ABSTRACT
The Arabidopsis thaliana MADS box transcription factors APETALA3 (AP3) and PISTILLATA (PI) heterodimerize and are required to specify petal identity, yet many details of how this regulatory process is effected are unclear. We have identified three related genes, BHLH136/BANQUO1 (BNQ1), BHLH134/BANQUO2 (BNQ2), and BHLH161/BANQUO3 (BNQ3), as being directly and negatively regulated by AP3 and PI in petals. BNQ1, BNQ2, and BNQ3 encode products belonging to a family of atypical non-DNA binding basic helix-loop-helix (bHLH) proteins that heterodimerize with and negatively regulate bHLH transcription factors. We show that bnq3 mutants have pale-green sepals and carpels and decreased chlorophyll levels, suggesting that BNQ3 has a role in regulating light responses. The ap3 bnq3 double mutant displays pale second-whorl organs, supporting the hypothesis that BNQ3 is downstream of AP3. Consistent with a role in light response, we show that the BNQ gene products regulate the function of HFR1 (for LONG HYPOCOTYL IN FAR-RED1), which encodes a bHLH protein that regulates photomorphogenesis through modulating phytochrome and cryptochrome signaling. The BNQ genes also are required for appropriate regulation of flowering time. Our results suggest that petal identity is specified in part through downregulation of BNQ-dependent photomorphogenic and developmental signaling pathways.
Subject(s)
Arabidopsis Proteins/metabolism , Arabidopsis/genetics , Light , MADS Domain Proteins/metabolism , Signal Transduction , Amino Acid Sequence , Arabidopsis/metabolism , Arabidopsis Proteins/genetics , Chlorophyll/analysis , Flowers/genetics , Flowers/metabolism , Gene Expression Regulation, Plant , MADS Domain Proteins/genetics , Molecular Sequence Data , Mutagenesis, Insertional , Mutation , Phylogeny , Plants, Genetically Modified/genetics , Plants, Genetically Modified/metabolismABSTRACT
Floral organogenesis is dependent on the combinatorial action of MADS-box transcription factors, which in turn control the expression of suites of genes required for growth, patterning, and differentiation. In Arabidopsis (Arabidopsis thaliana), the specification of petal and stamen identity depends on the action of two MADS-box gene products, APETALA3 (AP3) and PISTILLATA (PI). In a screen for genes whose expression was altered in response to the induction of AP3 activity, we identified GNC (GATA, nitrate-inducible, carbon-metabolism-involved) as being negatively regulated by AP3 and PI. The GNC gene encodes a member of the Arabidopsis GATA transcription factor family and has been implicated in the regulation of chlorophyll biosynthesis as well as carbon and nitrogen metabolism. In addition, we found that the GNC paralog, GNL (GNC-like), is also negatively regulated by AP3 and PI. Using chromatin immunoprecipitation, we showed that promoter sequences of both GNC and GNL are bound by PI protein, suggesting a direct regulatory interaction. Analyses of single and double gnc and gnl mutants indicated that the two genes share redundant roles in promoting chlorophyll biosynthesis, suggesting that in repressing GNC and GNL, AP3/PI have roles in negatively regulating this biosynthetic pathway in flowers. In addition, coexpression analyses of genes regulated by AP3, PI, GNC, and GNL indicate a complex regulatory interplay between these transcription factors in regulating a variety of light and nutrient responsive genes. Together, these results provide new insights into the transcriptional cascades controlling the specification of floral organ identities.
