the overexpression of LjHDZ7 in Arabidopsis for salt tolerance

The homeodomain-leucine zipper (HD-Zip) family participates in plant growth, development, and stress responses. Here, 40 HD-Zip transcription factors of Lotus japonicus were identified and gave an overview of the phylogeny and gene structures. The expression pattern of these candidate genes was determined in different organs and their response to abiotic stresses, including cold, heat, polyethylene glycol and salinity. The expression of the LjHDZ7 was strongly induced by abiotic stress, especially salt stress. Subsequently, LjHDZ7 gene was overexpressed in Arabidopsis. The transgenic plants grew obviously better than Col-0 plants under salt stress. Furthermore, LjHDZ7 transgenic lines accumulated higher proline contents and showed lower electrolyte leakage and MDA contents than Col-0 plants under salt stress. Antioxidant activities of the LjHDZ7 overexpression lines leaf were significantly higher than those of the Col-0 plants under salt stress. The concentration of Na⁺ ion in LjHDZ7 overexpression lines was significantly lower than that of Col-0 in leaf and root parts. The concentration of K⁺ ion in LjHDZ7 overexpression lines was significantly higher than that of Col-0 in the leaf parts. Therefore, these results showed that overexpression of LjHDZ7 increased resistance to salt stress in transgenic Arabidopsis plants, and certain genes of this family can be used as valuable tools for improving abiotic stresses.

A novel chemical inhibitor of polar auxin transport promotes shoot regeneration by local enhancement of HD-ZIP III transcription

De novo shoot organogenesis is a prerequisite for numerous applications in plant research and breeding but is often a limiting factor, for example, in genome editing approaches. Class III homeodomain-leucine zipper (HD-ZIP III) transcription factors have been characterized as crucial regulators of shoot specification, however up-stream components controlling their activity during shoot regeneration are only partially identified. In a chemical genetic screen, we isolated ZIC2, a novel activator of HD-ZIP III activity. Using molecular, physiological and hormone transport analyses in Arabidopsis and sunflower (Helianthus annuus), we examined the molecular mechanism by which the drug promotes HD-ZIP III expression. ZIC2-dependent upregulation of HD-ZIP III transcription promotes shoot regeneration in Arabidopsis and is accompanied by the induction of shoot specifying factors WUS and RAP2.6L and a subset of cytokinin biosynthesis enzymes. ZIC2's effect on HD-ZIP III expression and regeneration is based on its ability to limit polar auxin transport. We further provide evidence that chemical modulation of auxin efflux can enhance de novo shoot formation in the regeneration recalcitrant species sunflower. Activation of HD-ZIP III transcription during shoot regeneration depends on the local distribution of auxin and chemical modulation of auxin transport can be used to overcome poor shoot organogenesis in tissue culture.

Gene expression profiling before and after internode culture for adventitious shoot formation in ipecac

BACKGROUND

In ipecac (Carapichea ipecacuanha (Brot.) L. Andersson), adventitious shoots can be induced simply by placing internodal segments on phytohormone-free culture medium. The shoots form locally on the epidermis of the apical region of the segments, but not the basal region. Levels of endogenous auxin and cytokinin transiently increase in the segments after 1 week of culture.

RESULTS

Here, we conducted RNA-seq analysis to compare gene expression patterns in apical and basal regions of segments before culture and after 1 week of culture for adventitious shoot formation. The results revealed 8987 differentially expressed genes in a de novo assembly of 76,684 genes. Among them, 276 genes were upregulated in the apical region after 1 week of culture relative to before culture and the basal region after 1 week of culture. These genes include 18 phytohormone-response genes and shoot-formation-related genes. Validation of the gene expression by quantitative real-time PCR assay confirmed that the expression patterns were similar to those of the RNA-seq data.

CONCLUSIONS

The transcriptome data show that expression of cytokinin biosynthesis genes is induced along with the acquisition of cellular pluripotency and the initiation of cell division by wounding in the apical region of internodal segments, that trigger adventitious shoot formation without callusing.

Dual control of MAPK activities by AP2C1 and MKP1 MAPK phosphatases regulates defence responses in Arabidopsis

Mitogen-activated protein kinase (MAPK) cascades transmit environmental signals and induce stress and defence responses in plants. These signalling cascades are negatively controlled by specific Ser/Thr protein phosphatases of the type 2C (PP2C) and dual-specificity phosphatase (DSP) families that inactivate stress-induced MAPKs; however, the interplay between phosphatases of these different types has remained unknown. This work reveals that different Arabidopsis MAPK phosphatases, the PP2C-type AP2C1 and the DSP-type MKP1, exhibit both specific and overlapping functions in plant stress responses. Each single mutant, ap2c1 and mkp1, and the ap2c1 mkp1 double mutant displayed enhanced stress-induced activation of the MAPKs MPK3, MPK4, and MPK6, as well as induction of a set of transcription factors. Moreover, ap2c1 mkp1 double mutants showed an autoimmune-like response, associated with increased levels of the stress hormones salicylic acid and ethylene, and of the phytoalexin camalexin. This phenotype was reduced in the ap2c1 mkp1 mpk3 and ap2c1 mkp1 mpk6 triple mutants, suggesting that the autoimmune-like response is due to MAPK misregulation. We conclude that the evolutionarily distant MAPK phosphatases AP2C1 and MKP1 contribute crucially to the tight control of MAPK activities, ensuring appropriately balanced stress signalling and suppression of autoimmune-like responses during plant growth and development.

Jasmonate inhibits adventitious root initiation through repression of CKX1 and activation of RAP2.6L transcription factor in Arabidopsis

Adventitious rooting is a de novo organogenesis process that enables plants to propagate clonally and cope with environmental stresses. Adventitious root initiation (ARI) is controlled by interconnected transcriptional and hormonal networks, but there is little knowledge of the genetic and molecular programs orchestrating these networks. Thus, we have applied genome-wide transcriptome profiling to elucidate the transcriptional reprogramming events preceding ARI. These reprogramming events are associated with the down-regulation of cytokinin (CK) signaling and response genes, which could be triggers for ARI. Interestingly, we found that CK free base (iP, tZ, cZ, and DHZ) content declined during ARI, due to down-regulation of de novo CK biosynthesis and up-regulation of CK inactivation pathways. We also found that MYC2-dependent jasmonate (JA) signaling inhibits ARI by down-regulating the expression of the CYTOKININ OXIDASE/DEHYDROGENASE1 (CKX1) gene. We also demonstrated that JA and CK synergistically activate expression of the transcription factor RELATED to APETALA2.6 LIKE (RAP2.6L), and constitutive expression of this transcription factor strongly inhibits ARI. Collectively, our findings reveal that previously unknown genetic interactions between JA and CK play key roles in ARI.

Highly pleiotropic functions of CYP78As and AMP1 are regulated in non-cell-autonomous/organ-specific manners

The cytochrome P450 CYP78A5/KLUH in Arabidopsis thaliana is predicted to be involved in the synthesis of a mobile signal molecule that has a pleiotropic function that is distinct from classical phytohormones. CYP78A5 has five close relatives in Arabidopsis. We first investigated their functions, focusing on the plastochron, leaf size, and leaf senescence. Our analyses revealed that CYP78A5 and CYP78A7 are involved in the plastochron and leaf size, and CYP78A6 and CYP78A9 are involved in leaf senescence. Complementation analyses using heterologous promoters and expression analyses suggested that CYP78A isoforms have a common biochemical function and are functionally differentiated via organ-specific expression. The altered meristem program1 (amp1) carboxypeptidase mutant shows a phenotype very similar to that of the cyp78a5 mutant. Complementation analyses using boundary and organizing center-specific promoters suggested that both CYP78A5 and AMP1 act in a non-cell-autonomous manner. Analyses of multiple cyp78a mutants and crosses between cyp78a and amp1 mutants revealed that AMP1/LIKE AMP1 (LAMP1) and CYP78A isoforms regulate plastochron length and leaf senescence in the same genetic pathway, whereas leaf size is independently regulated. Furthermore, we detected feedback regulation between CYP78A6/CYP78A9 and AMP1 at the gene expression level. These observations raise the possibility that AMP1 and CYP78A isoforms are involved in the synthesis of the same mobile signal molecule, and suggest that AMP1 and CYP78A signaling pathways have a very close, albeit complex, functional relationship.

Post-translational modifications regulate the activity of the growth-restricting protease DA1

Plants are a primary food source and can form the basis for renewable energy resources. The final size of their organs is by far the most important trait to consider when seeking increased plant productivity. Being multicellular organisms, plant organ size is mainly determined by the coordination between cell proliferation and cell expansion. The protease DA1 limits the duration of cell proliferation and thereby restricts final organ size. Since its initial identification as a negative regulator of organ growth, various transcriptional regulators of DA1, but also interacting proteins, have been identified. These interactors include cleavage substrates of DA1, and also proteins that modulate the activity of DA1 through post-translational modifications, such as ubiquitination, deubiquitination, and phosphorylation. In addition, many players in the DA1 pathway display conserved phenotypes in other dicot and even monocot species. In this review, we provide a timely overview of the complex, but intriguing, molecular mechanisms that fine-tune the activity of DA1 and therefore final organ size. Moreover, we lay out a roadmap to identify and characterize substrates of proteases and frame the substrate cleavage events in their biological context.

Transcriptional landscapes of floral meristems in barley

Organ development in plants predominantly occurs postembryonically through combinatorial activity of meristems; therefore, meristem and organ fate are intimately connected. Inflorescence morphogenesis in grasses (Poaceae) is complex and relies on a specialized floral meristem, called spikelet meristem, that gives rise to all other floral organs and ultimately the grain. The fate of the spikelet determines reproductive success and contributes toward yield-related traits in cereal crops. Here, we examined the transcriptional landscapes of floral meristems in the temperate crop barley (Hordeum vulgare L.) using RNA-seq of laser capture microdissected tissues from immature, developing floral structures. Our unbiased, high-resolution approach revealed fundamental regulatory networks, previously unknown pathways, and key regulators of barley floral fate and will equally be indispensable for comparative transcriptional studies of grass meristems.

Molecular Analysis of the E2F/DP Gene Family of Daucus carota and Involvement of the DcE2F1 Factor in Cell Proliferation

E2F transcription factors are key components of the RB/E2F pathway that, through the action of cyclin-dependent kinases, regulates cell cycle progression in both plants and animals. Moreover, plant and animal E2Fs have also been shown to regulate other cellular functions in addition to cell proliferation. Based on structural and functional features, they can be divided into different classes that have been shown to act as activators or repressors of E2F-dependent genes. Among the first plant E2F factors to be reported, we previously described DcE2F1, an activating E2F which is expressed in cycling carrot (Daucus carota) cells. In this study, we describe the identification of the additional members of the E2F/DP family of D. carota, which includes four typical E2Fs, three atypical E2F/DEL genes, and three related DP genes. Expression analyses of the carrot E2F and DP genes reveal distinctive patterns and suggest that the functions of some of them are not necessarily linked to cell proliferation. DcE2F1 was previously shown to transactivate an E2F-responsive promoter in transient assays but the functional role of this protein in planta was not defined. Sequence comparisons indicate that DcE2F1 could be an ortholog of the AtE2FA factor of Arabidopsis thaliana. Moreover, ectopic expression of the DcE2F1 cDNA in transgenic Arabidopsis plants is able to upregulate AtE2FB and promotes cell proliferation, giving rise to polycotyly with low frequency, effects that are highly similar to those observed when over-expressing AtE2FA. These results indicate that DcE2F1 is involved in the control of cell proliferation and plays important roles in the regulation of embryo and plant development.

Cytokinin Signaling and De Novo Shoot Organogenesis

The ability to restore or replace injured tissues can be undoubtedly named among the most spectacular achievements of plant organisms. One of such regeneration pathways is organogenesis, the formation of individual organs from nonmeristematic tissue sections. The process can be triggered in vitro by incubation on medium supplemented with phytohormones. Cytokinins are a class of phytohormones demonstrating pleiotropic effects and a powerful network of molecular interactions. The present study reviews existing knowledge on the possible sequence of molecular and genetic events behind de novo shoot organogenesis initiated by cytokinins. Overall, the review aims to collect reactions encompassed by cytokinin primary responses, starting from phytohormone perception by the dedicated receptors, to transcriptional reprogramming of cell fate by the last module of multistep-phosphorelays. It also includes a brief reminder of other control mechanisms, such as epigenetic reprogramming.

ETHYLENE RESPONSE FACTOR 115 integrates jasmonate and cytokinin signaling machineries to repress adventitious rooting in Arabidopsis

Adventitious root initiation (ARI) is a de novo organogenesis program and a key adaptive trait in plants. Several hormones regulate ARI but the underlying genetic architecture that integrates the hormonal crosstalk governing this process remains largely elusive. In this study, we use genetics, genome editing, transcriptomics, hormone profiling and cell biological approaches to demonstrate a crucial role played by the APETALA2/ETHYLENE RESPONSE FACTOR 115 transcription factor. We demonstrate that ERF115 functions as a repressor of ARI by activating the cytokinin (CK) signaling machinery. We also demonstrate that ERF115 is transcriptionally activated by jasmonate (JA), an oxylipin-derived phytohormone, which represses ARI in NINJA-dependent and independent manners. Our data indicate that NINJA-dependent JA signaling in pericycle cells blocks early events of ARI. Altogether, our results reveal a previously unreported molecular network involving cooperative crosstalk between JA and CK machineries that represses ARI.

Natural Variation in Plant Pluripotency and Regeneration

Plant regeneration is essential for survival upon wounding and is, hence, considered to be a strong natural selective trait. The capacity of plant tissues to regenerate in vitro, however, varies substantially between and within species and depends on the applied incubation conditions. Insight into the genetic factors underlying this variation may help to improve numerous biotechnological applications that exploit in vitro regeneration. Here, we review the state of the art on the molecular framework of de novo shoot organogenesis from root explants in Arabidopsis, which is a complex process controlled by multiple quantitative trait loci of various effect sizes. Two types of factors are distinguished that contribute to natural regenerative variation: master regulators that are conserved in all experimental systems (e.g., WUSCHEL and related homeobox genes) and conditional regulators whose relative role depends on the explant and the incubation settings. We further elaborate on epigenetic variation and protocol variables that likely contribute to differential explant responsivity within species and conclude that in vitro shoot organogenesis occurs at the intersection between (epi) genetics, endogenous hormone levels, and environmental influences.

AMP1 and CYP78A5/7 act through a common pathway to govern cell fate maintenance in Arabidopsis thaliana

Higher plants can continuously form new organs by the sustained activity of pluripotent stem cells. These stem cells are embedded in meristems, where they produce descendants, which undergo cell proliferation and differentiation programs in a spatiotemporally-controlled manner. Under certain conditions, pluripotency can be reestablished in descending cells and this reversion in cell fate appears to be actively suppressed by the existing stem cell pool. Mutation of the putative carboxypeptidase ALTERED MERISTEM PROGRAM1 (AMP1) in Arabidopsis causes defects in the suppression of pluripotency in cells normally programmed for differentiation, giving rise to unique hypertrophic phenotypes during embryogenesis as well as in the shoot apical meristem. A role of AMP1 in the miRNA-dependent control of translation has recently been established, however, how this activity is connected to its developmental functions is not resolved. Here we identify members of the cytochrome P450 clade CYP78A to act in parallel with AMP1 to control cell fate in Arabidopsis. Mutation of CYP78A5 and its close homolog CYP78A7 in a cyp78a5,7 double mutant caused suspensor-to-embryo conversion and ectopic stem cell pool formation in the shoot meristem, phenotypes characteristic for amp1. The tissues affected in the mutants showed pronounced expression levels of AMP1 and CYP78A5 in wild type. A comparison of mutant transcriptomic responses revealed an intriguing degree of overlap and highlighted alterations in protein lipidation processes. Moreover, we also found elevated protein levels of selected miRNA targets in cyp78a5,7. Based on comprehensive genetic interaction studies we propose a model in which both enzyme classes act on a common downstream process to sustain cell fate decisions in the early embryo and the shoot apical meristem.

ALTERED MERISTEM PROGRAM1 regulates leaf identity independently of miR156-mediated translational repression

In Arabidopsis, loss of the carboxypeptidase ALTERED MERISTEM PROGRAM1 (AMP1) produces an increase in the rate of leaf initiation, an enlarged shoot apical meristem and an increase in the number of juvenile leaves. This phenotype is also observed in plants with reduced levels of miR156-targeted SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factors, suggesting that AMP1 might promote SPL activity. However, we found that the amp1 mutant phenotype is only partially corrected by elevated SPL gene expression, and that amp1 has no significant effect on SPL transcript levels, or on the level or the activity of miR156. Although AMP1 has been reported to promote miRNA-mediated translational repression, amp1 did not prevent the translational repression of the miR156 target SPL9 or the miR159 target MYB33. These results suggest that AMP1 regulates vegetative phase change downstream of, or in parallel to, the miR156/SPL pathway, and that it is not universally required for miRNA-mediated translational repression.

The miRNA-mRNA Networks Involving Abnormal Energy and Hormone Metabolisms Restrict Tillering in a Wheat Mutant dmc

Tillers not only determine plant architecture but also influence crop yield. To explore the miRNA regulatory network restraining tiller development in a dwarf-monoculm wheat mutant (dmc) derived from Guomai 301 (wild type, WT), we employed miRNome and transcriptome integrative analysis, real-time qRT-PCR, histochemistry, and determinations of the key metabolites and photosynthesis parameters. A total of 91 differentially expressed miRNAs (DEMs) were identified between dmc and WT. Among them, 40 key DEMs targeted 45 differentially expressed genes (DEGs) including the key DEGs encode growth-regulating factors (GRF), auxin response factors (ARF), and other proteins involved in the metabolisms of hormones and carbohydrates, etc. Compared with WT, both the chlorophyll contents and the photosynthesis rate were lower in dmc. The contents of glucose, sucrose, fructose, and maltose were lower in dmc. The contents of auxin (IAA) and zeatin (ZA) were significantly lower, but gibberellin (GA) was significantly higher in the tiller tissues of dmc. This research demonstrated that the DEMs regulating hormone and carbohydrate metabolisms were important causes for dmc to not tiller. A primary miRNA-mRNA regulatory model for dmc tillering was established. The lower photosynthesis rate, insufficient energy, and abnormal hormone metabolisms restrict tillering in dmc.

Molecular Mechanisms of Plant Regeneration

Plants reprogram somatic cells following injury and regenerate new tissues and organs. Upon perception of inductive cues, somatic cells often dedifferentiate, proliferate, and acquire new fates to repair damaged tissues or develop new organs from wound sites. Wound stress activates transcriptional cascades to promote cell fate reprogramming and initiate new developmental programs. Wounding also modulates endogenous hormonal responses by triggering their biosynthesis and/or directional transport. Auxin and cytokinin play pivotal roles in determining cell fates in regenerating tissues and organs. Exogenous application of these plant hormones enhances regenerative responses in vitro by facilitating the activation of specific developmental programs. Many reprogramming regulators are epigenetically silenced during normal development but are activated by wound stress and/or hormonal cues.

Stem cells within the shoot apical meristem: identity, arrangement and communication

Stem cells are specific cells that renew themselves and also provide daughter cells for organ formation. In plants, primary stem cell populations are nurtured within shoot and root apical meristems (SAM and RAM) for the production of aerial and underground parts, respectively. This review article summarizes recent progress on control of stem cells in the SAM from studies of the model plant Arabidopsis thaliana. To that end, a brief overview of the RAM is provided first to emphasize similarities and differences between the two apical meristems, which would help in better understanding of stem cells in the SAM. Subsequently, we will discuss in depth how stem cells are arranged in an organized manner in the SAM, how dynamically the stem cell identity is regulated, what factors participate in stem cell control, and how intercellular communication by mobile signals modulates stem cell behaviors within the SAM. Remaining questions and perspectives are also presented for future studies.