The role of hormones in shoot apical meristem function (original) (raw)
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International Journal of Molecular Sciences
Plants, unlike animals, have developed a unique system in which they continue to form organs throughout their entire life cycle, even after embryonic development. This is possible because plants possess a small group of pluripotent stem cells in their meristems. The shoot apical meristem (SAM) plays a key role in forming all of the aerial structures of plants, including floral meristems (FMs). The FMs subsequently give rise to the floral organs containing reproductive structures. Studies in the past few decades have revealed the importance of transcription factors and secreted peptides in meristem activity using the model plant Arabidopsis thaliana. Recent advances in genomic, transcriptomic, imaging, and modeling technologies have allowed us to explore the interplay between transcription factors, secreted peptides, and plant hormones. Two different classes of plant hormones, cytokinins and auxins, and their interaction are particularly important for controlling SAM and FM developme...
Plant Cell, 2012
The shoot apical meristem (SAM) is a small population of stem cells that continuously generates organs and tissues. This review covers our current understanding of organ initiation by the SAM in Arabidopsis thaliana. Meristem function and maintenance involves two major hormones, cytokinins and auxins. Cytokinins appear to play a major role in meristem maintenance and in controlling meristematic properties, such as cell proliferation. Self-organizing transport processes, which are still only partially understood, lead to the patterned accumulation of auxin at particular positions, where organs will grow out. A major downstream target of auxin-mediated growth regulation is the cell wall, which is a determinant for both growth rates and growth distribution, but feedbacks with metabolism and the synthetic capacity of the cytoplasm are crucial as well. Recent work has also pointed at a potential role of mechanical signals in growth coordination, but the precise mechanisms at work remain to be elucidated.
L1 Division and Differentiation Patterns Influence Shoot Apical Meristem Maintenance
Plant Physiology, 2006
Plant development requires regulation of both cell division and differentiation. The class 1 KNOTTED1-like homeobox (KNOX) genes such as knotted1 (kn1) in maize (Zea mays) and SHOOTMERISTEMLESS in Arabidopsis (Arabidopsis thaliana) play a role in maintaining shoot apical meristem indeterminacy, and their misexpression is sufficient to induce cell division and meristem formation. KNOX overexpression experiments have shown that these genes interact with the cytokinin, auxin, and gibberellin pathways. The L1 layer has been shown to be necessary for the maintenance of indeterminacy in the underlying meristem layers. This work explores the possibility that the L1 affects meristem function by disrupting hormone transport pathways. The semidominant Extra cell layers1 (Xcl1) mutation in maize leads to the production of multiple epidermal layers by overproduction of a normal gene product. Meristem size is reduced in mutant plants and more cells are incorporated into the incipient leaf primordium. Thus, Xcl1 may provide a link between L1 division patterns, hormonal pathways, and meristem maintenance. We used double mutants between Xcl1 and dominant KNOX mutants and showed that Xcl1 suppresses the Kn1 phenotype but has a synergistic interaction with gnarley1 and rough sheath1, possibly correlated with changes in gibberellin and auxin signaling. In addition, double mutants between Xcl1 and crinkly4 had defects in shoot meristem maintenance. Thus, proper L1 development is essential for meristem function, and XCL1 may act to coordinate hormonal effects with KNOX gene function at the shoot apex.
Development (Cambridge, England), 2018
The homeodomain transcription factor SHOOT MERISTEMLESS (STM) is crucial for shoot apical meristem (SAM) function, yet the components and structure of the STM gene regulatory network (GRN) are largely unknown. Here, we show that transcriptional regulators are overrepresented among STM-regulated genes and, using these as GRN components in Bayesian network analysis, we infer STM GRN associations and reveal regulatory relationships between STM and factors involved in multiple aspects of SAM function. These include hormone regulation, TCP-mediated control of cell differentiation, AIL/PLT-mediated regulation of pluripotency and phyllotaxis, and specification of meristem-organ boundary zones via CUC1. We demonstrate a direct positive transcriptional feedback loop between STM and CUC1, despite their distinct expression patterns in the meristem and organ boundary, respectively. Our further finding that STM activates expression of the CUC1-targeting microRNA combined with mathematical modell...
Plant Cell Reports, 2012
Establishment of the shoot apical meristem (SAM) in Arabidopsis embryos requires the KNOXI transcription factor SHOOT MERISTEMLESS. In Norway spruce (Picea abies), four KNOXI family members (HBK1, HBK2, HBK3 and HBK4) have been identified, but a corresponding role in SAM development has not been demonstrated. As a first step to differentiate between the functions of the four Norway spruce HBK genes, we have here analyzed their expression profiles during the process of somatic embryo development. This was made both under normal embryo development and under conditions of reduced SAM formation by treatment with the polar auxin transport inhibitor NPA. Concomitantly with the formation of an embryonic SAM, the HBK2 and HBK4 genes displayed a significant up-regulation that was delayed by NPA treatment. In contrast, HBK1 and HBK3 were up-regulated prior to SAM formation, and their temporal expression was not affected by NPA. Ectopic expression of the four HBK genes in transgenic Arabidopsis plants further supported similar functions of HBK2 and HBK4, distinct from those of HBK1 and HBK3. Together, the results suggest that HBK2 and HBK4 exert similar functions related to the SAM differentiation and somatic embryo development in Norway spruce, while HBK1 and HBK3 have more general functions during embryo development.
Direct control of shoot meristem activity by a cytokinin-activating enzyme
Nature, 2007
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 1 . It has been assumed that the phytohormone cytokinin has a positive role in shoot meristem function 2-4 . 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 5,6 . 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 cytokininspecific 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.
Control of shoot and root meristem function by cytokinin
Current Opinion in Plant Biology, 2007
Plant hormones regulate a variety of processes fundamental for growth and development. Recent studies have clearly shown that establishing adequate spatial and temporal distribution of hormones is central in the control of development. The activity of cytokinins (CKs) is essential to maintain undifferentiated cells in shoot apical meristem (SAM) and to promote cell differentiation in the root meristem (RAM). Detailed mechanisms how the gradient of CK activities is established in the meristem has begun to be elucidated.
KNOXing on the BELL: TALE Homeobox Genes and Meristem Activity
All plant organs are derived from meristems. The shoot apical meristem (SAM) produces the aerial part of the plant. It has two main functions: the maintenance of a group of stem cells at the center of the meristem and the initiation of organs at its periphery. The organs are initiated in a regular spatial pattern, referred to as phyllotaxy, and are separated from the surrounding tissue by a boundary domain. The KNOTTED-like homeobox (KNOX) family of transcription factors plays a key role in the control of SAM activity. These proteins belong to the three amino acid loop extension (TALE) homeodomain superclass and form heterodimers with other TALE proteins belonging to the BEL1-like (BELL) family. The KNOX proteins regulate the different activities of the SAM. They control SAM maintenance, boundary establishment, the correct patterning of organ initiation and the development of axillary meristems. They exert their effects through the regulation of several hormonal pathways. KNOX proteins repress gibberellin (GA) biosynthesis and activate cytokinin (CK) synthesis and signaling. In addition to their role in the SAM, they contribute to leaf form diversity. In plants with simple leaves, KNOX genes are expressed in the SAM and downregulated in leaf primordia, whereas in plants with dissected leaves their expression is reactivated in leaves.
The success of many tissue culture techniques, including organogenesis and somatic embryogenesis depends upon the proper formation of de novo meristems, the function of which affects the ultimate ability to regenerate viable plants. Therefore studies on meristem formation and maintenance are crucial for improving in vitro techniques. Given the economic importance of ornamental plants, it surprising that very few species are commercially mass propagated via somatic embryogenesis. This limitation is mainly due to the lack of optimization of protocols and/or specific culture conditions. This review examines the physiological and molecular events occurring during shoot apical meristem (SAM) formation in vivo and outlines differences in the ontogeny of the SAM produced in vivo and in vitro. Evidence is also provided that the genetic network regulating the function of the SAM in vivo might also operate in culture during the formation of embryogenic and/or organogenic cells. Recent experiments reveal that altered expression of SAM marker genes affects the organogenic and somatic embryogenic processes in vitro with the potential to improve tissue culture in ornamental species.