The astonishingly long lives of plants and their regeneration capacity depend on the activity of plant stem cells. As in animals, stem cells reside in stem cell niches, which produce signals that ...regulate the balance between self-renewal and the generation of daughter cells that differentiate into new tissues. Plant stem cell niches are located within the meristems, which are organized structures that are responsible for most post-embryonic development. The continuous organ production that is characteristic of plant growth requires a robust regulatory network to keep the balance between pluripotent stem cells and differentiating progeny. Components of this network have now been elucidated and provide a unique opportunity for comparing strategies that were developed in the animal and plant kingdoms, which underlie the logic of stem cell behaviour.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
•Various gene networks involve AIL genes to steer plant development.•AILs regulate embryogenesis, meristem development, organ initiation and growth.•AILs interact with auxin pathways at multiple ...levels throughout plant development.•AIL proteins exert their functions in a dosage-dependent manner.
Members of the AINTEGUMENTA-LIKE (AIL) family of APETALA 2/ETHYLENE RESPONSE FACTOR (AP2/ERF) domain transcription factors are expressed in all dividing tissues in the plant, where they have central roles in developmental processes such as embryogenesis, stem cell niche specification, meristem maintenance, organ positioning, and growth. When overexpressed, AIL proteins induce adventitious growth, including somatic embryogenesis and ectopic organ formation. The Arabidopsis (Arabidopsis thaliana) genome contains eight AIL genes, including AINTEGUMENTA, BABY BOOM, and the PLETHORA genes. Studies on these transcription factors have revealed their intricate relationship with auxin as well as their involvement in an increasing number of gene regulatory networks, in which extensive crosstalk and feedback loops have a major role.
During plant growth, dividing cells in meristems must coordinate transitions from division to expansion and differentiation, thus generating three distinct developmental zones: the meristem, ...elongation zone and differentiation zone. Simultaneously, plants display tropisms, rapid adjustments of their direction of growth to adapt to environmental conditions. It is unclear how stable zonation is maintained during transient adjustments in growth direction. In Arabidopsis roots, many aspects of zonation are controlled by the phytohormone auxin and auxin-induced PLETHORA (PLT) transcription factors, both of which display a graded distribution with a maximum near the root tip. In addition, auxin is also pivotal for tropic responses. Here, using an iterative experimental and computational approach, we show how an interplay between auxin and PLTs controls zonation and gravitropism. We find that the PLT gradient is not a direct, proportionate readout of the auxin gradient. Rather, prolonged high auxin levels generate a narrow PLT transcription domain from which a gradient of PLT protein is subsequently generated through slow growth dilution and cell-to-cell movement. The resulting PLT levels define the location of developmental zones. In addition to slowly promoting PLT transcription, auxin also rapidly influences division, expansion and differentiation rates. We demonstrate how this specific regulatory design in which auxin cooperates with PLTs through different mechanisms and on different timescales enables both the fast tropic environmental responses and stable zonation dynamics necessary for coordinated cell differentiation.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Factors with a graded distribution can program fields of cells in a dose-dependent manner, but no evidence has hitherto surfaced for such mechanisms in plants. In the Arabidopsis thaliana root, two ...PLETHORA (PLT) genes encoding AP2-domain transcription factors have been shown to maintain the activity of stem cells. Here we show that a clade of four PLT homologues is necessary for root formation. Promoter activity and protein fusions of PLT homologues display gradient distributions with maxima in the stem cell area. PLT activities are largely additive and dosage dependent. High levels of PLT activity promote stem cell identity and maintenance; lower levels promote mitotic activity of stem cell daughters; and further reduction in levels is required for cell differentiation. Our findings indicate that PLT protein dosage is translated into distinct cellular responses.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Nodules harboring nitrogen-fixing rhizobia are a well-known trait of legumes, but nodules also occur in other plant lineages, with rhizobia or the actinomycete Frankia as microsymbiont. It is ...generally assumed that nodulation evolved independently multiple times. However, molecular-genetic support for this hypothesis is lacking, as the genetic changes underlying nodule evolution remain elusive. We conducted genetic and comparative genomics studies by using Parasponia species (Cannabaceae), the only nonlegumes that can establish nitrogen-fixing nodules with rhizobium. Intergeneric crosses between Parasponia andersonii and its nonnodulating relative Trema tomentosa demonstrated that nodule organogenesis, but not intracellular infection, is a dominant genetic trait. Comparative transcriptomics of P. andersonii and the legume Medicago truncatula revealed utilization of at least 290 orthologous symbiosis genes in nodules. Among these are key genes that, in legumes, are essential for nodulation, including NODULE INCEPTION (NIN) and RHIZOBIUM-DIRECTED POLAR GROWTH (RPG). Comparative analysis of genomes from three Parasponia species and related nonnodulating plant species show evidence of parallel loss in nonnodulating species of putative orthologs of NIN, RPG, and NOD FACTOR PERCEPTION. Parallel loss of these symbiosis genes indicates that these nonnodulating lineages lost the potential to nodulate. Taken together, our results challenge the view that nodulation evolved in parallel and raises the possibility that nodulation originated ∼100 Mya in a common ancestor of all nodulating plant species, but was subsequently lost in many descendant lineages. This will have profound implications for translational approaches aimed at engineering nitrogen-fixing nodules in crop plants.
Stem cells self-renew and produce daughter cells that differentiate. How stem cells are specified and maintained is a central question in developmental biology. Plant stem cells occupy a small region ...or niche in larger zones of mitotic activity called meristems. Here we provide molecular evidence that in the Arabidopsis root meristem, the stem cell population depends on a central group of cells, the quiescent center (QC), which positions the stem cell niche. We show that the putative transcription factor SCARECROW (SCR), first identified by its role in radial patterning, is required cell-autonomously for distal specification of the QC, which in turn regulates stem cell fate of immediately surrounding cells.
Plant postembryonic development takes place in the meristems, where stem cells self-renew and produce daughter cells that differentiate and give rise to different organ structures. For the ...maintenance of meristems, the rate of differentiation of daughter cells must equal the generation of new cells: How this is achieved is a central question in plant development. In the
Arabidopsis root meristem, stem cells surround a small group of organizing cells, the quiescent center. Together they form a stem cell niche
1, 2, whose position and activity depends on the combinatorial action of two sets of genes—
PLETHORA1 (PLT1) and
PLETHORA2 (PLT2)
3, 4 and
SCARECROW (SCR) and
SHORTROOT (SHR)
2—as well as on polar auxin transport. In contrast, the mechanisms controlling meristematic cell differentiation remain unclear. Here, we report that cytokinins control the rate of meristematic cell differentiation and thus determine root-meristem size via a two-component receptor histidine kinase-transcription factor signaling pathway. Analysis of the root meristems of cytokinin mutants, spatial cytokinin depletion, and exogenous cytokinin application indicates that cytokinins act in a restricted region of the root meristem, where they antagonize a non-cell-autonomous cell-division signal, and we provide evidence that this signal is auxin.
Nodules on the roots of legume plants host nitrogen-fixing Rhizobium bacteria. Several lines of evidence indicate that nodules are evolutionarily related to roots. We determined whether developmental ...control of the Medicago truncatula nodule meristem bears resemblance to that in root meristems through analyses of root meristem-expressed PLETHORA genes. In nodules, MtPLETHORA 1 and 2 are preferentially expressed in cells positioned at the periphery of the meristem abutting nodule vascular bundles. Their expression overlaps with an auxin response maximum and MtWOX5, which is a marker for the root quiescent center. Strikingly, the cells in the central part of the nodule meristem have a high level of cytokinin and display MtPLETHORA 3 and 4 gene expression. Nodule-specific knockdown of MtPLETHORA genes results in a reduced number of nodules and/or in nodules in which meristem activity has ceased. Our nodule gene expression map indicates that the nodule meristem is composed of two distinct domains in which different MtPLETHORA gene subsets are expressed. Our mutant studies show that MtPLETHORA genes function redundantly in nodule meristem maintenance. This indicates that Rhizobium has recruited root developmental programs for nodule formation.
In the Arabidopsis root, the SHORT-ROOT transcription factor moves outward to the ground tissue from its site of transcription in the stele and is required for the specification of the endodermis and ...the stem cell organizing quiescent center cells. In addition, SHORT-ROOT and the downstream transcription factor SCARECROW control an oriented cell division in ground tissue stem cell daughters. Here, we show that the JACKDAW and MAGPIE genes, which encode members of a plant-specific family of zinc finger proteins, act in a SHR-dependent feed-forward loop to regulate the range of action of SHORT-ROOT and SCARECROW. JACKDAW expression is initiated independent of SHORT-ROOT and regulates the SCARECROW expression domain outside the stele, while MAGPIE expression depends on SHORT-ROOT and SCARECROW. We provide evidence that JACKDAW and MAGPIE regulate tissue boundaries and asymmetric cell division and can control SHORT-ROOT and SCARECROW activity in a transcriptional and protein interaction network.
Local accumulation of the plant growth regulator auxin mediates pattern formation in Arabidopsis roots and influences outgrowth and development of lateral root- and shoot-derived primordia. However, ...it has remained unclear how auxin can simultaneously regulate patterning and organ outgrowth and how its distribution is stabilized in a primordium-specific manner. Here we show that five PIN genes collectively control auxin distribution to regulate cell division and cell expansion in the primary root. Furthermore, the joint action of these genes has an important role in pattern formation by focusing the auxin maximum and restricting the expression domain of PLETHORA (PLT) genes, major determinants for root stem cell specification. In turn, PLT genes are required for PIN gene transcription to stabilize the auxin maximum at the distal root tip. Our data reveal an interaction network of auxin transport facilitators and root fate determinants that control patterning and growth of the root primordium.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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