Petunia is both a genus of enormous horticultural interest and a model plant that is subject to numerous scientific research projects. I recall the deep purple variety from which I harvested the pollen during my PhD studies in what must be one of the Botanical Gardens with the most gorgeous views: that of the city of Siena in Tuscany, Italy. Nestled into a hillside behind one of the university's institutes this garden houses much more exotic plants than Petunia, of course. However, in the context of my PhD project I used the pollen of Petunia hybrida for cell growth experiments, thereby exploiting the fact that the tubes of this species show significant, periodic changes in growth rate. I have not used Petunia pollen or any other part of the plant much since, and therefore I was curious to learn about the state of the art of Petunia research in the new book edited by Tom Gerats and Judith Strommer.
The book consists of a collection of chapters written for a scientific audience that highlight various aspects of Petunia genetics. It starts out with a historical review on the genus Petunia, elaborating on the geographical origin and providing a taxonomic guide for the 14 currently recognized Petunia species. Very helpfully, pictures are provided representing typical specimens of each species. The second chapter summarizes why Petunia has become a model system for the evolution of pollination syndromes. Traits such as flower colour, morphology, scent and nectar production that vary significantly between the different species of this genus seem to fit the morphology and behavior of specific pollinator types ranging from insects to birds. Petunia thus represents an excellent model system to study and test the hypothesis that selection by pollinators drives floral evolution. Among other emerging model systems for the study of pollination syndromes, Petunia is the only genus that is genetically accessible, and hence it plays a special role for evolutionary science.
One of the parameters for pollinator specificity is scent, the chemistry of which is analysed in the third chapter of the book. The authors summarize how scent emitted from living plants is measured and they provide an overview of the biochemical reactions leading to the synthesis of volatile benzenoids/phenylpropanoids, as well as of the diurnal changes in fragrance production. Enzyme activities involved in alcoholic fermentation are analysed in the fourth chapter. Pyruvate decarboxylase and alcohol dehydrogenase are important enzymes, but their roles in Petunia were found to not necessarily correspond to those recognized in other plant species. The study of these two enzymes in Petunia have therefore contributed to our understanding of the adaptation strategies of plants to their environment.
The study of Petunia reproduction would not be complete without talking about the gametophytic self-incompatibility system operating in this genus, which prevents self-fertilization. As pointed out in chapter 5, research in the past two decades has succeeded in identifying many genes associated with self-incompatibility and the operation of the complex mechanism that governs the distinction between self and non-self in this genus in particular and in RNase-based self-incompatibility mechanisms in general. The pollen is also the subject of another chapter that deals with cytoplasmic male sterility. The case discussed here is based on an aberrant chimeric gene that is encoded in the mitochondria and that influences primarily pollen development.
The series of chapters pertaining to reproduction is then interrupted by two chapters that cover the vegetative development of the plant. Chapter 8 details the symbiotic relationship between the Petunia root and arbuscular mycorrhizal fungi and focuses in particular on the phosphate transporters that play a crucial role in this interaction. The other vegetative development is that of the branching pattern that ultimately determines the form of the adult organism. Axillary bud development is a crucial architectural process that is governed by a number of genes, the modification of which has a huge potential to manipulate the overall phenotype of the plant. Commercially probably even more relevant in this genus is the potential modification of inflorescence and flower morphology, however. Chapters 9 through 11 show that the diversity in these architectural traits that can be observed within the genus Petunia and between Petunia and other genera is based on a number of genes encoding conserved proteins whose expression patterns and regulatory interactions can change dramatically. While these chapters focus on the development of flower geometry, chapter 13 discusses flower colour and the biochemical pathways involved in pigment production. The end of a flower's life is a well co-ordinated, active process and chapter 14 elaborates this example of senescence and the concerted action of the plant hormones that are involved in controlling it.
Plant morphogenesis is a product of cellular division, cell growth and cell differentiation. Cell shape and size are determined by manipulating the mechanical properties of the cell wall and chapter 12 summarizes the important role of expansins, a set of proteins controlling the expansion of plant cell walls and thus potentially affecting overall plant structure.
The last section of the book is dedicated to genome mapping and functional gene analysis. Chapter 15 provides an overview of the efforts to map the Petunia genome and, again, a nice historical review is included. The transposable element system that is one of the factors responsible for the genic instability in Petunia is subsequently described in chapter 17. Another particularity, retroelements, defined by their dependence on reverse transcription for replication, are the topic of chapter 16. These transposable elements have been exploited for gene identification and isolation, as also evident from several other chapters of the book. These methods will eventually enable the construction of a saturated insertion library, an invaluable tool for functional gene analysis. Another powerful element in the tool box of functional gene analysis is the use of virus-induced gene silencing, described in chapter 18. The authors used a modified tobacco rattle virus vector for Petunia to silence up to five independent genes at the same time.
The final two chapters summarize the great advantages of the Petunia system that allows relatively easy Agrobacterium-mediated transformation and has in fact been used to prove the feasibility of this method since its first development. Gene transfer is, of course, a key tool that is used to add alleles or genes, or to silence endogenous genes through antisense or RNAi strategies. These approaches are invaluable for the development of new cultivars for the ornamental horticulture industry, as finally explored in chapter 20.
I would like to point out that this book is more than an update of recent research activities in the style of ‘what's new’ since many chapters provide very appreciated historical reviews of the particular field of research. This makes this book very valuable and the individual contributions more complete than conventional review articles. The quality of figures is generally very good though somewhat uneven, which is not surprising in a multi-author work. The book is a useful resource for scientists beginning research work on the genus Petunia, but it also serves as an excellent compendium that, although it might not be completely up to date anymore five years from now, will still remain a valuable long-term resource of information. Having said this, I kept wondering why my favourite cell, the pollen tube, was not covered in more detail in this book other than in the context of self-incompatibility and, somewhat indirectly, in the context of metabolic pathways in the developing pollen grain. However, a quick literature search informed me that I was not the only ‘pollen tube scientist’ who had given up Petunia pollen for Arabidopsis in recent years. This is a surprising development really, given the fickle growth behaviour, the small size of the pollen grain and tube, and the minute amount of pollen per plant that are typical for Arabidopsis. Perhaps I should give Petunia pollen tubes a second chance after all – if only to reminisce about that beautiful Botanical Garden in Siena.