The Delayed Terminal Flower Phenotype Is Caused by a Conditional Mutation in the CENTRORADIALIS Gene of Snapdragon

The snapdragon (Antirrhinum majus) centroradialis mutant (cen) is characterized by the development of a terminal flower, thereby replacing the normally open inflorescence by a closed inflorescence. In contrast to its Arabidopsis counterpart, terminal flower1, the cen-null mutant displays an almost constant number of lateral flowers below the terminal flower. Some partial revertants of an X-radiation-induced cen mutant showed a delayed formation of the terminal flower, resulting in a variable number of lateral flowers. The number of lateral flowers formed was shown to be environmentally controlled, with the fewer flowers formed under the stronger flower-inducing conditions. Plants displaying this "Delayed terminal flower" phenotype were found to be heterozygous for a mutant allele carrying a transposon in the coding region and an allele from which the transposon excised, leaving behind a 3-bp duplication as footprint. As a consequence, an iso-leucine is inserted between Asp148 and Gly149 in the CENTRORADIALIS protein. It is proposed that this mutation results in a low level of functional CEN activity, generating a phenotype that is more similar to the Arabidopsis Terminal flower phenotype.     

Molecular genetic basis of flower colour variegation in Linaria

To identify transposons that may be of use for mutagenesis we investigated the genetic molecular basis of a case of flower colour variegation in Linaria, a close relative of the model species Antirrhinum majus. We show that this variegation is attributable to an unstable mutant allele of the gene encoding dihydroflavonol-4-reductase, one of the enzymes required for anthocyanin biosynthesis. This allele carries an insertion of a transposon belonging to the CACTA family (Tl1, Transposon Linaria 1) which blocks its expression thus conferring an ivory flower colour phenotype. Tl1 is occasionally excised in dividing epidermal cells to produce clonal patches of red tissue on the ivory background, and in cells giving rise to gametes to generate reversion alleles conferring a fully coloured phenotype. This finding may open the way for targeted transposon-mutagenesis in Linaria, and hence for using this genus in comparative genetic studies.     

The transposon Tip100 from the common morning glory is an autonomous element that can transpose in tobacco plants

The mutable flaked or a (flaked) (a(f)) line of the common morning glory (Ipomoea purpurea) displays white flowers with colored flakes, and the a(f) mutation is caused by the insertion of a transposable element named Tip100 into the CHS-D gene for anthocyanin biosynthesis. The 3.9-kb Tip100 element belongs to the Ac/Ds family and contains an ORF encoding a polypeptide of 808 amino acids. The frequency and timing of flower variegation vary in different a(f) lines, and a genetic element termed Modulator has been postulated to affect the variegation pattern. Since the pattern of flower variegation is determined by the frequency and timing of excision of Tip100 from the CHS-D gene, we wished to determine whether Tip100 is an autonomous element that is itself capable of transposition in a heterologous host. To do this, we introduced the element into the genome of tobacco plants by Agrobacterium-mediated transformation. The intact Tip100 element was able to excise from its original position in the chromosome and reinsert into new sites in the tobacco genome, whereas an internal deletion derivative was not. Based on these results, we conclude that Tip100 is an autonomous element. We also discuss the nature of the putative Modulator element affecting flower and leaf variegation in various mutable lines of the morning glory.     

植物花青素合成相关的bHLH转录因子

花青素是由类黄酮途径一个特异的分支合成的,它不仅能够决定花和果实的颜色,还能保护植物免受各种生物和非生物胁迫损伤。bHLH转录因子广泛存在于植物中,在植物的生长发育与形态建成、次级代谢产物的合成及对外界环境胁迫应答中起着重要的调控作用。近年来,随着大规模基因组测序技术和分子生物学的发展,植物中越来越多的bHLH转录因子得到鉴定。本文主要对花青素合成相关的bHLH转录因子及其在调节结构基因表达和花青素合成中的作用进行了综述。     

Gene duplication and mobile genetic elements in the morning glories

We review gene duplication and subsequent structural and functional divergence in the anthocyanin biosynthesis genes in the Japanese and common morning glories and discuss their evolutionary implications. These plants appear to contain at least six copies of the CHS gene and three tandem copies of the DFR gene. Of these, the CHS-D and DFR-B genes are mainly responsible for flower pigmentation and mutations in these genes confer white flowers. We compared the genomic sequences of these duplicated genes between the two morning glories and found small mobile element-like sequences (MELSs) and direct repeats (DRs) in introns and intergenic regions. The results indicate that the MELS elements and DRs play significant roles in divergence after gene duplication. We also discuss DNA rearrangements occurring before and after speciation of these morning glories. DNA transposable elements belonging to the Ac/Ds or En/Spm families have acted as major spontaneous mutagens in these morning glories. We also describe the structural features of the first Mu-related element found in the morning glories and polymorphisms found in the same species.