Transcription factors controlling plant secondary metabolism: what regulates the regulators?

Plants produce secondary metabolites, among others, to protect themselves against microbial and herbivore attack or UV irradiation. Certain metabolite classes also function in beneficial interactions with other organisms. For example, anthocyanin pigments and terpenoid essential oils have key roles in attraction of flower pollinators. Secondary metabolites also have direct uses for man. Flavonoids and terpenoids for example have health-promoting activities as food ingredients, and several alkaloids have pharmacological activities. Controlled transcription of biosynthetic genes is one major mechanism regulating secondary metabolite production in plant cells. Several transcription factors involved in the regulation of metabolic pathway genes have been isolated and studied. There are indications that transcription factor activity itself is regulated by internal or external signals leading to controlled responses. The aim of this review is to discuss the regulation of transcription factors involved in secondary metabolism in plants at gene and protein levels, using phenylpropanoid and terpenoid indole alkaloid pathways as two well-studied examples.

Expression of a chitinase gene from Metarhizium anisopliae in tobacco plants confers resistance against Rhizoctonia solani.

The chit1 gene from the entomopathogenic fungus Metarhizium anisopliae, encoding the endochitinase CHIT42, was placed under the control of the CaMV 35S promoter, and the resulting construct was transferred to tobacco. Seventeen kanamycin-resistant transgenic lines were recovered, and the presence of the transgene was confirmed by polymerase chain reactions and Southern blot hybridization. The number of chit1 copies was determined to be varying from one to four. Copy number had observable effects neither on plant growth nor development. Substantial heterogeneity concerning production of the recombinant chitinase, and both general and specific chitinolytic activities were detected in leaf extracts from primary transformants. The highest chitinase activities were found in plants harboring two copies of chit1 inserts at different loci. Progeny derived from self-pollination of the primary transgenics revealed a stable inheritance pattern, with transgene segregation following a mendelian dihybrid ratio. Two selected plants expressing high levels of CHIT42 were consistently resistant to the soilborne pathogen Rhizoctonia solani, suggesting a direct relationship between enzyme activity and reduction of foliar area affected by fungal lesions. To date, this is the first report of resistance to fungal attack in plants mediated by a recombinant chitinase from an entomopathogenic and acaricide fungus.

Identification of a bipartite jasmonate-responsive promoter element in the Catharanthus roseus ORCA3 transcription factor gene that interacts specifically with AT-Hook DNA-binding proteins.

Jasmonates are plant signaling molecules that play key roles in defense against certain pathogens and insects, among others, by controlling the biosynthesis of protective secondary metabolites. In Catharanthus roseus, the APETALA2-domain transcription factor ORCA3 is involved in the jasmonate-responsive activation of terpenoid indole alkaloid biosynthetic genes. ORCA3 gene expression is itself induced by jasmonate. By loss- and gain-of-function experiments, we located a 74-bp region within the ORCA3 promoter, which contains an autonomous jasmonate-responsive element (JRE). The ORCA3 JRE is composed of two important sequences: a quantitative sequence responsible for a high level of expression and a qualitative sequence that appears to act as an on/off switch in response to methyl jasmonate. We isolated 12 different DNA-binding proteins having one of four different types of DNA-binding domains, using the ORCA3 JRE as bait in a yeast (Saccharomyces cerevisiae) one-hybrid transcription factor screening. The binding of one class of proteins bearing a single AT-hook DNA-binding motif was affected by mutations in the quantitative sequence within the JRE. Two of the AT-hook proteins tested had a weak activating effect on JRE-mediated reporter gene expression, suggesting that AT-hook family members may be involved in determining the level of expression of ORCA3 in response to jasmonate.