In Trifolium subterraneum, chalcone synthase is encoded by a multigene family

Flavonoids: new roles for old molecules

Flavonoids are ubiquitous in the plant kingdom and have many diverse functions including defense, UV protection, auxin transport inhibition, allelopathy, and flower coloring. Interestingly, these compounds also have considerable biological activity in plant, animal and bacterial systems - such broad activity is accomplished by few compounds. Yet, for all the research over the last three decades, many of the cellular targets of these secondary metabolites are unknown. The many mutants available in model plant species such as Arabidopsis thaliana and Medicago truncatula are enabling the intricacies of the physiology of these compounds to be deduced. In the present review, we cover recent advances in flavonoid research, discuss deficiencies in our understanding of the physiological processes, and suggest approaches to identify the cellular targets of flavonoids.

Molecular cloning and sequence analysis of a novel chalcone synthase cDNA from Ginkgo biloba

A chalcone synthase (CHS) gene was cloned from Ginkgo biloba for the first time and it was also the first cloned gene involved in flavonoids metabolic pathway in G. biloba. The full-length cDNA of G. biloba CHS (designated as Gbchs) was 1608bp with poly(A) tailing and it contained a 1173bp open reading frame (ORF) encoding a 391 amino acid protein. Gbchs was found to have extensive homology with those of other plant chs genes via multiple alignments. The active sites of the CoA binding, coumaroyl pocket and cyclization pocket in CHS protein of Medicago sativa were also found in GbCHS. Molecular modeling of GbCHS indicated that the three-dimensional structure of GbCHS strongly resembled that of M. sativa (MsCHS2), implying GbCHS may have similar functions with MsCHS2. Phylogenetic tree analysis revealed that GbCHS had closer relationship with CHSs from gymnosperm plants than from other plants. Gbchs is a useful tool to study the regulation of flavonoids metabolism in G. biloba.

Rhizobia can induce nodules in white clover by "hijacking" mature cortical cells activated during lateral root development

We examined a range of responses of root cortical cells to Rhizobium sp. inoculation to investigate why rhizobia preferentially nodulate legume roots in the zone of emerging root hairs, but generally fail to nodulate the mature root. We tested whether the inability to form nodules in the mature root is due to a lack of plant flavonoids to induce the bacterial genes required for nodulation or a failure of mature cortical cells to respond to Rhizobium spp. When rhizobia were inoculated in the zone of emerging root hairs, changes in beta-glucuronidase (GUS) expression from an auxin-responsive promoter (GH3), expression from three chalcone synthase promoters, and the accumulation of specific flavonoid compounds occurred in cortical cells prior to nodule formation. Rhizobia failed to induce these responses when inoculated in the mature root, even when co-inoculated with nod gene-inducing flavonoids. However, mature root hairs remained responsive to rhizobia and could support infection thread formation. This suggests that a deficiency in signal transduction is the reason for nodulation failure in the mature root. However, nodules could be initiated in the mature root at sites of lateral root emergence. A comparison between lateral root and nodule formation showed that similar patterns of GH3:gusA expression, chalcone synthase gene expression, and accumulation of a particular flavonoid compound occurred in the cortical cells involved in both processes. The results suggest that rhizobia can "hijack" cortical cells next to lateral root emergence sites because some of the early responses required for nodule formation have already been activated by the plant in those cells.

Characterization of the structure and determination of mRNA levels of the phenylalanine ammonia-lyase gene family from Populus kitakamiensis

We isolated two new PAL genes, palg2b and palg4, from Populus kitakamiensis, palg2a and palg2b are clustered and palg2b encodes a polypeptide of 710 amino acids. The nucleotide sequence in the coding region of palg2b was 94.6% identical to that of palg2a. The promoter regions of palg1, palg2a and palg2b have several elements conserved among many phenylpropanoid biosynthetic genes. We measured the mRNA levels of the four PAL genes by S1 mapping using total RNA from stem tissues developing secondary xylem. Results showed that the transcript level of palg2b was higher than that of the other PAL genes.

Evolution of the chalcone synthase gene family in the genus Ipomoea

The evolution of the chalcone synthase [CHS; malonyl-CoA:4-coumaroyl-CoA malonyltransferase (cyclizing), EC 2.3.1.74] multigene family in the genus Ipomoea is explored. Thirteen CHS genes from seven Ipomoea species (family Convolvulaceae) were sequenced--three from genomic clones and the remainder from PCR amplification with primers designed from the 5' flanking region and the end of the 3' coding region of Ipomoea purpurea Roth. Analysis of the data indicates a duplication of CHS that predates the divergence of the Ipomoea species in this study. The Ipomoea CHS genes are among the most rapidly evolving of the CHS genes sequenced to date. The CHS genes in this study are most closely related to the Petunia CHS-B gene, which is also rapidly evolving and highly divergent from the rest of the Petunia CHS sequences.

Characterization of a phenylalanine ammonia-lyase multigene family in Trifolium subterraneum

The enzyme phenylalanine ammonia-lyase (PAL) was found to be encoded by a small gene family in the legume Trifolium subterraneum (subterranean clover). At least three of the family members are tightly clustered within approx. 20 kb of DNA. Sequencing of one of the genes established that it possesses two exons, the position of the single intron being identical to that found for PAL genes from other plants. The PAL protein consists of 725 amino acids, as deduced from the nucleotide sequence.