Search

Endoplasmic reticulum (ER)–derived organelles, ER bodies, participate in the defense against herbivores in Brassicaceae plants. ER bodies accumulate β-glucosidases, which hydrolyze specialized thioglucosides known as glucosinolates to generate bioactive substances. In Arabidopsis thaliana , the leaf ER (LER) bodies are formed in large pavement cells, which are found in the petioles, margins and blades of rosette leaves. However, the regulatory mechanisms involved in establishing large pavement cells are unknown. Here, we show that the ARABIDOPSIS THALIANA MERISTEM L1 LAYER (ATML1) transcription factor regulates the formation of LER bodies in large pavement cells of rosette leaves. Overexpression of ATML1 enhanced the expression of LER body–related genes and the number of LER body–containing large pavement cells, whereas its knock-out resulted in opposite effects. ATML1 enhances endoreduplication and cell size through LOSS OF GIANT CELLS FROM ORGANS (LGO). Although the overexpression and knock-out of LGO affected the appearance of large pavement cells in Arabidopsis , the effect on LER body–related gene expression and LER body formation was weak. LER body–containing large pavement cells were also found in Eutrema salsugineum , another Brassicaceae species. Our results demonstrate that ATML1 establishes large pavement cells to induce LER body formation in Brassicaceae plants and thereby possibly contribute to the defense against herbivores. [ABSTRACT FROM AUTHOR]

In many plants, only the outermost cells are specified into the epidermis, with underlying mechanisms unknown. Here, the authors show that a key epidermis identity gene is activated in surface cells, via positional cues involving mechanical signals.

The cloning of the ATML1 gene, encoding an HD-ZIP class IV transcription factor, was first reported in 1996. Because ATML1 mRNA was preferentially detected in the shoot epidermis, cis-regulatory sequences of ATML1 have been used to drive gene expression in the outermost cells of the shoot apical meristem and leaves, even before the function of ATML1 was understood. Later studies revealed that ATML1 is required for developmental processes related to shoot epidermal specification and differentiation. Consistent with its central role in epidermal development, ATML1 activity has been revealed to be restricted to the outermost cells via several regulatory mechanisms. In this review, we look back on the history of ATML1 research and provide a perspective for future studies.

Multicellular development produces patterns of specialized cell types. Yet, it is often unclear how individual cells within a field of identical cells initiate the patterning process. Using live imaging, quantitative image analyses and modeling, we show that during Arabidopsis thaliana sepal development, fluctuations in the concentration of the transcription factor ATML1 pattern a field of identical epidermal cells to differentiate into giant cells interspersed between smaller cells. We find that ATML1 is expressed in all epidermal cells. However, its level fluctuates in each of these cells. If ATML1 levels surpass a threshold during the G2 phase of the cell cycle, the cell will likely enter a state of endoreduplication and become giant. Otherwise, the cell divides. Our results demonstrate a fluctuation-driven patterning mechanism for how cell fate decisions can be initiated through a random yet tightly regulated process.

Cell fate determination in plants relies on positional cues. To investigate the position-dependent gene regulation in plants, we focused on shoot epidermal cell specification, which occurs only in the outermost cells. ATML1, which encodes an HD-ZIP class IV transcription factor, is a positive regulator of shoot epidermal cell identity. Despite the presence of a weak ATML1 promoter activity in the inner cells, ATML1 protein was detected mostly in the outermost cells, which suggests that ATML1 accumulation is inhibited in the inner cells. ATML1 nuclear localization was reduced in the epidermis and therewas a positive, albeit weak, correlation between the amount of ATML1 in the nuclei and the expression of a direct target of ATML1. Nuclear accumulation of ATML1 was more strongly inhibited in the inner cells than in the outermost cells. Domain deletion analyses revealed that the ZLZ-coding sequence was necessary and partially sufficient for the post-transcriptional repression of ATML1. Our results suggest that post-transcriptional repressions contribute to the restriction of master transcriptional regulator activity in specific cells to enable position-dependent cell differentiation. [ABSTRACT FROM AUTHOR]

In Arabidopsis thaliana, the epidermis is the outermost cell layer composed of many specialized types of epidermal cells, such as pavement cells, trichomes, and guard cells. The homeodomain‐leucine zipper (HD‐ZIP) class Ⅳ transcription factors (TFs), which are unique to the plant kingdom, have been recognized as key regulators of epidermis development. Unlike animal HD proteins, which can bind to DNA as monomers, plant HD‐ZIP class Ⅳ TFs bind to DNA as dimers, although little is known about the regulation of their dimerization process. Here, we show that the homodimerization of ARABIDOPSIS THALIANA MERISTEM LAYER 1 (ATML1) — HD‐ZIP class Ⅳ TF that is required for protoderm development — is regulated by the lipid‐binding steroidogenic acute regulatory protein‐related lipid transfer (START) domain. We found that ATML1 forms homodimer through interaction via its ZIP motif in yeast and plant cells, although the interaction is abolished by generating a mutation into the lipid‐binding START domain to disrupt the lipid‐binding ability. These results suggest that lipidic ligands function as key regulators of protoderm development via modulating the dimerization of ATML1. [ABSTRACT FROM AUTHOR]

The epidermis of shoot organs in plants develops from the outermost layer (L1) of the shoot apical meristem. In Arabidopsis, a pair of homeobox genes, ARABIDOPSIS THALIANA MERISTEM LAYER1 (ATML1) and PROTODERMAL FACTOR2 (PDF2), play a role in regulating the expression of L1-specific genes. atml1-1 pdf2-1 double mutants show striking defects in the differentiation of shoot epidermal cells. However, because atml1-1 and pdf2-1 have a T-DNA inserted downstream of the respective homeobox sequences, these alleles may not represent null mutations. Here we characterized additional mutant alleles that have a T-DNA insertion at different positions of each gene. Double mutants of a strong atml1-3 allele with each pdf2 allele were found to cause embryonic arrest at the globular stage. Although with low frequency, all double mutant combinations of a weak atml1-1 allele with each pdf2 allele germinated and showed phenotypes defective in shoot epidermal cell differentiation. We further confirmed that transgenic induction of PDF2 fused to the Drosophila Engrailed repressor domain temporarily interferes with epidermal cell differentiation in the wild-type background. These results indicate that ATML1 and PDF2 act redundantly as a positive regulator of shoot epidermal cell differentiation and at least one copy of these genes is essential for embryo development. [ABSTRACT FROM AUTHOR]

The surface of plants is covered by the epidermis, which protects the plant's body from the external environment and mediates inter-cell layer signaling to regulate plant development. Therefore, the manifestation of epidermal traits at a precise location is a prerequisite for their normal growth and development. In Arabidopsis thaliana, class IV homeodomain-leucine zipper transcription factors PROTODERMAL FACTOR2 (PDF2) and ARABIDOPSIS THALIANA MERISTEM LAYER1 (ATML1) play redundant roles in epidermal cell differentiation. Nevertheless, several pieces of evidence suggest that the activity and/or function of PDF2 and ATML1 are regulated differently. The role of the steroidogenic acute regulatory protein-related lipid transfer (START) domain of ATML1 in restricting this protein's activity has been demonstrated; however, whether this lipid-dependent mechanism regulates PDF2 expression is unknown. In this study, we demonstrated that the START domains of PDF2 and ATML1, regulate protein turnover in a position-dependent manner and affect the dimeric proteins. Our results show that a conserved mechanism provides the basis for the functional redundancy of PDF2 and ATML1 in epidermal cell differentiation and that an unidentified regulatory layer specific to PDF2 or ATML1 is responsible for the difference in the activity and/or function of PDF2 and ATML1. [ABSTRACT FROM AUTHOR]

The cloning of the ATML1 gene, encoding an HD-ZIP class IV transcription factor, was first reported in 1996. Because ATML1 mRNA was preferentially detected in the shoot epidermis, cis-regulatory sequences of ATML1 have been used to drive gene expression in the outermost cells of the shoot apical meristem and leaves, even before the function of ATML1 was understood. Later studies revealed that ATML1 is required for developmental processes related to shoot epidermal specification and differentiation. Consistent with its central role in epidermal development, ATML1 activity has been revealed to be restricted to the outermost cells via several regulatory mechanisms. In this review, we look back on the history of ATML1 research and provide a perspective for future studies., Competing Interests: The authors declare no conflict of interest.

The differentiation of distinct cell types in appropriate patterns is a fundamental process in the development of multicellular organisms. In Arabidopsis thaliana, protoderm/epidermis differentiates as a single cell layer at the outermost position. However, little is known about the molecular nature of the positional signals that achieve correct epidermal cell differentiation. Here, we propose that verylong- chain fatty acid-containing ceramides (VLCFA-Cers) mediate positional signals by stimulating the function of ARABIDOPSIS THALIANA MERISTEM LAYER1 (ATML1), a master regulator of protoderm/epidermis differentiation, during lateral root development. We show that VLCFA-Cers, which are synthesized predominantly in the outermost cells, bind to the lipid-binding domain of ATML1. Importantly, this cell type-specific protein-lipid association alters the activity of ATML1 protein and consequently restricts its expression to the protoderm/epidermis through a transcriptional feedback loop. Furthermore, establishment of a compartment, enriched with VLCFAcontaining sphingolipids, at the outer lateral membrane facing the external environment may function as a determinant of protodermal cell fate. Taken together, our results indicate that VLCFA-Cers play a pivotal role in directing protoderm/epidermis differentiation by mediating positional signals to ATML1. [ABSTRACT FROM AUTHOR]

Cell fate determination in plants relies on positional cues. To investigate the position-dependent gene regulation in plants, we focused on shoot epidermal cell specification, which occurs only in the outermost cells. ATML1 , which encodes an HD-ZIP class IV transcription factor, is a positive regulator of shoot epidermal cell identity. Despite the presence of a weak ATML1 promoter activity in the inner cells, ATML1 protein was detected mostly in the outermost cells, which suggests that ATML1 accumulation is inhibited in the inner cells. ATML1 nuclear localization was reduced in the epidermis and there was a positive, albeit weak, correlation between the amount of ATML1 in the nuclei and the expression of a direct target of ATML1. Nuclear accumulation of ATML1 was more strongly inhibited in the inner cells than in the outermost cells. Domain deletion analyses revealed that the ZLZ-coding sequence was necessary and partially sufficient for the post-transcriptional repression of ATML1 Our results suggest that post-transcriptional repressions contribute to the restriction of master transcriptional regulator activity in specific cells to enable position-dependent cell differentiation., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2019. Published by The Company of Biologists Ltd.)

The epidermal cell layer of plants has important functions in regulating plant growth and development. We have studied the impact of an altered epidermal cytokinin metabolism on Arabidopsis shoot development. Increased epidermal cytokinin synthesis or breakdown was achieved through expression of the cytokinin synthesis gene LOG4 and the cytokinin-degrading CKX1 gene, respectively, under the control of the epidermis-specific AtML1 promoter. During vegetative growth, increased epidermal cytokinin production caused an increased size of the shoot apical meristem and promoted earlier flowering. Leaves became larger and the shoots showed an earlier juvenile-to-adult transition. An increased cytokinin breakdown had the opposite effect on these phenotypic traits indicating that epidermal cytokinin metabolism can be a factor regulating these aspects of shoot development. The phenotypic consequences of abbreviated cytokinin signaling in the epidermis achieved through expression of the ARR1-SRDX repressor were generally milder or even absent indicating that the epidermal cytokinin acts, at least in part, cell non-autonomously. Enhanced epidermal cytokinin synthesis delayed cell differentiation during leaf development leading to an increased cell proliferation and leaf growth. Genetic analysis showed that this cytokinin activity was mediated mainly by the AHK3 receptor and the transcription factor ARR1. We also demonstrate that epidermal cytokinin promotes leaf growth in a largely cell-autonomous fashion. Increased cytokinin synthesis in the outer layer of reproductive tissues and in the placenta enhanced ovule formation by the placenta and caused the formation of larger siliques. This led to a higher number of seeds in larger pods resulting in an increased seed yield per plant. Collectively, the results provide evidence that the cytokinin metabolism in the epidermis is a relevant parameter determining vegetative and reproductive plant growth and development. [ABSTRACT FROM AUTHOR]

Molecular mechanisms that generate distinct tissue layers in plant shoots are not well understood. ATML1, an Arabidopsis homeobox gene, is expressed in the outermost cell layer, beginning at an early stage of development. The promoters of many epidermis-specific genes, including ATML1, contain an ATML1-binding site called an L1 box, suggesting that ATML1 regulates epidermal cell fate. Here, we show that overexpression of ATML1 was sufficient to activate the expression of epidermal genes and to induce epidermis-related traits such as the formation of stomatal guard cells and trichome-like cells in non-epidermal seedling tissues. Detailed observation of the division planes of these ectopic stomatal cells suggested that a near-surface position, as well as epidermal cell identity, were required for regular anticlinal cell division, as seen in wild-type epidermis. Moreover, analyses of a loss-of-function mutant and overexpressors implied that differentiation of epidermal cells was associated with repression of mesophyll cell fate. Collectively, our studies contribute new information about the molecular basis of cell fate determination in different layers of plant aerial organs.

The transcription factors ARABIDOPSIS THALIANA MERISTEM L1 ( ATML1) and PROTODERMAL FACTOR2 ( PDF2) are indispensable for epidermal cell-fate specification in Arabidopsis embryos. However, the mechanisms of regulation of these genes, particularly their relationship with cell-cell signalling pathways, although the subject of considerable speculation, remain unclear. Here we demonstrate that the receptor kinase ARABIDOPSIS CRINKLY4 ( ACR4) positively affects the expression of ATML1 and PDF2 in seedlings. In contrast, ATML1- and PDF2-containing complexes directly and negatively affect both their own expression and that of ACR4. By modelling the resulting feedback loop, we demonstrate a network structure that is capable of maintaining robust epidermal cell identity post-germination. We show that a second seed-specific signalling pathway involving the subtilase ABNORMAL LEAFSHAPE1 ( ALE1) and the receptor kinases GASSHO1 ( GSO1) and GASSHO2 ( GSO2) acts in parallel to the epidermal loop to control embryonic surface formation via an ATML1/ PDF2-independent pathway. Genetic interactions between components of this linear pathway and the epidermal loop suggest that an intact embryo surface is necessary for initiation and/or stabilization of the epidermal loop, specifically during early embryogenesis. [ABSTRACT FROM AUTHOR]

In multicellular organisms, a long-standing question is how spatial patterns of distinct cell types are initiated and maintained during continuous cell division and proliferation. Along the vertical axis of plant shoot apical meristems (SAMs), stem cells are located at the top while cells specifying the stem cells are located more basally, forming a robust apical-basal pattern. We previously found that in Arabidopsis SAMs, the HAIRY MERISTEM (HAM) family transcription factors form a concentration gradient from the epidermis to the interior cell layers, and this gradient is essential for the stem cell specification and the apical-basal patterning of the SAMs. Here, we uncover that epidermis specific transcription factors, ARABIDOPSIS THALIANA MERISTEM LAYER 1 (ATML1) and its close homolog, define the concentration gradient of HAM in the SAM through activating a group of microRNAs. This study provides a molecular framework linking the epidermis-derived signal to the stem cell homeostasis in plants. A concentration gradient of HAM transcription factors specifies apical-basal patterning in the Arabidopsis shoot apical meristem. Here, the authors show that epidermal expression of the ATML1 transcription factor defines this concentration gradient via activation of mobile micro RNA. [ABSTRACT FROM AUTHOR]

Endoreduplication is a specialized cell cycle in which DNA replication occurs, but mitosis is skipped creating enlarged polyploid cells. Endoreduplication is associated with the differentiation of many specialized cell types. In the Arabidopsis thaliana sepal epidermis endoreduplicated giant cells form interspersed between smaller cells. Both the transcription factor Arabidopsis thaliana MERISTEM LAYER1 (ATML1) and the plant-specific cyclin dependent kinase inhibitor LOSS OF GIANT CELLS FROM ORGANS (LGO)/SIAMESE RELATED1 (SMR1) are required for the formation of giant cells. Overexpression of LGO is sufficient to produce sepals covered in highly endoreduplicated giant cells. Here we ask whether overexpression of LGO changes the transcriptome of these mature sepals. We show that overexpression of LGO in the epidermis (LGOoe) drives giant cell formation even in atml1 mutant sepals. Using RNAseq we show that LGOoe has significant effects on the mature sepal transcriptome that are primarily ATML1-independent changes of gene activity. Genes activated by LGOoe, directly or indirectly, predominantly encode proteins involved in defense responses, including responses to wounding, insects (a predator of Arabidopsis), and fungus. They also encode components of the glucosinolate biosynthesis pathway, a key biochemical pathway in defense against herbivores. LGOoe-activated genes include previously known marker genes of systemic acquired resistance such as PR1 through PR5. The defensive functions promoted by LGOoe in sepals overlap with functions recently shown to be transcriptionally activated by hyperimmune cpr5 mutants in a LGO-dependent manner. Our findings show that the cell cycle regulator LGO can directly or indirectly drive specific states of gene expression; in particular, they are consistent with recent findings showing LGO to be necessary for transcriptional activation of many defense genes in Arabidopsis. [ABSTRACT FROM AUTHOR]