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Regulation of gene expression in plants: the role of transcript structure alternative transcript initiation and processing, polyadenylation, RNA.
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Many WRI1-regulated genes are also under the control of the master regulators. However, overexpression of WRI1 does not fully restore the seed oil content of the lec2 mutant, suggesting the requirement of other factors in regulating LEC2-mediated seed oil synthesis.
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WRI1 has been shown to bind directly to the AW box of several fatty acid synthesis genes in vitro , and mutation of the AW box abolishes WRI-mediated transcriptional activation in transient protoplast assays. Together, these data indicate that, in addition to WRI1, other key transcription factors are required for the activation of genes involved in triacylglycerol and oil body assembly. Transcript levels of a putative WRI1 gene from oil palm are substantially higher fold in the mesocarp than in its relative date palm, which does not accumulate triacylglycerol.
Levels of plastidial fatty acid gene transcripts were also elevated in oil palm, on average fold, and their temporal expression profile mirrored that of WRI1. Transcripts of genes involved in triacylglycerol assembly were similar between oil and date palm mesocarp, suggesting that factors other than WRI1 are involved in the regulation of these pathways. Interestingly, expressed sequence tags ESTs of the seed maturation master regulators were not detected in either oil or date palm mesocarp, indicating a possible unique mode of WRI1 activation in these tissues.
Searches for putative cis -element binding motifs enriched in promoters of fatty acid synthesis genes suggest the involvement of DOF, GATA, and MYB classes of transcription factors, while searches of oleosin promoters identified cognate binding sites for basic leucin zipper bZIP factors. Analysis of several DOF factors from soybean Glycine max identified two GmDof4 and GmDof11 capable of enhancing seed oil content and increasing seed weight when constitutively expressed in Arabidopsis.
Transgenic expression of GmDof4 decreased the proportion of in seed oil, while GmDof11 did not alter fatty acid composition. These genes are differentially expressed in the respective transgenic lines, suggesting that they are direct targets of the DOF proteins. Levels of seed storage proteins may be negatively regulated by GmDof4 and GmDof11, as these bind to the promoter of a gene CRA1 encoding a seed storage protein which is down-regulated in the transgenic seeds.
Mutation of Arabidopsis GL2 or antisense suppression of BnGL2 in canola lead to increases in seed oil, implicating this transcription factor as a negative regulator. Loss of MUM4 and other genes required for mucilage formation also result in increased seed oil content, suggesting that the phenomenon is related to greater carbon availability in the absence of mucilage. However, a mutant at the TTG2 locus, which encodes a WRKY class of transcription factor and is defective in mucilage formation, is not compromised in MUM4 expression and does not have elevated seed oil.
It was speculated that in the ttg2 mutant, carbon may already be committed to rhamnose and no longer readily diverted to fatty acid synthesis.
The AP2 transcription factor also has been shown to act maternally, as a negative regulator of seed size. Arabidopsis ap2 mutants produce larger seeds with concomitant increases in seed oil. The mutants display a longer maturation phase and higher concentrations of hexoses in maturing seeds than the wild-type. One possibility is that AP2 modulates seed oil synthesis indirectly through the sugar metabolism in the seed coat.
Given that several of the transcription factors discussed above have strong positive roles in promoting seed-specific programs, mechanisms have evolved to ensure that their expression is suppressed during vegetative growth. Genes contributing to this suppression have been identified as loss-of-function mutants that ectopically express the master regulators outside of the seed. These suppressors all appear to be involved in determining the chromatin conformation of target genes. They have generated interest as a possible means to metabolically engineer storage lipids in vegetative tissues such as leaves, tubers and roots to increase their energy index.
Levels of LEC1 , LEC2 and FUS3 mRNA are close to fold higher in roots of pkl mutants than wild-type plants, leading to the ectopic expression of embryogenic genes, the synthesis of storage lipids and swollen green primary root tips. CHD3 proteins are known to act as transcriptional repressors, often as part of complexes possessing histone deacetylase activity.
However, chromatin of pkl mutants shows no differences in histone acetylation but is depleted in trimethylated histone H3 lysine 27 H3K27m3 , a modification widely associated with gene silencing. In germinating wild-type seeds, PKL is physically associated with promoters enriched in H3K27m3, including LEC1 , LEC2 , and FUS3 , suggesting that it acts to directly promote this repressive chromatin mark on the genes encoding master regulators of seed maturation at the transition from seed to vegetative growth.
PKL function appears to be transient, and is no longer required two weeks following germination in Arabidopsis. It is clear, however, that PCR2 contributes to repressing the master seed regulators in vegetative plant tissues, as mutation of several of its components result in the derepression of master regulator genes and the production of seed oil in seedlings.
A clade of B3-domain transcription factors, separate but related to the AFL clade discussed above, also acts redundantly to suppress seed maturation programs in vegetative tissues. Although it displays a number of developmental abnormalities, the asil1 mutant does not produce pkl roots or ectopic embryos reminiscent of the pkl or val mutants. WRI1 and the master regulators of seed maturation are presently the best understood transcription factors that contribute in seed oil deposition.
There are certainly additional transcription factors involved, acting either directly on lipid biosynthetic genes or indirectly to influence seed size, development or the allocation of resources. Of particular relevance are the further characterization of WRI1 function and the identification of other transcription factors that act specifically on different steps of lipid biosynthesis. Rapid developments in the field of genomics, for example the emergence of next-generation sequencing technologies, should facilitate the discovery of transcription factors in crop species and their associated transcriptional cascades.
Introduction With few exceptions, oil accumulates in seeds as triacylglycerols; three fatty acid molecules esterified to a glycerol backbone. Transcription Factors Transcription factors are sequence-specific DNA binding proteins that help recruit the transcriptional machinery to gene promoters. Table 1. Summary of transcription factors that have been implicated in the control of seed oil deposition. The list is not intended to be comprehensive. Negative Vibe: Repressing Seed Oil Biosynthesis in Vegetative Tissues Given that several of the transcription factors discussed above have strong positive roles in promoting seed-specific programs, mechanisms have evolved to ensure that their expression is suppressed during vegetative growth.
Closing Comments WRI1 and the master regulators of seed maturation are presently the best understood transcription factors that contribute in seed oil deposition. Some Key References Baud, S. Baud, S. Physiological and developmental regulation of seed oil production. Berger, N. Plant Cell , 23 , DOI: Bourgis, F. Comparative transcriptome and metabolite analysis of oil palm and date palm mesocarp that differ dramatically in carbon partitioning. Braybrook, S. LECs go crazy in embryo development.ilearntennis.com/wp-content/220/qufo-sie-sucht-ihn.php
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Trends Plant Sci. Gao, M. Repression of seed maturation genes by a trihelix transcriptional repressor in Arabidopsis seedlings. Plant Cell , 21 , DOI: Liu, J. Increasing seed mass and oil content in transgenic Arabidopsis by the overexpression of a wri1 -like gene from Brassica napus. Plant Physiol. Maeo, K. Mu, J. Ohto, M. Plant Repro. Peng, F. Gene coexpression clusters and putative regulatory elements underlying seed storage reserve accumulation in Arabidopsis. Pouvreau, B. Duplicate maize Wrinkled1 transcription factors activate target genes involved in seed oil biosynthesis.
Shi, L. Arabidopsis glabra2 mutant seeds deficient in mucilage biosynthesis produce more oil. Suzuki, M. Functional symmetry of the B3 network controlling seed development. Plant Biol.
Tan, H. The repressor binds to the operator gene and prevents it from initiating the synthesis of the protein called for by the operon. The presence or absence of certain repressor molecules determines whether the operon is off or on. As mentioned, this model applies to bacteria. The genes of eukaryotes, which do not have operons, are regulated independently. The series of events associated with gene expression in higher organisms involves multiple levels of regulation and is often influenced by the presence or absence of molecules called transcription factors.
These factors influence the fundamental level of gene control, which is the rate of transcription, and may function as activators or enhancers. Specific transcription factors regulate the production of RNA from genes at certain times and in certain types of cells. Transcription factors often bind to the promoter, or regulatory region, found in the genes of higher organisms.
Following transcription, introns noncoding nucleotide sequences are excised from the primary transcript through processes known as editing and splicing. The result of these processes is a functional strand of mRNA.
For most genes this is a routine step in the production of mRNA, but in some genes there are multiple ways to splice the primary transcript, resulting in different mRNAs, which in turn result in different proteins. Some genes also are controlled at the translational and posttranslational levels. Mutations occur when the number or order of bases in a gene is disrupted. Nucleotides can be deleted, doubled, rearranged, or replaced, each alteration having a particular effect.
Mutation generally has little or no effect, but, when it does alter an organism, the change may be lethal or cause disease. A beneficial mutation will rise in frequency within a population until it becomes the norm. For more information on the influence of genetic mutations in humans and other organisms, see human genetic disease and evolution. Article Media. Info Print Print. Table Of Contents. Submit Feedback. Thank you for your feedback. Introduction Chemical structure of genes Gene transcription and translation Gene regulation Gene mutations.
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