Role of the regulatory gene pl in the photocontrol of maize anthocyanin pigmentation

Sequence variations associated with novel purple-pericarp super-sweetcorn compared to its purple-pericarp maize and white super-sweetcorn parents

Recently, a novel purple-pericarp super-sweetcorn line, 'Tim1' (A1A1.sh2sh2) was derived from the purple-pericarp maize 'Costa Rica' (A1Sh2.A1Sh2) and white shrunken2 (sh2) super-sweetcorn 'Tims-white' (a1sh2.a1sh2), however, information regarding anthocyanin biosynthesis genes controlling purple colour and sweetness gene is lacking. Specific sequence differences in the CDS (coding DNA sequence) and promoter regions of the anthocyanin biosynthesis structural genes, anthocyanin1 (A1), purple aleurone1 (Pr1) and regulatory genes, purple plant1 (Pl1), plant colour1 (B1), coloured1 (R1), and the sweetcorn structural gene, shrunken2 (sh2) were investigated using the publicly available annotated yellow starchy maize, B73 (NAM5.0) as a reference genome. In the CDS region, the A1, Pl1 and R1 gene sequence differences of 'Tim1' and 'Costa Rica' were similar, as they control purple-pericarp pigmentation. However, the B1 gene showed similarity between the 'Tim1' and 'Tims-white' lines, which may indicate that it does not have a role in controlling pericarp colour, unlike the report of a previous study. In the case of the Pr1 gene, in contrast to 'Costa Rica', 6- and 8-bp dinucleotide (TA) repeats were observed in the promoter region of the 'Tims-white' and 'Tim1' lines, respectively, indicating the defective functionality (redder colour in 'Tim1' rather than purple in 'Costa Rica') of the recessive pr1 allele. In sweetcorn, the structural gene (sh2), sequence showed similarity between purple-sweet 'Tim1' and its white-sweet parent 'Tims-white', as both display a shrunken phenotype in their mature kernels. These findings revealed that the developed purple-sweet line is different to the reference yellow-nonsweet line in both the anthocyanin biosynthesis and sweetcorn genes.

DhMYB22 and DhMYB60 regulate pigment intensity and floral organ shape in Dendrobium hybrid

BACKGROUND

Flower pigment and shape are determined by the coordinated expression of a set of structural genes during flower development. R2R3-MYB transcription factors are known regulators of structural gene expression. The current study focused on two members of this large family of transcription factors that were predicted to have roles in pigment biosynthesis and organ shape development in orchids.

METHODS

Phylogenetic analysis was used to identify candidate Dendrobium catenatum R2R3-MYB (DcaMYB) sequences associated with pigment and cell shape development. Gene silencing of candidate DhMYBs in Dendrobium hybrid by direct application of dsRNA to developing flowers was followed by observation of gene expression level and flower phenotypes. Silencing of the structural gene chalcone synthase was used as a comparative control.

KEY RESULTS

Ten candidate flower-associated DcaMYBs were identified. Flowers treated with dsRNA of DhMYB22 and DhMYB60 sequences were less pigmented and had relatively low expression of anthocyanin biosynthetic genes (F3'H and DFR), lower total anthocyanin concentration and markedly lower levels of cyanidin-3-glucoside and cyanidin-3-rutinoside. Petals of DhMYB22-treated flowers and sepals of DhMYB60-treated flowers showed the greatest colour difference relative to the same organs in untreated flowers. DhMYB22-treated flowers had relatively narrow and constricted lips, while DhMYB60-treated flowers had narrow and constricted sepals. No significant difference in shape was observed for DhCHS-treated or untreated flowers.

CONCLUSIONS

Our results demonstrate that DhMYB22 and DhMYB60 regulate pigment intensity and floral organ shape in Dendrobium. This is a first report of MYB regulation of floral organ shape in orchids.

Maize Flavonoid Biosynthesis, Regulation, and Human Health Relevance: A Review

Maize is one of the most important crops for human and animal consumption and contains a chemical arsenal essential for survival: flavonoids. Moreover, flavonoids are well known for their beneficial effects on human health. In this review, we decided to organize the information about maize flavonoids into three sections. In the first section, we include updated information about the enzymatic pathway of maize flavonoids. We describe a total of twenty-one genes for the flavonoid pathway of maize. The first three genes participate in the general phenylpropanoid pathway. Four genes are common biosynthetic early genes for flavonoids, and fourteen are specific genes for the flavonoid subgroups, the anthocyanins, and flavone C-glycosides. The second section explains the tissue accumulation and regulation of flavonoids by environmental factors affecting the expression of the MYB-bHLH-WD40 (MBW) transcriptional complex. The study of transcription factors of the MBW complex is fundamental for understanding how the flavonoid profiles generate a palette of colors in the plant tissues. Finally, we also include an update of the biological activities of C3G, the major maize anthocyanin, including anticancer, antidiabetic, and antioxidant effects, among others. This review intends to disclose and integrate the existing knowledge regarding maize flavonoid pigmentation and its relevance in the human health sector.

Locus-specific paramutation in Zea mays is maintained by a PICKLE-like chromodomain helicase DNA-binding 3 protein controlling development and male gametophyte function

Paramutations represent directed and meiotically-heritable changes in gene regulation leading to apparent violations of Mendelian inheritance. Although the mechanism and evolutionary importance of paramutation behaviors remain largely unknown, genetic screens in maize (Zea mays) identify five components affecting 24 nucleotide RNA biogenesis as required to maintain repression of a paramutant purple plant1 (pl1) allele. Currently, the RNA polymerase IV largest subunit represents the only component also specifying proper development. Here we identify a chromodomain helicase DNA-binding 3 (CHD3) protein orthologous to Arabidopsis (Arabidopsis thaliana) PICKLE as another component maintaining both pl1 paramutation and normal somatic development but without affecting overall small RNA biogenesis. In addition, genetic tests show this protein contributes to proper male gametophyte function. The similar mutant phenotypes documented in Arabidopsis and maize implicate some evolutionarily-conserved gene regulation while developmental defects associated with the two paramutation mutants are largely distinct. Our results show that a CHD3 protein responsible for normal plant ontogeny and sperm transmission also helps maintain meiotically-heritable epigenetic regulatory variation for specific alleles. This finding implicates an intersection of RNA polymerase IV function and nucleosome positioning in the paramutation process.

Genes are switched “on” and “off” during normal development by regulating DNA accessibility within the chromosomes. How certain gene variants permanently maintain “off” states from one generation to the next remains unclear, but studies in multiple eukaryotes implicate roles for specific types of small RNAs, some of which define cytosine methylation patterns. In corn, these RNAs come from at least two RNA polymerase II-derived complexes sharing a common catalytic subunit (RPD1). Although RPD1 both controls the normal developmental switching of many genes and permanently maintains some of these “off” states across generations, how RPD1 function defines heritable DNA accessibility is unknown. We discovered that a protein (CHD3a) belonging to a group known to alter nucleosome positioning is also required to help maintain a heritable “off” state for one particular corn gene variant controlling both plant and flower color. We also found CHD3a necessary for normal plant development and sperm transmission consistent with the idea that proper nucleosome positioning defines evolutionarily-important gene expression patterns. Because both CHD3a and RPD1 maintain the heritable “off” state of a specific gene variant, their functions appear to be mechanistically linked.

Alternatively Spliced BnaPAP2.A7 Isoforms Play Opposing Roles in Anthocyanin Biosynthesis of Brassica napus L

Brassica napus L. (rapeseed, oilseed rape, and canola) and varieties of its two diploid parents, B. oleracea and B. rapa, display a large amount of variation in anthocyanin pigmentation of the leaf, stem, and fruit. Here, we demonstrate that BnaPAP2.A7, an ortholog of the B. oleracea anthocyanin activator BoMYB2 that confers purple traits, positively regulates anthocyanin biosynthesis in leaves of B. napus. Sequencing of BnaPAP2.A7 and transgenic analysis suggests that activation of this gene in purple rapeseed may result from a single nucleotide and/or 2bp insertion in its promoter region. BnaPAP2.A7 gives rise to three splice variants, designated BnaPAP2.A7-744, BnaPAP2.A7-910, and BnaPAP2.A7-395 according to the length of the transcripts. While BnaPAP2.A7-744 encodes a full-length R2R3-MYB, both BnaPAP2.A7-910 and BnaPAP2.A7-395 encode truncated proteins that lack both a partial R3 repeat and the complete C terminal domain, and so in vitro are unable to interact with the Arabidopsis bHLH protein AtTT8. Although expression of either BnaPAP2.A7-910 or BnaPAP2.A7-395 in green rapeseed does not result in purple leaves, both genes do modify genome-wide gene expression, with a strong repression of anthocyanin-related genes. We have demonstrated that BnaPAP.A7 regulates anthocyanin accumulation in leaves of B. napus and propose a potential mechanism for modulation of anthocyanin biosynthesis by alternative splicing.

Methylation of MdMYB1 locus mediated by RdDM pathway regulates anthocyanin biosynthesis in apple

Methylation at the MdMYB1 promoter in apple sports has been reported as a regulator of the anthocyanin pathway, but little is known about how the locus is recognized by the methylation machinery to regulate anthocyanin accumulation. In this study, we analysed three differently coloured 'Fuji' apples and found that differences in the transcript levels of MdMYB1, which encodes a key regulator of anthocyanin biosynthesis, control the anthocyanin content (and therefore colour) in fruit skin. The CHH methylation levels in the MR3 region (-1246 to -780) of the MdMYB1 promoter were found to be negatively correlated with MdMYB1 expression. Thus, they were ideal materials to study DNA methylation in apple sports. The protein of RNA-directed DNA methylation (RdDM) pathway responsible for CHH methylation, MdAGO4, was found to interact with the MdMYB1 promoter. MdAGO4s can interact with MdRDM1 and MdDRM2s to form an effector complex, fulfilling CHH methylation. When MdAGO4s and MdDRM2s were overexpressed in apple calli and Arabidopsis mutants, those proteins increase the CHH methylation of AGO4-binding sites. In electrophoretic mobility shift assays, MdAGO4s were found to specifically bind to sequence containing ATATCAGA. Knockdown of MdNRPE1 did not affect the binding of MdAGO4s to the c3 region of the MdMYB1 promoter in 35S::AGO4 calli. Taken together, our data show that the MdMYB1 locus is methylated through binding of MdAGO4s to the MdMYB1 promoter to regulate anthocyanin biosynthesis by the RdDM pathway.

A genomic region harboring the Pl1 allele from the Peruvian cultivar JC072A confers purple cob on Japanese flint corn (Zea mays L.)

Purple corn is a maize variety (Zea mays L.) with high anthocyanin content. When purple corn is used as forage, its anthocyanins may mitigate oxidative stresses causing lower milk production in dairy cows. In this study, we analyzed quantitative trait loci (QTLs) for anthocyanin pigmentation of maize organs in an F₂ population derived from a cross between the Peruvian cultivar 'JC072A' (purple) and the inbred line 'Ki68' (yellowish) belonged to Japanese flint. We detected 17 significant QTLs on chromosomes 1-3, 6, and 10. Because the cob accounts for most of the fresh weight of the plant ear, we focused on a significant QTL for purple cob on chromosome 6. This QTL also conferred pigmentation of anther, spikelet, leaf sheath, culm, and bract leaf, and was confirmed by using two F₃ populations. The gene Pl1 (purple plant 1) is the most likely candidate gene in this QTL region because the amino acid sequence encoded by Pl1-JC072A is similar to that of an Andean allele, Pl-bol3, which is responsible for anthocyanin production. The markers designed for the Pl1 alleles will be useful for the breeding of F₁ lines with anthocyanin pigmentation in cobs.

Failure to launch: the self-regulating Md-MYB10 R6 gene from apple is active in flowers but not leaves of Petunia

The Md - MYB10 R6 gene from apple is capable of self-regulating in heterologous host species and enhancing anthocyanin pigmentation, but the activity of MYB10 is dependent on endogenous protein partners. Coloured foliage due to anthocyanin pigments (bronze/red/black) is an attractive trait that is often lacking in many bedding, ornamental and horticultural plants. Apples (Malus × domestica) containing an allelic variant of the anthocyanin regulator, Md-MYB10 R6 , are highly pigmented throughout the plant, due to autoregulation by MYB10 upon its own promoter. We investigated whether Md-MYB10 R6 from apple is capable of functioning within the heterologous host Petunia hybrida to generate plants with novel pigmentation patterns. The Md-MYB10 R6 transgene (MYB10-R6 pro :MYB10:MYB10 term ) activated anthocyanin synthesis when transiently expressed in Antirrhinum rosea (dorsea) petals and petunia leaf discs. Stable transgenic petunias containing Md-MYB10 R6 lacked foliar pigmentation but had coloured flowers, complementing the an2 phenotype of 'Mitchell' petunia. The absence of foliar pigmentation was due to the failure of the Md-MYB10 R6 gene to self-activate in vegetative tissues, suggesting that additional protein partners are required for Md-MYB10 to activate target genes in this heterologous system. In petunia flowers, where endogenous components including MYB-bHLH-WDR (MBW) proteins were present, expression of the Md-MYB10 R6 promoter was initiated, allowing auto-regulation to occur and activating anthocyanin production. Md-MYB10 is capable of operating within the petunia MBW gene regulation network that controls the expression of the anthocyanin biosynthesis genes, AN1 (bHLH) and MYBx (R3-MYB repressor) in petals.

MYB Transcription Factors as Regulators of Phenylpropanoid Metabolism in Plants

Phenylpropanoid-derived compounds represent a diverse family of secondary metabolites that originate from phenylalanine. These compounds have roles in plant growth and development, and in defense against biotic and abiotic stress. Many of these compounds are also beneficial to human health and welfare. V-myb myeloblastosis viral oncogene homolog (MYB) proteins belong to a large family of transcription factors and are key regulators of the synthesis of phenylpropanoid-derived compounds. This review summarizes the current understanding of MYB proteins and their roles in the regulation of phenylpropanoid metabolism in plants.

Maize RNA polymerase IV defines trans-generational epigenetic variation

The maize (Zea mays) RNA Polymerase IV (Pol IV) largest subunit, RNA Polymerase D1 (RPD1 or NRPD1), is required for facilitating paramutations, restricting expression patterns of genes required for normal development, and generating small interfering RNA (siRNAs). Despite this expanded role for maize Pol IV relative to Arabidopsis thaliana, neither the general characteristics of Pol IV-regulated haplotypes, nor their prevalence, are known. Here, we show that specific haplotypes of the purple plant1 locus, encoding an anthocyanin pigment regulator, acquire and retain an expanded expression domain following transmission from siRNA biogenesis mutants. This conditioned expression pattern is progressively enhanced over generations in Pol IV mutants and then remains heritable after restoration of Pol IV function. This unusual genetic behavior is associated with promoter-proximal transposon fragments but is independent of sequences required for paramutation. These results indicate that trans-generational Pol IV action defines the expression patterns of haplotypes using co-opted transposon-derived sequences as regulatory elements. Our results provide a molecular framework for the concept that induced changes to the heterochromatic component of the genome are coincident with heritable changes in gene regulation. Alterations of this Pol IV-based regulatory system can generate potentially desirable and adaptive traits for selection to act upon.

Paramutagenicity of a p1 epiallele in maize

Complex silencing mechanisms in plants and other kingdoms target transposons, repeat sequences, invasive viral nucleic acids and transgenes, but also endogenous genes and genes involved in paramutation. Paramutation occurs in a heterozygote when a transcriptionally active allele heritably adopts the epigenetic state of a transcriptionally and/or post-transcriptionally repressed allele. P1-rr and its silenced epiallele P1-pr, which encode a Myb-like transcription factor mediating pigmentation in floral organs of Zea mays, differ in their cytosine methylation pattern and chromatin structure at a complex enhancer site. Here, we tested whether P1-pr is able to heritably silence its transcriptionally active P1-rr allele in a heterozygote and whether DNA methylation is associated with the establishment and maintenance of P1-rr silencing. We found that P1-pr participates in paramutation as the repressing allele and P1-rr as the sensitive allele. Silencing of P1-rr is highly variable compared to the inducing P1-pr resulting in a wide range of gene expression. Whereas cytosine methylation at P1-rr is negatively correlated with transcription and pigment levels after segregation of P1-pr, methylation lags behind the establishment of the repressed p1 gene expression. We propose a model in which P1-pr paramutation is triggered by changing epigenetic states of transposons immediately adjacent to a P1-rr enhancer sequence. Considering the vast amount of transposable elements in the maize genome close to regulatory elements of genes, numerous loci could undergo paramutation-induced allele silencing, which could also have a significant impact on breeding agronomically important traits.

RNA polymerase IV functions in paramutation in Zea mays

Plants have distinct RNA polymerase complexes (Pol IV and Pol V) with largely unknown roles in maintaining small RNA-associated gene silencing. Curiously, the eudicot Arabidopsis thaliana is not affected when either function is lost. By use of mutation selection and positional cloning, we showed that the largest subunit of the presumed maize Pol IV is involved in paramutation, an inherited epigenetic change facilitated by an interaction between two alleles, as well as normal maize development. Bioinformatics analyses and nuclear run-on transcription assays indicate that Pol IV does not engage in the efficient RNA synthesis typical of the three major eukaryotic DNA-dependent RNA polymerases. These results indicate that Pol IV employs abnormal RNA polymerase activities to achieve genome-wide silencing and that its absence affects both maize development and heritable epigenetic changes.

Expression analysis of anthocyanin regulatory genes in response to different light qualities in Arabidopsis thaliana

In this work we analysed, at the transcript level, the response of Arabidopsis anthocyanin regulatory genes of the MYB (PAP1 and PAP2), bHLH (TT8, EGL3 and GL3) and WD40 (TTG1) families to white light in seedlings and to different light qualities in rosette leaves. Our experiments showed strong light induction of the MYB genes PAP1 and PAP2. In particular, the kinetics of PAP1 expression preceded those of PAP2 and all of the structural genes (CHS, DFR, F3H, LDOX), consistent with the hypothesis that it has a key role in light induction of anthocyanin biosynthesis. All bHLH genes analysed showed light induction, and in seedlings their expression preceded that of the late structural genes, suggesting their possible role in light regulation of these structural genes. TTG1 expression is essentially constitutive in both systems. Experiments with transgenic lines over-expressing the MYB factors show that PAP1, but not PAP2, strongly stimulates expression of the anthocyanin structural gene encoding dihydroflavonol reductase, but neither factor affected expression of the early flavonoid biosynthesis gene encoding chalcone synthase. Consistent with these findings, PAP1, but not PAP2, stimulated light induction of anthocyanin biosynthesis in seedlings. We conclude that specific members of the MYB and bHLH families play important roles in regulating anthocyanin biosynthesis in response to different light qualities in Arabidopsis.

A novel Snf2 protein maintains trans-generational regulatory states established by paramutation in maize

Paramutations represent heritable epigenetic alterations that cause departures from Mendelian inheritance. While the mechanism responsible is largely unknown, recent results in both mouse and maize suggest paramutations are correlated with RNA molecules capable of affecting changes in gene expression patterns. In maize, multiple required to maintain repression (rmr) loci stabilize these paramutant states. Here we show rmr1 encodes a novel Snf2 protein that affects both small RNA accumulation and cytosine methylation of a proximal transposon fragment at the Pl1-Rhoades allele. However, these cytosine methylation differences do not define the various epigenetic states associated with paramutations. Pedigree analyses also show RMR1 does not mediate the allelic interactions that typically establish paramutations. Strikingly, our mutant analyses show that Pl1-Rhoades RNA transcript levels are altered independently of transcription rates, implicating a post-transcriptional level of RMR1 action. These results suggest the RNA component of maize paramutation maintains small heterochromatic-like domains that can affect, via the activity of a Snf2 protein, the stability of nascent transcripts from adjacent genes by way of a cotranscriptional repression process. These findings highlight a mechanism by which alleles of endogenous loci can acquire novel expression patterns that are meiotically transmissible.

Multiple trans-sensing interactions affect meiotically heritable epigenetic states at the maize pl1 locus

Interactions between specific maize purple plant1 (pl1) alleles result in heritable changes of gene regulation that are manifested as differences in anthocyanin pigmentation. Transcriptionally repressed states of Pl1-Rhoades alleles (termed Pl') are remarkably stable and invariably facilitate heritable changes of highly expressed states (termed Pl-Rh) in Pl'/Pl-Rh plants. However, Pl' can revert to Pl-Rh when hemizygous, when heterozygous with pl1 alleles other than Pl1-Rhoades, or in the absence of trans-acting factors required to maintain repressed states. Cis-linked features of Pl1-Rhoades responsible for these trans-sensing behaviors remain unknown. Here, genetic tests of a pl1 allelic series identify two potentially separate cis-linked features: one facilitating repression of Pl-Rh and another stabilizing Pl' in trans. Neither function is affected in ethyl-methanesulfonate-induced Pl1-Rhoades derivatives that produce truncated PL1 peptides, indicating that PL1 is unlikely to mediate trans interactions. Both functions, however, are impaired in a spontaneous Pl1-Rhoades derivative that fails to produce detectable pl1 RNA. Pl'-like states can also repress expression of a pl1-W22 allele, but this repression is not meiotically heritable. As the Pl' state is not associated with unique small RNA species representing the pl1-coding region, the available data suggest that interactions between elements required for transcription underlie Pl1-Rhoades epigenetic behaviors.

A novel R2R3 MYB transcription factor NtMYBJS1 is a methyl jasmonate-dependent regulator of phenylpropanoid-conjugate biosynthesis in tobacco

Target metabolic and large-scale transcriptomic analyses of tobacco (Nicotiana tabacum L.) Bright Yellow-2 (BY-2) cells were employed to identify novel gene(s) involved in methyl jasmonate (MJ)-dependent function in plants. At the metabolic level, we describe the specific accumulation of several phenylpropanoid-polyamine conjugates in MJ-treated BY-2 cells. Furthermore, global gene expression analysis of MJ-treated cells using a 16K cDNA microarray containing expressed sequence tags (ESTs) from BY-2 cells revealed 828 genes that were upregulated by MJ treatment within 48 h. Using time-course expression data we identified a novel MJ-inducible R2R3 MYB-type transcription factor (NtMYBJS1) that was co-expressed in a close temporal pattern with the core phenylpropanoid genes phenylalanine ammonia-lyase (PAL) and 4-coumarate:CoA ligase (4CL). Overexpression of NtMYBJS1 in tobacco BY-2 cells caused accumulation of specific phenylpropanoid conjugates in the cells. Subsequent microarray analysis of NtMYBJS1 transgenic lines revealed that a limited number of genes, including PAL and 4CL, were specifically induced in the presence of the NtMYBJS1 transgene. These results, together with results of both antisense expression analysis and of gel mobility shift assays, strongly indicate that the NtMYBJS1 protein functions in tobacco MJ signal transduction, inducing phenylpropanoid biosynthetic genes and the accumulation of phenylpropanoid-polyamine conjugates during stress.

The dominant inhibitory chalcone synthase allele C2-Idf (inhibitor diffuse) from Zea mays (L.) acts via an endogenous RNA silencing mechanism

The flavonoid pigment pathway in plants has been used as a model system for studying gene regulatory mechanisms. C2-Idf is a stable dominant mutation of the chalcone synthase gene, c2, which encodes the first dedicated enzyme in this biosynthetic pathway of maize. Homozygous C2-Idf plants show no pigmentation. This allele also inhibits expression of functional C2 alleles in heterozygotes, producing a less pigmented condition instead of the normal deeply pigmented phenotype. To explore the nature of this effect, the C2-Idf allele was cloned. The gene structure of the C2-Idf haplotype differs substantially from that of the normal c2 gene in that three copies are present. Two of these are located in close proximity to each other in a head-to-head orientation and the third is closely linked. Previous experiments showed that the lower level of pigmentation in heterozygotes is correlated with reduced enzyme activity and low steady-state mRNA levels. We found that c2 transcription occurs in nuclei of C2-Idf/C2 heterozygotes, but mRNA does not accumulate, suggesting that the inhibition is mediated by RNA silencing. Infection of C2-Idf/C2 heterozygotes with viruses that carry suppressors of RNA silencing relieved the phenotypic inhibition, restoring pigment production and mRNA levels. Finally, we detected small interfering RNAs (siRNAs) in plants carrying C2-Idf, but not in plants homozygous for the wild-type C2 allele. Together, our results indicate that the inhibitory effect of C2-Idf occurs through RNA silencing.

pl-bol3, a complex allele of the anthocyanin regulatory pl1 locus that arose in a naturally occurring maize population

The pl1 gene encodes a MYB-related transcriptional activator committed to the regulation of anthocyanin biosynthesis in maize. Here, we report the genetic and molecular characterisation of pl-bol3, an Andean allele displaying features that make it different from all the known pl1 alleles. pl-bol3 has partial, light-independent expression, and it is active mainly in the juvenile phase of growth. It has a complex molecular structure, containing multiple pl1 gene copies, thus being the first complex locus discovered in the c1/pl1 family. Although the composite genes of the complex locus encode proteins identical to other functional PL1 proteins, the putative promoters of the pl-bol3 gene are different from the promoters of Pl-Rhoades (Pl-Rh) and pl1 sun-red alleles. The intensity and the tissue specificity of anthocyanin production directed by pl-bol3 differ significantly from that of Pl-Rh and the original pl-W22, and are specified by the interaction of pl-bol3 with the different r1/b1 gene family members and the competence of pl-bol3 to different pigment tissues. This allele represents a natural example of gene duplication and diversification of expression, giving rise to a significant change in phenotype and, in this way, is analogous to the complex r1 locus in maize. Analysis of the pl-bol3 allele contributes to understanding the generation of diversity associated with multiple-copy genes and the molecular basis of allele-specific gene expression.

Interpreting spatial variation in ozone symptoms shown by cutleaf cone flower, Rudbeckia laciniata L

Visible injury caused by ozone is recorded every year in native plant species growing in Great Smoky Mountains National Park (USA). One of the most sensitive species, cutleaf coneflower (Rudbeckia laciniata L.), shows great variation in symptoms between and within populations but the causes of this variation and its ecological significance are currently unknown. This paper presents data relating to genetic variation, ozone concentrations, stomatal conductance and light (PAR) within populations. The data show that populations differ in genetic diversity, one consisting of only three genets while another was very diverse. In the former population, symptoms varied greatly within a single genet, pointing to a large micro-environmental influence. Measurements of ozone, stomatal conductance and PAR within plant canopies suggest that variation in symptom expression is unlikely to be due to differences in ozone flux and more likely to be due to variation in light. The variation in visible symptoms raises the question of what bioindicators actually indicate, and it suggests that symptoms should be interpreted with great caution until the underlying causes of that variation are fully understood.

A maize r1 gene is regulated post-transcriptionally by differential splicing of its leader

Anthocyanin biosynthesis in Zea mays is controlled by regulatory genes of the r1/b1 family that encode bHLH transcription factors. Analysis of the 381 nucleotide leader sequence of a member of this family, Sn, discloses the presence of five ATG triplets upstream of the coding region and three upstream open reading frames (uORFs) of 38, 15 and 13 amino acids respectively. RT-PCR studies revealed that a splicing event occurs in the leader region in the different tissues tested. Splicing deletes 146 nucleotides which include uORF2 and uORF3. By trans-activation experiments in maize protoplasts we find that the spliced leader, compared to the non-spliced one, reduces the number of pigmented protoplasts by four-fold. We suggest a multilevel regulation of the Sn transcription factor acting not only at the transcriptional but also at the post-transcriptional level.

Members of the c1/pl1 regulatory gene family mediate the response of maize aleurone and mesocotyl to different light qualities and cytokinins

We investigated the role of transcription factors (R, SN, C1, and PL) in the regulation of anthocyanin biosynthesis by different light qualities (white, red, blue, and ultraviolet) and by cytokinin in maize (Zea mays). We analyzed anthocyanin accumulation, structural gene expression, and regulatory gene expression in the seed aleurone and the seedling mesocotyl. In the mesocotyl, white, blue, and ultraviolet-B light strongly induced anthocyanin accumulation and expression of two key structural genes. In contrast, red light had little effect. Cytokinin enhanced the response to light but was not sufficient to induce anthocyanin accumulation in darkness. Plants with the pl-bol3 allele showed high levels of anthocyanin accumulation in response to light, whereas those with the pl-W22 allele did not, demonstrating the importance of pl1 in the light response. The expression of the pl-bol3 gene, encoding an MYB-related transcription factor, was induced by light and enhanced by cytokinin in a very similar manner to the structural genes and anthocyanin accumulation. Expression of the bHLH (basic helix-loop-helix) Sn1-bol3 gene was stimulated by several light qualities, but not enhanced by cytokinin, and was less well correlated with the induction of anthocyanin biosynthesis. In the aleurone, white, red, and blue light were effective in stimulating anthocyanin accumulation and expression of the MYB-related gene C1. The bHLH R gene was constitutively expressed. We conclude that specific members of the MYB-related c1/pl1 gene family play important roles in the regulation of anthocyanin synthesis in maize in response to different light qualities and cytokinin.

MYB61 is required for mucilage deposition and extrusion in the Arabidopsis seed coat

We have undertaken a systematic reverse genetic approach to understand R2R3-MYB gene function in Arabidopsis. Here, we report the functional characterization of MYB61 based on the phenotype of three independent insertion alleles. Wide-ranging phenotype screens indicated that MYB61 mutants were deficient in seed mucilage extrusion upon imbibition. This phenotype was expressed in the sporophytic tissues of the seed. Deposition and extrusion of the principal component of the mucilage, a relatively unbranched rhamnogalacturonan, were reduced in the MYB61 mutant seed coats. Additional defects in the maturation of the testa epidermal cells suggested a potential deficiency in extracellular secretion in myb61 lines. Consistent with a proposed role in testa development, reverse transcription-polymerase chain reaction analysis showed the highest MYB61 expression in siliques, which was localized to the seed coat by a beta-glucuronidase (GUS) reporter gene fusion. Lower levels of GUS expression were detected in developing vascular tissue. Parallel analysis of the ttg1-1 mutant phenotype indicated that this mutant showed more severe developmental defects than myb61 and suggested that MYB61 may function in a genetic pathway distinct from that of TTG1. The transient nature of seed epidermal characteristics in the ttg1-1 mutant suggested that TTG1 was required for maintenance rather than initiation of testa epidermal differentiation. Germination and seedling establishment were compromised in the myb61 and ttg1-1 mutants under conditions of reduced water potential, suggesting a function for Arabidopsis seed mucilage during germination in dry conditions.

MYB61 is required for mucilage deposition and extrusion in the Arabidopsis seed coat

We have undertaken a systematic reverse genetic approach to understand R2R3-MYB gene function in Arabidopsis. Here, we report the functional characterization of MYB61 based on the phenotype of three independent insertion alleles. Wide-ranging phenotype screens indicated that MYB61 mutants were deficient in seed mucilage extrusion upon imbibition. This phenotype was expressed in the sporophytic tissues of the seed. Deposition and extrusion of the principal component of the mucilage, a relatively unbranched rhamnogalacturonan, were reduced in the MYB61 mutant seed coats. Additional defects in the maturation of the testa epidermal cells suggested a potential deficiency in extracellular secretion in myb61 lines. Consistent with a proposed role in testa development, reverse transcription-polymerase chain reaction analysis showed the highest MYB61 expression in siliques, which was localized to the seed coat by a beta-glucuronidase (GUS) reporter gene fusion. Lower levels of GUS expression were detected in developing vascular tissue. Parallel analysis of the ttg1-1 mutant phenotype indicated that this mutant showed more severe developmental defects than myb61 and suggested that MYB61 may function in a genetic pathway distinct from that of TTG1. The transient nature of seed epidermal characteristics in the ttg1-1 mutant suggested that TTG1 was required for maintenance rather than initiation of testa epidermal differentiation. Germination and seedling establishment were compromised in the myb61 and ttg1-1 mutants under conditions of reduced water potential, suggesting a function for Arabidopsis seed mucilage during germination in dry conditions.

MYB61 is required for mucilage deposition and extrusion in the Arabidopsis seed coat

We have undertaken a systematic reverse genetic approach to understand R2R3-MYB gene function in Arabidopsis. Here, we report the functional characterization of MYB61 based on the phenotype of three independent insertion alleles. Wide-ranging phenotype screens indicated that MYB61 mutants were deficient in seed mucilage extrusion upon imbibition. This phenotype was expressed in the sporophytic tissues of the seed. Deposition and extrusion of the principal component of the mucilage, a relatively unbranched rhamnogalacturonan, were reduced in the MYB61 mutant seed coats. Additional defects in the maturation of the testa epidermal cells suggested a potential deficiency in extracellular secretion in myb61 lines. Consistent with a proposed role in testa development, reverse transcription-polymerase chain reaction analysis showed the highest MYB61 expression in siliques, which was localized to the seed coat by a beta-glucuronidase (GUS) reporter gene fusion. Lower levels of GUS expression were detected in developing vascular tissue. Parallel analysis of the ttg1-1 mutant phenotype indicated that this mutant showed more severe developmental defects than myb61 and suggested that MYB61 may function in a genetic pathway distinct from that of TTG1. The transient nature of seed epidermal characteristics in the ttg1-1 mutant suggested that TTG1 was required for maintenance rather than initiation of testa epidermal differentiation. Germination and seedling establishment were compromised in the myb61 and ttg1-1 mutants under conditions of reduced water potential, suggesting a function for Arabidopsis seed mucilage during germination in dry conditions.

The maize CRINKLY4 receptor kinase controls a cell-autonomous differentiation response

The maize (Zea mays) CRINKLY4 (cr4) gene encodes a receptor-like kinase that controls a variety of cell differentiation responses, particularly in the leaf epidermis and in the aleurone of the endosperm. In situ hybridization indicated that the cr4 transcript is present throughout the shoot apical meristem and young leaf primordia. A genetic mosaic analysis was conducted to test whether CR4 signal transduction directly regulated the cellular processes associated with differentiation or whether differentiation was controlled through the production of a secondary signal. Genetic mosaics were created using gamma-rays to induce chromosome breakage in a cr4/Cr4+ heterozygote. The mutant cr4 allele was marked with the albino mutation, Oy-700. Breakage and loss of the chromosome arm carrying the wild-type alleles created a sector of albino, cr4 mutant tissue in an otherwise normal leaf. Analysis of such sectors indicated that cr4 functions cell autonomously to regulate cell morphogenesis, implying that CR4 signal transduction regulates cell differentiation through strictly intracellular functions and not the production of secondary intercellular signals. However, several sectors altered cell patterning in wild-type tissue adjacent to the sectors, suggesting that cr4 mutant cells are defective in the production of other lateral signals.

Molecular analysis of the Doppia transposable element of maize

Doppia (Dop) transposable elements were first identified from element termini found in the upstream portions of certain alleles of the pl1 and r1 loci of maize. At the r1 locus, these Dop end sequences are present in a region called sigma, which functions as the promoter for the S genes of the R-r haplotype, and which is required for efficient epigenetic modification of the S genes during paramutation. In order to better understand the significance of the Dop element sequences at R-r, and to investigate the Dop-encoded products that might regulate r1 genes in this haplotype, we have cloned a more complete Dop element, Dop4. The Dop4 element can encode two proteins that have strong sequence similarity to the TnpA and TnpD proteins of the well characterized maize transposable element En/Spm. The DOPA protein, which is similar to TnpA of En/Spm, is shown to bind to short, subterminal repeat motifs located in the Dop element ends. Like TnpA, DOPA promotes intermolecular associations between DNA molecules. In contrast to the activity of TnpA, which is a transcriptional repressor of En/Spm, DOPA activates expression of reporter genes driven by either the Dop promoter or sigma in transient expression assays.

Activation tagging identifies a conserved MYB regulator of phenylpropanoid biosynthesis

Plants produce a wide array of natural products, many of which are likely to be useful bioactive structures. Unfortunately, these complex natural products usually occur at very low abundance and with restricted tissue distribution, thereby hindering their evaluation. Here, we report a novel approach for enhancing the accumulation of natural products based on activation tagging by Agrobacterium-mediated transformation with a T-DNA that carries cauliflower mosaic virus 35S enhancer sequences at its right border. Among approximately 5000 Arabidopsis activation-tagged lines, we found a plant that exhibited intense purple pigmentation in many vegetative organs throughout development. This upregulation of pigmentation reflected a dominant mutation that resulted in massive activation of phenylpropanoid biosynthetic genes and enhanced accumulation of lignin, hydroxycinnamic acid esters, and flavonoids, including various anthocyanins that were responsible for the purple color. These phenotypes, caused by insertion of the viral enhancer sequences adjacent to an MYB transcription factor gene, indicate that activation tagging can overcome the stringent genetic controls regulating the accumulation of specific natural products during plant development. Our findings suggest a functional genomics approach to the biotechnological evaluation of phytochemical biodiversity through the generation of massively enriched tissue sources for drug screening and for isolating underlying regulatory and biosynthetic genes.

Activation tagging identifies a conserved MYB regulator of phenylpropanoid biosynthesis

Plants produce a wide array of natural products, many of which are likely to be useful bioactive structures. Unfortunately, these complex natural products usually occur at very low abundance and with restricted tissue distribution, thereby hindering their evaluation. Here, we report a novel approach for enhancing the accumulation of natural products based on activation tagging by Agrobacterium-mediated transformation with a T-DNA that carries cauliflower mosaic virus 35S enhancer sequences at its right border. Among approximately 5000 Arabidopsis activation-tagged lines, we found a plant that exhibited intense purple pigmentation in many vegetative organs throughout development. This upregulation of pigmentation reflected a dominant mutation that resulted in massive activation of phenylpropanoid biosynthetic genes and enhanced accumulation of lignin, hydroxycinnamic acid esters, and flavonoids, including various anthocyanins that were responsible for the purple color. These phenotypes, caused by insertion of the viral enhancer sequences adjacent to an MYB transcription factor gene, indicate that activation tagging can overcome the stringent genetic controls regulating the accumulation of specific natural products during plant development. Our findings suggest a functional genomics approach to the biotechnological evaluation of phytochemical biodiversity through the generation of massively enriched tissue sources for drug screening and for isolating underlying regulatory and biosynthetic genes.

The developmental expression of the maize regulatory gene Hopi determines germination-dependent anthocyanin accumulation

The Hopi gene is a member of the maize r1 gene family. By genetic and molecular analyses we report that Hopi consists of a single gene residing on chromosome 10 approximately 4.5 cM distal to r1. Hopi conditions anthocyanin deposition in aleurone, scutellum, pericarp, root, mesocotyl, leaves, and anthers, thus representing one of the broadest specifications of pigmentation pattern reported to date of all the r1 genes. A unique feature of the Hopi gene is that seeds are completely devoid of pigment at maturity but show a photoinducible germination-dependent anthocyanin accumulation in aleurone and scutellum. Our analysis has shown that the Hopi transcript is not present in scutellum of developing seeds but is induced only upon germination and that the simultaneous presence of both C1 and Hopi mRNAs is necessary to achieve A1 activation in scutella. We conclude that the expression pattern of the Hopi gene accounts for the germination-dependent anthocyanin synthesis in scutella, whereas the developmental competence of germinating seeds to induce anthocyanin production in scutella results from the combination of the light-inducible expression of C1 and the developmentally regulated expression of the Hopi gene.

Comparative genomics of chalcone synthase and Myb genes in the grass family

Most plant genes occur as members of multigene families where new copies arise through duplication. Duplicate genes that do not confer an adaptive advantage to the plant are expected to rapidly erode into pseudogenes owing to the accumulation of transpositions, insertion/deletion mutations and nucleotide changes. Nonfunctional copies will drift to fixation within a few million years and ultimately erode beyond recognition. Duplicate genes that are retained over longer periods of evolutionary time must be positively selected based on some adaptive advantage conferred on the plant species. We explore the dynamics of the recruitment of new duplicate genes for chalcone synthase, the enzyme that catalyzes the first committed step of flavonoid biosynthesis, and for the myb family of transcriptional activators. Our analyses show that new chs genes are recruited into the genome of grasses at a rate of one new copy every 15 to 25 million years. In contrast, the myb gene family is much older and many duplicate copies appear to predate the separation of the angiosperm lineage from other seed plants. The general pattern suggests a rapid adaptive proliferation of new chs genes but a more ancient elaboration of regulatory gene functions. Our analyses also reveal accelerated rates of protein evolution following gene duplication and evidence is presented for interlocus exchange among duplicate gene loci.

ABI3 affects plastid differentiation in dark-grown Arabidopsis seedlings

The Arabidopsis ABSCISIC ACID-INSENSITIVE3 (ABI3) protein has been identified previously as a crucial regulator of late seed development. Here, we show that dark-grown abi3 plants, or abi3 plants returned to the dark after germination in the light, developed and maintained an etioplast with a prominent prolamellar body at developmental stages in which the wild type did not. Overexpression of ABI3 led to the preservation of the plastid ultrastructure that was present at the onset of darkness. These observations suggest that ABI3 plays a role in plastid differentiation pathways in vegetative tissues. Furthermore, the analysis of deetiolated (det1) abi3 double mutants revealed that DET1 and ABI3 impinge on a multitude of common processes. During seed maturation, ABI3 required DET1 to achieve its full expression. Mature det1 abi3 seeds were found to be in a highly germinative state, indicating that germination is controlled by both DET1 and ABI3. During plastid differentiation in leaves of dark-grown plants, DET1 is required for the action of ABI3 as it is during seed development. Together, the results suggest that ABI3 is at least partly regulated by light.

The AmMYB308 and AmMYB330 transcription factors from antirrhinum regulate phenylpropanoid and lignin biosynthesis in transgenic tobacco

MYB-related transcription factors are known to regulate different branches of flavonoid metabolism in plants and are believed to play wider roles in the regulation of phenylpropanoid metabolism in general. Here, we demonstrate that overexpression of two MYB genes from Antirrhinum represses phenolic acid metabolism and lignin biosynthesis in transgenic tobacco plants. The inhibition of this branch of phenylpropanoid metabolism appears to be specific to AmMYB308 and AmMYB330, suggesting that they recognize their normal target genes in these transgenic plants. Experiments with yeast indicate that AmMYB308 can act as a very weak transcriptional activator so that overexpression may competitively inhibit the activity of stronger activators recognizing the same target motifs. The effects of the transcription factors on inhibition of phenolic acid metabolism resulted in complex modifications of the growth and development of the transgenic plants. The inhibition of monolignol production resulted in plants with at least 17% less lignin in their vascular tissue. This reduction is of importance when designing strategies for the genetic modification of woody crops.

The AmMYB308 and AmMYB330 transcription factors from antirrhinum regulate phenylpropanoid and lignin biosynthesis in transgenic tobacco

MYB-related transcription factors are known to regulate different branches of flavonoid metabolism in plants and are believed to play wider roles in the regulation of phenylpropanoid metabolism in general. Here, we demonstrate that overexpression of two MYB genes from Antirrhinum represses phenolic acid metabolism and lignin biosynthesis in transgenic tobacco plants. The inhibition of this branch of phenylpropanoid metabolism appears to be specific to AmMYB308 and AmMYB330, suggesting that they recognize their normal target genes in these transgenic plants. Experiments with yeast indicate that AmMYB308 can act as a very weak transcriptional activator so that overexpression may competitively inhibit the activity of stronger activators recognizing the same target motifs. The effects of the transcription factors on inhibition of phenolic acid metabolism resulted in complex modifications of the growth and development of the transgenic plants. The inhibition of monolignol production resulted in plants with at least 17% less lignin in their vascular tissue. This reduction is of importance when designing strategies for the genetic modification of woody crops.

Analysis of bHLH and MYB domain proteins: species-specific regulatory differences are caused by divergent evolution of target anthocyanin genes

The regulatory anthocyanin loci, an1, an2, an4 and an11 of Petunia hybrida, and r and c1 from Zea mays, control transcription of different sets of target genes. Both an2 and c1 encode a MYB-type protein. This study reports the isolation of a P. hybrida gene, jaf13, encoding a basic helix-loop-helix protein that, on the basis of sequence homology and intron/exon structure, represents the P. hybrida orthologue of the Z. mays r genes. Ectopic expression of an2 and jaf13 is sufficient for activation of the dihydroflavonol 4-reductase-A (dfrA) promoter and enhanced pigment accumulation in P. hybrida. This indicates that an2 and jaf13 play a key role in determining the tissue-specific expression pattern of structural genes. However, because chalcone synthase (chs) and flavanone-3-hydroxylase (f3h) are not activated, the pattern of pigmentation is not fundamentally altered. Expression of an2 in Z. mays complements a mutation in pl, a c1 paralogue, indicating that an2 activates a wider set of target genes in this host. Transient expression assays in Z. mays and P. hybrida tissues showed that C1 and R or AN2 and JAF13 can activate the promoter of the c2 gene, encoding Z. mays CHS, but not the chsA promoter from P. hybrida. These results indicate that regulatory anthocyanin genes are conserved between species and that divergent evolution of the target gene promoters is responsible for the species-specific differences in regulatory networks.

Paramutation and related allelic interactions

Paramutation is an allelic interaction that results in meiotically heritable changes in gene expression. Until recently, the few documented cases in higher plants seemed unusual and rare. This perception is rapidly fading because of the discovery of related examples and the growing recognition of epigenetic changes in a wide variety of biological systems.

MOLECULAR GENETIC ANALYSIS OF TRICHOME DEVELOPMENT IN ARABIDOPSIS

Two basic questions in developmental biology are: How does a cell know when it should or should not differentiate, and once a cell is committed to differentiate, how is that process controlled? The first process regulates the arrangement or pattern of the various cell types, whereas the second makes cells functionally distinct. Together, these two processes define plant morphogenesis. Trichome development in Arabidopsis provides an excellent model to analyze these questions. First, trichome development in Arabidopsis is a relatively simple process. A single epidermal cell differentiates into a unicellular trichome. Second, this differentiation occurs in a nonrandom pattern on the plant surface. Finally, the process is amenable to genetic analysis because many mutations that affect trichome differentiation do not alter other aspects of plant development. Thus far, more than 20 genes affecting trichome development have been identified. This review examines the current state of our understanding of these genes.

Localization and interaction of the cis-acting elements for abscisic acid, VIVIPAROUS1, and light activation of the C1 gene of maize

The C1 regulatory gene of the maize anthocyanin pathway is regulated by a combination of developmental and environmental signals that include the Viviparous1 (Vp1) gene, abscisic acid (ABA), and light. Using protoplast electroporation and particle bombardment assays, we have defined c/s-acting elements that are necessary and sufficient for the activation of C1 by ABA, VP1, and light, respectively. The sequence from positions -142 to -132 (CGTCCATGCAT) is essential for VP1 activation, whereas a larger overlapping element from -147 to -132 (CGTGTCGTCCATGCAT) is necessary and sufficient for activation by ABA. A separate light (blue and red)-responsive c/s element is located between positions -116 and -59. Light interacts synergistically with the ABA and VP1 responses in transient expression assays, suggesting that combinatorial interaction between modules plays a role in integrating these signals in the developing seed.

Allelic interactions heritably alter the activity of a metastable maize pl allele

The maize pl locus encodes a transcriptional activator of anthocyanin biosynthetic genes. The Pl-Rhoades (Pl-Rh) allele confers robust purple anthocyanin pigment in several tissues. Spontaneous derivatives of Pl-Rh, termed Pl'-mahogany (Pl'-mah), arise that confer reduced pigment and are meiotically heritable. These derivatives influence other Pl-Rh alleles such that only Pl'-mah alleles are transmitted form a Pl-Rh/Pl'mah heterozygote. Genetic crosses establish that chromosomal segregation distortion does not explain this exclusive transmission and suggest that Pl-Rh invariably changes to Pl'-mah when exposed to Pl'-mah. Such behavior is a hallmark of paramutation. Cosegregation experiments demonstrate that this paramutagenic activity is genetically linked to the pl locus. By visually quantifying pl action through successive crosses, we find that phenotypic expression is inversely related to paramutation at two other maize loci, b and r. Previous analysis of b and r paramutation revealed extensive differences and led to suggestions of distinct molecular mechanisms. Consideration of the common features of all three systems reinvigorates the interpretation that the mechanistic processes of these three allelic interactions are similar.

Overlap of Viviparous1 (VP1) and abscisic acid response elements in the Em promoter: G-box elements are sufficient but not necessary for VP1 transactivation

The relationship between promoter sequences that mediate Viviparous1 (VP1) transactivation and regulation by abscisic acid (ABA) in the wheat Em promoter was investigated using deletion analysis and directed mutagenesis. The Em1a G-box is strongly coupled to VP1 transactivation as well as to ABA regulation; however, the Em promoter includes additional components that can support VP1 transactivation without ABA responsiveness or synergism. Oligonucleotide tetramers of several G-box sequences, including Em1a, Em1b, and the dyad G-box element from the UV light-regulated parsley chalcone synthase gene, were sufficient to confer VP1 transactivation and the synergistic interaction with ABA to the -45 cauliflower mosaic virus 35S core promoter. These data suggest that VP1 can activate transcription through at least two classes of cis-acting sequences, including the G-box elements and the Sph regulatory motif found in the C1 promoter. The contrasting roles of these motifs in the Em and C1 promoters suggest a basis for the differential regulation of the corresponding genes by VP1.

Low Temperature Induces the Accumulation of Phenylalanine Ammonia-Lyase and Chalcone Synthase mRNAs of Arabidopsis thaliana in a Light-Dependent Manner

Anthocyanins, which accumulate in leaves and stems in response to low temperature and changes in light intensity, are synthesized through the phenylpropanoid pathway that is controlled by key enzymes that include phenylalanine ammonia-lyase (PAL) and chalcone synthase (CHS). In this work we demonstrate that PAL and CHS mRNAs accumulate in leaves of Arabidopsis thaliana (L.) Heynh. upon exposure to low temperature in a light-dependent manner. The regulation of the PAL1 gene expression by low temperature and light was examined by analyzing the expression of the [beta]-glucuronidase (uidA) reporter gene in transgenic Arabidopsis plants containing the uidA gene of Escherichia coli under the control of the PAL1 promoter. The results indicate that the accumulation of PAL1 mRNA is transcriptionally regulated. Histochemical staining for [beta]-glucuronidase activity showed that the PAL1 promoter is preferentially activated in photosynthetically active cells, paralleling anthocyanin accumulation. Moreover, we show that light may also be implicated in the regulation of the CHS gene in response to bacterial infiltration. Finally, using two transparent testa Arabidopsis mutants that are unable to accumulate anthocyanins, we demonstrate that these pigments are not required for successful development of freezing tolerance in this species.

Maize anthocyanin regulatory gene pl is a duplicate of c1 that functions in the plant

Genetic studies in maize have identified several regulatory genes that control the tissue-specific synthesis of purple anthocyanin pigments in the plant. c1 regulates pigmentation in the aleurone layer of the kernel, whereas pigmentation in the vegetative and floral tissues of the plant body depends on pl. c1 encodes a protein with the structural features of eukaryotic transcription factors and functions to control the accumulation of transcripts for the anthocyanin biosynthetic genes. Previous genetic and molecular observations have prompted the hypothesis that c1 and pl are functionally duplicate, in that they control the same set of anthocyanin structural genes but in distinct parts of the plant. Here, we show that this proposed functional similarity is reflected by DNA sequence homology between c1 and pl. Using a c1 DNA fragment as a hybridization probe, genomic and cDNA clones for pl were isolated. Comparison of pl and c1 cDNA sequences revealed that the genes encode proteins with 90% or more amino acid identity in the amino- and carboxyl-terminal domains that are known to be important for the regulatory function of the C1 protein. Consistent with the idea that the pl gene product also acts as a transcriptional activator is our finding that a functional pl allele is required for the transcription of at least three structural genes in the anthocyanin biosynthetic pathway.