Separable whorl-specific expression and negative regulation by enhancer elements within the AGAMOUS second intron.
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We analyzed the 4-kb intragenic control region of the AGAMOUS (AG) gene to gain insight into the mechanisms controlling its expression during early flower development. We identified three major expression patterns conferred by 19 AG::reporter gene constructs: the normal AG pattern, a stamen-specific pattern, and a predominantly carpel pattern. To determine whether these three expression patterns were under negative control by APETALA2 (AP2) or LEUNIG (LUG), we analyzed beta-glucuronidase staining patterns in Arabidopsis plants homozygous for strong ap2 and lug mutations. Our results indicated that the stamen-specific pattern was independent of AP2 but dependent on LUG; conversely, the carpel-specific pattern was independent of LUG but dependent on AP2. These results lead to a model of control of AG expression such that expression in each of the two inner whorls is under independent positive and negative control. (+info)
A leaf-derived signal is a quantitative determinant of floral form in Impatiens.
(42/1585)
The completion of flower development in Impatiens balsamina requires continuous inductive (short-day) conditions. We have previously shown that a leaf-derived signal has a role in floral maintenance. The research described here analyzes the role of the leaf in flower development. Leaf removal treatments, in which plants were restricted to a specified number of leaves, resulted in flowers with increased petal number, up to double that of the undefoliated control. Similar petal number increases (as well as changes in bract number or morphology) were recorded when plants began their inductive treatment at a late developmental age or when plants of a nonreverting line (capable of floral maintenance in the absence of continuous short days) were transferred from short days to long days. Our data imply that the increased petal number was neither a response to stress effects associated with leaf removal nor a result of alterations in primordium initiation rates or substitutions of petals for stamens. Rather, the petal initiation phase was prolonged when the amounts of a leaf-derived signal were limiting. We conclude that a leaf-derived signal has a continuous and quantitative role in flower development and propose a temporal model for the action of organ identity genes in Impatiens. This work adds a new dimension to the prevailing ABC model of flower development and may provide an explanation for the wide variety and instabilities of floral form seen among certain species in nature. (+info)
Regulation of gynoecium marginal tissue formation by LEUNIG and AINTEGUMENTA.
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The carpel is the female reproductive organ of flowering plants. In Arabidopsis, congenital fusion of two carpels leads to the formation of an enclosed gynoecium. The margins of the two fused carpels are meristematic in nature and give rise to placentas, ovules, septa, abaxial repla, and the majority of the stylar and stigmatic tissues. Thus, understanding how the marginal tissues are specified and identifying genes that direct their development may provide important insight into higher plant reproductive development. In this study, we show that LEUNIG and AINTEGUMENTA are two critical regulators of marginal tissue development. Double mutants of leunig aintegumenta fail to develop placentas, ovules, septa, stigma, and style. This effect is specific to the leunig aintegumenta double mutant and is not found in other double mutant combinations such as leunig apetala2 or aintegumenta apetala2. Additional analyses indicate that the absence of marginal tissues in leunig aintegumenta double mutants is not mediated by ectopic AGAMOUS. We propose that LEUNIG and AINTEGUMENTA act together to control the expression of common target genes that regulate cell proliferation associated with marginal tissue development. (+info)
GRCD1, an AGL2-like MADS box gene, participates in the C function during stamen development in Gerbera hybrida.
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Despite the differences in flower form, the underlying mechanism in determining the identity of floral organs is largely conserved among different angiosperms, but the details of how the functions of A, B, and C are specified varies greatly among plant species. Here, we report functional analysis of a Gerbera MADS box gene, GRCD1, which is orthologous to AGL2-like MADS box genes. Members of this group of genes are being reported in various species in growing numbers, but their functions remained largely unsettled. GRCD1 expression is detected in all four whorls, but the strongest signal is seen in the developing stamen and carpel. Downregulating GRCD1 expression by antisense transformation revealed that lack of GRCD1 caused homeotic changes in one whorl only: sterile staminodes, which normally develop in whorl 3 of marginal female florets, were changed into petals. This indicates that the GRCD1 gene product is active in determining stamen identity. Transgenic downregulation of GRCD1 causes a homeotic change similar to that in the downregulation of the Gerbera C function genes GAGA1 and GAGA2, but one that is limited to whorl 3. Downregulation of GRCD1 expression does not reduce expression of GAGA1 or GAGA2, or vice versa; and in yeast two-hybrid analysis, GRCD1 is able to interact with GAGA1 and GAGA2. We propose that a heterodimer between the GRCD1 and GAGA1/2 gene products is needed to fulfill the C function in whorl 3 in Gerbera. (+info)
Alterations in CER6, a gene identical to CUT1, differentially affect long-chain lipid content on the surface of pollen and stems.
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Very long chain lipids contribute to the hydrophobic cuticle on the surface of all land plants and are an essential component of the extracellular pollen coat in the Brassicaceae. Mutations in Arabidopsis CER genes eliminate very long chain lipids from the cuticle surface and, in some cases, from the pollen coat. In Arabidopsis, the loss of pollen coat lipids can disrupt interactions with the stigma, inhibiting pollen hydration and causing sterility. We have positionally cloned CER6 and demonstrate that a wild-type copy complements the cer6-2 defect. In addition, we have identified a fertile, intragenic suppressor, cer6-2R, that partially restores pollen coat lipids but does not rescue the stem wax defect, suggesting an intriguing difference in the requirements for CER6 activity on stems and the pollen coat. Importantly, analysis of this suppressor demonstrates that low amounts of very long chain lipids are sufficient for pollen hydration and germination. The predicted CER6 amino acid sequence resembles that of fatty acid-condensing enzymes, consistent with its role in the production of epicuticular and pollen coat lipids >28 carbons long. DNA sequence analysis revealed the nature of the cer6-1, cer6-2, and cer6-2R mutations, and segregation analysis showed that CER6 is identical to CUT1, a cDNA previously mapped to a different chromosome arm. Instead, we have determined that a new gene, CER60, with a high degree of nucleotide and amino acid similarity to CER6, resides at the original CUT1 locus. (+info)
Quantification of water transport in plants with NMR imaging.
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A new nuclear magnetic resonance imaging (NMRi) method is described to calculate the characteristics of water transport in plant stems. Here, dynamic NMRi is used as a non-invasive technique to record the distribution of displacements of protons for each pixel in the NMR image. Using the NMR-signal of the stationary water in a reference tube for calibration, the following characteristics can be calculated per pixel without advance knowledge of the flow-profile in that pixel: the amount of stationary water, the amount of flowing water, the cross-sectional area of flow, the average linear flow velocity of the flowing water, and the volume flow. The accuracy of the method is demonstrated with a stem segment of a chrysanthemum flower by comparing the volume flow, measured with NMR, with the actual volumetric uptake, measured with a balance. NMR measurements corresponded to the balance uptake measurements with a rms error of 0.11 mg s(-1) in a range of 0 to 1.8 mg s(-1). Local changes in flow characteristics of individual voxels of a sample (e.g. intact plant) can be studied as a function of time and of any conceivable changes the sample experiences on a time-scale, longer than the measurement time of a complete set of pixel-propagators (17 min). (+info)
Quantitative trait loci for floral morphology in Arabidopsis thaliana.
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A central question in biology is how genes control the expression of quantitative variation. We used statistical methods to estimate genetic variation in eight Arabidopsis thaliana floral characters (fresh flower mass, petal length, petal width, sepal length, sepal width, long stamen length, short stamen length, and pistil length) in a cosmopolitan sample of 15 ecotypes. In addition, we used genome-wide quantitative trait locus (QTL) mapping to evaluate the genetic basis of variation in these same traits in the Landsberg erecta x Columbia recombinant inbred line population. There was significant genetic variation for all traits in both the sample of naturally occurring ecotypes and in the Ler x Col recombinant inbred line population. In addition, broad-sense genetic correlations among the traits were positive and high. A composite interval mapping (CIM) analysis detected 18 significant QTL affecting at least one floral character. Eleven QTL were associated with several floral traits, supporting either pleiotropy or tight linkage as major determinants of flower morphological integration. We propose several candidate genes that may underlie these QTL on the basis of positional information and functional arguments. Genome-wide QTL mapping is a promising tool for the discovery of candidate genes controlling morphological development, the detection of novel phenotypic effects for known genes, and in generating a more complete understanding of the genetic basis of floral development. (+info)
The late flowering phenotype of fwa mutants is caused by gain-of-function epigenetic alleles of a homeodomain gene.
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The transition to flowering in Arabidopsis thaliana is delayed in fwa mutant plants. FWA was identified by loss-of-function mutations in normally flowering revertants of the fwa mutant, and it encodes a homeodomain-containing transcription factor. The DNA sequence of wild-type and fwa mutant alleles was identical in the genomic region of FWA. Furthermore, the FWA gene is ectopically expressed in fwa mutants and silenced in mature wild-type plants. This silencing is associated with extensive methylation of two direct repeats in the 5' region of the gene. The late flowering phenotype, ectopic FWA expression, and hypomethylation of the repeats were also induced in the ddm1 hypomethylated background. Mechanisms for establishment and maintenance of the epigenetic mark on FWA are discussed. (+info)