Functional characterization of PhGR and PhGRL1 during flower senescence in the petunia

Shikimate Kinase Plays Important Roles in Anthocyanin Synthesis in Petunia

In plants, the shikimate pathway is responsible for the production of aromatic amino acids L-tryptophan, L-phenylalanine, and L-tyrosine. L-Phenylalanine is the upstream substrate of flavonoid and anthocyanin synthesis. Shikimate kinase (SK) catalyzes the phosphorylation of the C3 hydroxyl group of shikimate to produce 3-phosphate shikimate (S3P), the fifth step of the shikimate pathway. However, whether SK participates in flavonoid and anthocyanin synthesis is unknown. This study characterized the single-copy PhSK gene in the petunia (Petunia hybrida) genome. PhSK was localized in chloroplasts. PhSK showed a high transcription level in corollas, especially in the coloring stage of flower buds. Suppression of PhSK changed flower color and shape, reduced the content of anthocyanins, and changed the flavonoid metabolome profile in petunia. Surprisingly, PhSK silencing caused a reduction in the shikimate, a substrate of PhSK. Further qPCR analysis showed that PhSK silencing resulted in a reduction in the mRNA level of PhDHQ/SDH, which encodes the protein catalyzing the third and fourth steps of the shikimate pathway, showing a feedback regulation mechanism of gene expression in the shikimate pathway.

Suppression of chorismate synthase, which is localized in chloroplasts and peroxisomes, results in abnormal flower development and anthocyanin reduction in petunia

In plants, the shikimate pathway generally occurs in plastids and leads to the biosynthesis of aromatic amino acids. Chorismate synthase (CS) catalyses the last step of the conversion of 5-enolpyruvylshikimate 3-phosphate (EPSP) to chorismate, but the role of CS in the metabolism of higher plants has not been reported. In this study, we found that PhCS, which is encoded by a single-copy gene in petunia (Petunia hybrida), contains N-terminal plastidic transit peptides and peroxisomal targeting signals. Green fluorescent protein (GFP) fusion protein assays revealed that PhCS was localized in chloroplasts and, unexpectedly, in peroxisomes. Petunia plants with reduced PhCS activity were generated through virus-induced gene silencing and further characterized. PhCS silencing resulted in reduced CS activity, severe growth retardation, abnormal flower and leaf development and reduced levels of folate and pigments, including chlorophylls, carotenoids and anthocyanins. A widely targeted metabolomics analysis showed that most primary and secondary metabolites were significantly changed in pTRV2-PhCS-treated corollas. Overall, the results revealed a clear connection between primary and specialized metabolism related to the shikimate pathway in petunia.

Petal senescence: a hormone view

Flowers are highly complex organs that have evolved to enhance the reproductive success of angiosperms. As a key component of flowers, petals play a vital role in attracting pollinators and ensuring successful pollination. Having fulfilled this function, petals senesce through a process that involves many physiological and biochemical changes that also occur during leaf senescence. However, petal senescence is distinct, due to the abundance of secondary metabolites in petals and the fact that petal senescence is irreversible. Various phytohormones are involved in regulating petal senescence, and are thought to act both synergistically and antagonistically. In this regard, there appears to be developmental point during which such regulatory signals are sensed and senescence is initiated. Here, we review current understanding of petal senescence, and discuss associated regulatory mechanisms involving hormone interactions and epigenetic regulation.

The acyl-activating enzyme PhAAE13 is an alternative enzymatic source of precursors for anthocyanin biosynthesis in petunia flowers

Anthocyanins, a class of flavonoids, are responsible for the orange to blue coloration of flowers and act as visual attractors to aid pollination and seed dispersal. Malonyl-CoA is the precursor for the formation of flavonoids and anthocyanins. Previous studies have suggested that malonyl-CoA is formed almost exclusively by acetyl-CoA carboxylase, which catalyzes the ATP-dependent formation of malonyl-CoA from acetyl-CoA and bicarbonate. In the present study, the full-length cDNA of Petunia hybrida acyl-activating enzyme 13 (PhAAE13), a member of clade VII of the AAE superfamily that encodes malonyl-CoA synthetase, was isolated. The expression of PhAAE13 was highest in corollas and was down-regulated by ethylene. Virus-induced gene silencing of petunia PhAAE13 significantly reduced anthocyanin accumulation, fatty acid content, and cuticular wax components content, and increased malonic acid content in flowers. The silencing of PhAAE3 and PhAAE14, the other two genes in clade VII of the AAE superfamily, did not change the anthocyanin content in petunia flowers. This study provides strong evidence indicating that PhAAE13, among clade VII of the AAE superfamily, is specifically involved in anthocyanin biosynthesis in petunia flowers.

Molecular Characterization and Functional Analysis of Two Petunia PhEILs

Ethylene plays an important role in flower senescence of many plants. Arabidopsis ETHYLENE INSENSITIVE3 (EIN3) and its homolog EIL1 are the downstream component of ethylene signaling transduction. However, the function of EILs during flower senescence remains unknown. Here, a petunia EIL gene, PhEIL2, was isolated. Phylogenetic tree showed that PhEIL1, whose coding gene is previously isolated, and PhEIL2 are the homologs of Arabidopsis AtEIL3 and AtEIL1, respectively. The expression of both PhEIL1 and PhEIL2 is the highest in corollas and increased during corolla senescence. Ethylene treatment increased the mRNA level of PhEIL1 but reduced that of PhEIL2. VIGS-mediated both PhEIL1 and PhEIL2 silencing delayed flower senescence, and significantly reduced ethylene production and the expression of PhERF3 and PhCP2, two senescence-associated genes in petunia flowers. The PhEIL2 protein activating transcription domain is identified in the 353-612-amino acids at C-terminal of PhEIL2 and yeast two-hybrid and bimolecular fluorescence complementation assays show that PhEIL2 interacts with PhEIL1, suggesting that PhEIL1 and PhEIL2 might form heterodimers to recognize their targets. These molecular characterizations of PhEIL1 and PhEIL2 in petunia are different with those of in Vigna radiata and Arabidopsis.