Petunia hybrida

The South American genus Petunia (Solanaceae) consists of about 30 species. The name Petunia is derived from the Brazilian word for tobacco: petun. The origin of Petunia hybrida (), which seems to be a hybrid of several Petunia species, is not fully understood; different Petunia hybrida cultivars may have originated from different ancestors. Restriction fragment polymorphism analysis, using the chalcon isomerase A gene, indicates that the cultivar Violet 30 (V30), with which most of the described research is carried out, is most similar to the species P. axillaris and P. parodii ().

Phyllotaxy of seedling growth is opposite, four-ranked, and when flowering commences, it changes to alternate usually with a solitary flower arising from each leaf node. Flowering in Petunia is photoperiodically controlled with long days or a night interruption with incandescent light promoting early flowering. Petunia species bear an indeterminate number of solitary axillary flowers, and the flowers do not always arise at every node. The pentamerous flower has five epipetalous stamens. The pollen grains are bilateral in origin, and usually have three germ pores.

The fruit is a capsule containing 30-1000 seeds (depending on the species), is 7-8 mm in diameter, conic, widest at the base and tapering to the apex, brown, glabrous, and smooth. The mature fruit is surrounded by an enlarged, glandular-hairy calyx composed of five strap-shaped lobes. Petunia seeds are 0.6-0.7 mm in length, 0.5-0.6 mm in diameter. Light to dark brown seed contains considerable endosperm with a well-formed embryo.

Floriculturally, Petunia is a bedding plant. It is generally grown as an annual from seed each year for outdoor decorative purposes in prepared ground beds and window boxes. Some cultivars show a pendulous growth-flowering habit, which makes them suitable for growing as flowering hanging basket plants. Petunia is one of the most sold bedding plant species. It is probably most economically important in terms of worldwide use as an ornamental plant. With respect to plant type, most plants are single-flowered grandifloras. With respect to flower color, pink, rose, salmon, and red are most popular.

Furthermore, Petunia plants are widely used as model systems for (molecular) research on floral development and several mutants in developmental genes are obtained, e.g., by modern techniques such as transposon tagging and particle bombardment. By mutation, also variation in flower color can be obtained. An example is the line of Petunia that was constructed containing a copy of the maize Al gene that enhances the production of the red pelargonidin pigment. The flower colors depend on the growth conditions, which apparently influence the degree of methylation of the DNA.

In this chapter the secondary metabolite production and physiology of cell and tissue cultures of Petunia hybrida are described as a model system for the study of the regulation of secondary metabolism. From this plant species cell and tissue cultures can easily be obtained in which the phenyl propanoid pathway (PPP) can be induced by different treatments. Already much research has been carried out on the different steps and branches of the PPP, it is widespread in the plant kingdom and most of the end products are easily detectable.

Summary

In Petunia hybrida cv. Violet 30 cell suspensions, the regulation of the production of the secondary metabolites anthocyanin and lignin was studied. PAL, the first enzyme of the PPP, can be induced by various treatments, e.g., biotic elicitors or abiotic elicitors. Orthovanadate (inhibiting plasma membrane ATPases) activated the branch of the PPP, leading to production of (guaiacyl) lignin-like material. Increase of the pH in continuous cell cultures also led to production of lignin-like material. Subculturing cells using small inoculum sizes (< 2 g fresh weight/1) gives rise to lignin and anthocyanin production. Reduction of the NAA concentration of the medium, subsequent to the elicitation by orthovanadate or dilution stress, gave rise to a further increase in the production of lignin and anthocyanins respectively.

A variety of ATPase inhibitors and ionophores were tested for the ability to elicit the PPP. Concomitantly, changes in cytoplasmic pH were followed with the fluorescent probe oxonol VI. These data indicate that a decrease of the cytoplasmic pH was sufficient to influence PAL activity and lignin production of P. hybrida cells and could therefore be an intermediate in signal transduction.

Orthovanadate-treated cells showed a higher respiration and a higher activity of the pentose phosphate pathway. Also, in these cells a decrease in soluble carbohydrates and an increase in insoluble carbohydrates, protein, EFW, and DW/FW ratio were observed. Possibly, lignifying cells remain smaller than nonlignifying cells, due to a firmer cell wall caused by extra lignin deposition.

Glucose-limited and phosphate-limited continuous cultures were established. In these cultures induction of secondary metabolism was also studied and several results of experiments using cells from batch cultures were corroborated.

Selections from the book: “Medicinal and Aromatic Plants IX” (1996).