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PubMed2005

Regulation of developmental transitions.

Plants undergo a series of profound developmental changes throughout their lifetimes in response to both external environmental factors and internal intrinsic ones. When these changes are abrupt and dramatic, the process is referred to as phase change. Recently, several genes have been discovered that play a role in these developmental transitions. Their sequence and expression patterns shed new light on the mechanisms of phase change, and provide a link between the external and internal factors that control them. Examples of these transitions include changes from juvenile to adult leaf formation, vegetative to inflorescence meristem development, and inflorescence to floral meristem initiation.

Flowers↗

PubMed2005

Formation of primordia and phyllotaxy.

Leaves are made in an iterative pattern by the shoot apical meristem. The mechanism of this pattern formation has fascinated biologists, mathematicians and poets for centuries. Over the past year, fundamental insights into the molecular basis of this process have been gained. Patterns of auxin polar transport dictate when and where new leaf primordia are formed on the surface of the apical meristem. Subsequent events are still obscure but appear to involve both alteration of cell wall characteristics (to facilitate a new vector of growth) and a cascade of spatially co-ordinated transcription factor activity (to determine the fate of cells that are incorporated into new lateral organs). The co-ordinated signalling events involved in these processes are beginning to be elucidated.

Gene Expression Regulation, Developmental↗

PubMed2005

Genetic control of shoot organ boundaries.

The initiation of plant lateral organs from the shoot meristem is associated with the formation of boundaries that separate the primordia from surrounding tissue. A distinctive set of cells is present along the boundary, and these 'boundary cells' display characteristic patterns of cell division, morphology and gene expression. A certain class of the NAC transcription factors is important for growth suppression at the boundary, and auxin and microRNAs participate in boundary formation by regulating NAC gene expression. Other factors regulate different aspects of boundary functions, such as the establishment of the border of different cell identities, the initiation of axillary meristems, or the proper development of organs and tissues in adjacent regions.

Gene Expression Regulation, Plant↗

PubMed1999

Digging out roots: pattern formation, cell division, and morphogenesis in plants.

The analysis of plant development by genetic, molecular, and surgical approaches has accumulated a large body of data, and yet it remains a challenge to uncover the basic mechanisms that are operating. Early steps of development, when the zygote and its daughter cells organize the embryonic plant, are poorly understood despite considerable efforts toward the identification of relevant genes. Reported cases of genetic redundancy suggest that the difficulty in uncovering patterning genes may reflect overlapping gene activities. Our current knowledge on plant embryo development still leaves open whether mechanisms for axis formation and subsequent pattern formation are fundamentally different in animals and plants. Axis formation may follow the general principle of establishing a peripheral asymmetric cue and mobilizing the cytoskeleton toward this cue--in the case of plants possibly located in the cell wall--but the molecules involved may be entirely different. Embryonic pattern formation involves the establishment of different domains, but although there are candidates, it is not clear whether genes that define these domains are identified yet. Pattern formation continues postembryonically in the meristem, and the flexibility of this process may be explained by a feed-forward system of patterning cues originating from more mature cells. Control of cell division and differentiation, which is important in the meristems--regions of continuous development--has been studied intensively and appears to involve short-range signaling and transmembrane receptor kinase activation. Finally, although high importance of control of cell division rates and planes for plant morphogenesis have been often inferred, recent genetic studies as well as comparative morphological data point to a less decisive role of cell division and to global controls of as yet unknown nature.

Body Patterning↗

PubMed1998

The PHANTASTICA gene encodes a MYB transcription factor involved in growth and dorsoventrality of lateral organs in Antirrhinum.

The organs of a higher plant show two fundamental axes of asymmetry: proximodistal and dorsoventral. Dorsoventrality in leaves, bracts, and petal lobes of Antirrhinum majus requires activity of the PHANTASTICA (PHAN) gene. Conditional mutants revealed that PHAN is also required for earlier elaboration of the proximodistal axis. PHAN was isolated and shown to encode a MYB transcription factor homolog. PHAN mRNA is first detected in organ initials before primordium initiation. The structure and expression pattern of PHAN, together with its requirement in two key features of organ development, are consistent with a role in specifying lateral organ identity as distinct from that of the stem or meristem. PHAN also appears to maintain meristem activity in a non-cell-autonomous manner.

Alleles↗

PubMed2002

AtPIN4 mediates sink-driven auxin gradients and root patterning in Arabidopsis.

In contrast to animals, little is known about pattern formation in plants. Physiological and genetic data suggest the involvement of the phytohormone auxin in this process. Here, we characterize a novel member of the PIN family of putative auxin efflux carriers, Arabidopsis PIN4, that is localized in developing and mature root meristems. Atpin4 mutants are defective in establishment and maintenance of endogenous auxin gradients, fail to canalize externally applied auxin, and display various patterning defects in both embryonic and seedling roots. We propose a role for AtPIN4 in generating a sink for auxin below the quiescent center of the root meristem that is essential for auxin distribution and patterning.

Amino Acid Sequence↗

PubMed2003

Rice UV-damaged DNA binding protein homologues are most abundant in proliferating tissues.

Ultraviolet-damaged DNA binding protein (UV-DDB) is an important factor involved in DNA repair. To study the role of UV-DDB, we attempted to obtain the cDNA and the protein of a plant UV-DDB. We succeeded in isolating both genes for UV-DDB subunits from rice (Oryza sativa cv. Nipponbare), designated as OsUV-DDB1 and OsUV-DDB2. OsUV-DDB2 (65 kDa) was much larger than human UV-DDB2, but immunoprecipitation and gel mobility shift assay suggested that OsUV-DDB2 is a plant counterpart of UV-DDB2. The transcripts were expressed in proliferating tissues such as the meristem, but were detected at only low levels in the mature leaves, although the leaves are strongly exposed to UV. These transcripts were induced in the meristem after UV-irradiation. The expression levels of OsUV-DDB were significantly reduced when cell proliferation was temporarily halted. These results indicated that the level of OsUV-DDB expression is correlated with cell proliferation, and its expression may be required mostly for DNA repair in DNA replication.

Amino Acid Sequence↗

PubMed1999

A TM3-like MADS-box gene from Eucalyptus expressed in both vegetative and reproductive tissues.

MADS-box genes in plants are a diverse class of transcription factors that are involved in regulating developmental processes, particularly meristem and organ identity during floral development. They are characterized by a highly conserved MADS-box domain of 59 amino acids that binds to specific DNA sequences. We report the characterization of a cDNA clone, ETL (Eucalyptus TM3 Like), from Eucalyptus globulus subspecies bicostata encoding a putative transcription factor of the MADS-box class that is strongly expressed in both vegetative and floral tissues, suggesting that it regulates processes other than floral development. The clone was isolated from a floral bud cDNA library with a probe generated from Eucalyptus genomic DNA by PCR using degenerate primers to the MADS-box of the floral regulatory gene APETALA 1. The ETL cDNA clone encodes a putative protein of 206 amino acids that contains an N-terminal MADS-box and a helical domain of approx. 60 amino acids predicted to form a coiled-coil (K-box). These structural features are characteristic of plant MADS-box proteins. The MADS-box domain contains all the signature residues of a class of MADS-box genes typified by the tomato gene TM3 and overall, ETL shows 56% amino acid identity to TM3. Like TM3, the ETL gene is expressed in both vegetative and reproductive organs, predominantly in root and shoot meristems and organ primordia, as well as in developing male and female floral organs.

Amino Acid Sequence↗

PubMed2003

Mechanism underlying regulated expression of RFL, a conserved transcription factor, in the developing rice inflorescence.

LFY and its orthologues are necessary for flower specification in diverse dicotyledonous plants. The spatial and temporal RNA expression pattern of a rice LFY-like gene: RFL differs significantly from that in several other species studied thus far. The onset of RFL expression coincides with inflorescence meristem (panicle meristem) initiation, and continues during panicle branching. Notably, incipient flower primordia have lower expression levels, and during floral organogenesis the expression is restricted to second-whorl lodicules. To explore mechanisms underlying this distinct expression pattern, we have tested the transcription regulatory functions of sequences upstream to RFL coding sequences either alone, or together with intragenic segments. Sequences 3.0 kb upstream of the RFL reading frame do not confer correctly regulated reporter gene expression in transgenic rice. In contrast, RFL intron1 or 2 can individually confer the expected profile in the developing panicle and floret. However, the additional repression of expression in vegetative tissues, is a pattern achieved by intron2 together with far-upstream sequences. Strikingly, RFL intron2 sequences can even utilize the Arabidopsis thaliana LFY promoter to confer regulated transcription in young rice panicles. By sequence comparison of RFL upstream sequences, intron1, intron2 and the Arabidopsis LFY promoter, we identify putative cis-regulatory elements unique to RFL. These data exemplify the use of regulatory circuits specific to rice RFL that may underlie its divergent function.

Arabidopsis↗

PubMedDate not supplied

Post-transcriptional control of the Arabidopsis auxin efflux carrier EIR1 requires AXR1.

The auxin efflux carrier EIR1 (also known as AGR and AtPIN2) is a key mediator of the response of Arabidopsis roots to gravity [1,2]. This response is thought to require the establishment of a transient auxin gradient in the root meristem, resulting in differential cell elongation [3]. Recent reports suggest that EIR1 is essential for the asymmetric distribution of auxin in the root meristem [4-7], but the regulatory aspects of this process are still not fully understood. Here, we studied the regulation of EIR1 in Arabidopsis using two reporters: one was a translational fusion that contained the entire EIR1 coding sequence, and the other a transcriptional fusion that had no EIR1 coding sequence. We found that EIR1 is controlled at the post-transcriptional level. The translational fusion was unstable in response to changes in auxin homeostasis, and was destabilized by cycloheximide. In contrast, the protein was stabilized in the axr1-3 mutant, which is auxin resistant and defective in auxin responses such as root gravitropism [8,9]. AXR1 is thought to participate in ubiquitin-mediated control of protein stability [10-12]. The dependence of EIR1 reporter expression on auxin concentrations and AXR1 suggests that auxin transport is regulated through a feedback regulatory loop that affects protein stability in response to auxin.

Arabidopsis↗

PubMed1997

A LEAFY co-regulator encoded by UNUSUAL FLORAL ORGANS.

BACKGROUND: . Development of petals and stamens in Arabidopsis flowers requires the function of the organ-identity gene APETALA3 (AP3), whose RNA is expressed specifically in petal and stamen primordia. AP3 expression is positively regulated by the meristem-identity gene LEAFY (LFY), which is expressed ubiquitously in young flowers. It is unknown how the transition from ubiquitous expression of LFY to region-specific expression of AP3 is made. It has previously been proposed for Antirrhinum that another gene, FIMBRIATA (FIM), mediates between the LFY and AP3 orthologs, with the three genes acting in a simple regulatory hierarchy. FIM is activated later than the LFY ortholog, and its expression is more restricted than that of the LFY ortholog. RESULTS: . We have tested whether the model proposed for Antirrhinum applies to Arabidopsis, by creating transgenic plants in which the FIM ortholog UNUSUAL FLORAL ORGANS (UFO) was expressed constitutively from the promoter of the cauliflower mosaic virus 35S gene. In 35S::UFO flowers, AP3 was expressed precociously and ectopically, confirming that UFO is an upstream regulator of AP3. However, 35S::UFO could not restore petal and stamen development in lfy mutants, indicating that UFO can only function in the presence of LFY activity. The failure of 35S::UFO to rescue lfy mutants is consistent with our observation that UFO expression levels are not markedly changed in lfy mutants. CONCLUSIONS: . We conclude that UFO is not a simple mediator between meristem- and organ-identity genes, but is likely to be a partially dispensable co-regulator that acts together with LFY. The interplay between LFY and UFO provides a paradigm for how a global regulator such as LFY activates selected target genes only in restricted regions within its expression domain.

Arabidopsis↗

PubMedDate not supplied

Intrinsic oxygen use kinetics of transformed plant root culture.

Root meristem oxygen uptake, root tip extension rate, and specific growth rate are assessed as a function of dissolved oxygen level for three transformed root cultures. The influence of hydrodynamic boundary layer was considered for all measurements to permit correlation of oxygen-dependent kinetics with the concentration of oxygen at the surface of the root meristem. Oxygen uptake rate is shown to be saturated at ambient conditions, and a saturation level of approximately 300 micromole O2/(cm(3) tissue.hr) was observed for all three of these morphologically diverse root types. In nearly all cases, the observation of a minimum oxygen pressure, below which respiration, extension, or root growth would not occur, could be accounted for as a boundary layer mass transfer resistance. The critical oxygen pressure below which respiration declines is below saturated ambient oxygen conditions. In contrast, critical oxygen pressures for root tip extension were much higher; extension was nearly linear for the two thicker root types (Hyoscyamus muticus, henbain; Solanum tuberosum, potato) above ambient oxygen levels. The performance of the thinnest root, Brassica juncea (Indian mustard) was consistent with reduced internal limitations for oxygen transport. Extension rates did not correlate with biomass accumulation. The fastest growing henbain culture micro = 0.44 day(-)(1)) displayed the slowest extension rate (0.16 mm/hr), and the slowest growing mustard culture (micro = 0.22 day(-)(1)) had the fastest tip extension rate (0.3 mm/hr). This apparent paradox is explained in terms of root branching patterns, where the root branching ratio is shown to be dependent upon the oxygen-limited mersitem extension rate. The implications of these observations on the performance of root culture in bioreactors is discussed.

Brassica↗

PubMed2007

Picloram resistance in transgenic tobacco expressing an anti-picloram scFv antibody is due to reduced translocation.

Picloram resistance exhibited by transgenic tobacco (Nicotiana tabacum) plants expressing an anti-picloram single-chain variable fragment (scFv) antibody was investigated through the study of homozygous lines expressing the antibody. Dose-response bioassays, using foliar application of picloram, showed that these homozygous transgenic plants were resistant to at least 5 g of ai ha-1 picloram and grew normally to produce seed, whereas wild-type plants did not survive. Although these lines had improved resistance compared with those previously reported, significant improvements are still required to achieve field-level resistance. Uptake and translocation studies demonstrated that [14C]picloram translocation from treated leaves to the apical meristem was reduced in transgenic versus wild-type plants. The presence of [14C]picloram visualized by autoradiography and quantified by liquid scintillation spectrometry, demonstrated the distribution of more picloram in the treated leaf and less in the apical meristem of transgenic plants when compared to wild-type plants. No differences between transgenic and wild-type plants were found in the distribution of [14C]clopyralid, a herbicide with structural similarity to picloram as well as the same mechanism of action. No differences were found in the metabolism of [14C]picloram. Taken together, these results suggest that reduced translocation to the site of action is a major mechanism responsible for picloram resistance in tobacco plants expressing this anti-picloram antibody.

Antibodies↗

PubMed1998

Functional analysis of the promoter region of a maize (Zea mays L.) H3 histone gene in transgenic Arabidopsis thaliana.

A 1023 bp fragment and truncated derivatives of the maize (Zea mays L.) histone H3C4 gene promoter were fused to the beta-glucuronidase (GUS) gene and introduced via Agrobacterium tumefaciens into the genome of Arabidopsis thaliana. GUS activity was found in various meristems of transgenic plants as for other plant histone promoters, but unexplained activity also occurred at branching points of both stems and roots. Deletion of the upstream 558 bp of the promoter reduced its activity to an almost basal expression. Internal deletion of a downstream fragment containing plant histone-specific sequence motifs reduced the promoter activity in all tissues and abolished the expression in meristems. Thus, both the proximal and distal regions of the promoter appear necessary to achieve the final expression pattern in dicotyledonous plant tissues. In mesophyll protoplasts isolated from the transformed Arabidopsis plants, the full-length promoter showed both S phase-dependent and -independent activity, like other plant histone gene promoters. Neither of the 5'-truncated nor the internal-deleted promoters were able to direct S phase-dependent activity, thus revealing necessary cooperation between the proximal and distal parts of the promoter to achieve cell cycle-regulated expression. The involvement of the different regions of the promoter in the different types of expression is discussed.

Arabidopsis↗

PubMed2002

Alterations in the distribution of histone H3 phosphorylation in mitotic plant chromosomes in response to cold treatment and the protein phosphatase inhibitor cantharidin.

The function of the phosphorylation of histone H3 at Ser 10 in plant cell division is uncertain. The timing correlates with chromosome condensation, and studies in plant meiosis suggest that it is involved in sister chromatid cohesion. In mitosis, plant chromosomes are highly phosphorylated in the pericentromeric region only. In order to modulate H3 phosphorylation, root meristems of different plant species were treated with the protein phosphatase inhibitor cantharidin or with ice-water. Immunostaining using an antibody specific to phosphorylated H3 at Ser 10 revealed a high level of H3 phosphorylation along the whole mitotic chromosome after cantharidin treatment, which resembles the distribution seen exclusively in first meiotic division. In chromosomes that were isolated from meristems treated with ice-water, the heterochromatic regions and nucleolar organizer regions, in addition to the pericentromeric region, were highly phosphorylated at H3. Cantharidin and ice-water also affected spindle assembly and chromosome length, but these effects did not seem to be directly linked to changes in H3 phosphorylation.

Cantharidin↗

PubMedDate not supplied

Asymmetric leaves1 mediates leaf patterning and stem cell function in Arabidopsis.

Meristem function in plants requires both the maintenance of stem cells and the specification of founder cells from which lateral organs arise. Lateral organs are patterned along proximodistal, dorsoventral and mediolateral axes. Here we show that the Arabidopsis mutant asymmetric leaves1 (as1) disrupts this process. AS1 encodes a myb domain protein, closely related to PHANTASTICA in Antirrhinum and ROUGH SHEATH2 in maize, both of which negatively regulate knotted-class homeobox genes. AS1 negatively regulates the homeobox genes KNAT1 and KNAT2 and is, in turn, negatively regulated by the meristematic homeobox gene SHOOT MERISTEMLESS. This genetic pathway defines a mechanism for differentiating between stem cells and organ founder cells within the shoot apical meristem and demonstrates that genes expressed in organ primordia interact with meristematic genes to regulate shoot morphogenesis.

Arabidopsis↗

PubMed2004

Control of phyllotaxy by the cytokinin-inducible response regulator homologue ABPHYL1.

Phyllotaxy describes the geometric pattern of leaves and flowers, and has intrigued botanists and mathematicians for centuries. How these patterns are initiated is poorly understood, and this is partly due to the paucity of mutants. Signalling by the plant hormone auxin appears to determine the site of leaf initiation; however, this observation does not explain how distinct patterns of phyllotaxy are initiated. abphyl1 (abph1) mutants of maize initiate leaves in a decussate pattern (that is, paired at 180 degrees), in contrast to the alternating or distichous phyllotaxy observed in wild-type maize and other grasses. Here we show that ABPH1 is homologous to two-component response regulators and is induced by the plant hormone cytokinin. ABPH1 is expressed in the embryonic shoot apical meristem, and its spatial expression pattern changes rapidly with cytokinin treatment. We propose that ABPH1 controls phyllotactic patterning by negatively regulating the cytokinin-induced expansion of the shoot meristem, thereby limiting the space available for primordium initiation at the apex.

Alleles↗

PubMed2006

Intracellular trafficking and proteolysis of the Arabidopsis auxin-efflux facilitator PIN2 are involved in root gravitropism.

Root gravitropism describes the orientation of root growth along the gravity vector and is mediated by differential cell elongation in the root meristem. This response requires the coordinated, asymmetric distribution of the phytohormone auxin within the root meristem, and depends on the concerted activities of PIN proteins and AUX1 - members of the auxin transport pathway. Here, we show that intracellular trafficking and proteasome activity combine to control PIN2 degradation during root gravitropism. Following gravi-stimulation, proteasome-dependent variations in PIN2 localization and degradation at the upper and lower sides of the root result in asymmetric distribution of PIN2. Ubiquitination of PIN2 occurs in a proteasome-dependent manner, indicating that the proteasome is involved in the control of PIN2 turnover. Stabilization of PIN2 affects its abundance and distribution, and leads to defects in auxin distribution and gravitropic responses. We describe the effects of auxin on PIN2 localization and protein levels, indicating that redistribution of auxin during the gravitropic response may be involved in the regulation of PIN2 protein.

Arabidopsis↗