The Matryoshka dolls of plant polarity

In its broadest sense, polarity can be described as asymmetry. And, if you look at a plant, you will immediately notice a variety of asymmetric features at different levels of complexity. There is a root at the bottom and a shoot at the top. Leaves are attached to the shoot at one end and are free at the other. The upper side of the leaf may have hairs on it, while the underside is smooth. When you take a closer look at a hair, you might see that it is asymmetrically branched. If you happen to have a microscope handy, a cross-section of the plant's stem will reveal that the cells at the core look different from those further out. Even better equipped, you will recognize polarity at the cellular level, which manifests itself in the directions of vesicle movement and in the asymmetric distribution of organelles,cytoskeletal strands, and even of single proteins. Finally, if you are really inquisitive and do not mind leaving the realm of biology, you can go further and look at the polarity of single molecules, atoms and elementary particles,and end up in the dark and vast space of the ultimate microcosmos (but please be back for lunch). You have just discovered different levels of polarity at different orders of magnitude, nested like Russian Matryoshka dolls, and you may speculate that polarities at lower levels in some way underlie the polarity found in bigger structures.

In general, polarity is a very common phenomenon and it is an intrinsic property of matter at every level of complexity. However, for plants, polarity is something more – it is the means by which they maintain developmental continuity, communicate, expand and adapt. For plants, polarity is a real theme of life. In his new book Polarity in Plants, Keith Lindsey explores the multiple levels at which polarity arises and the integral role that polarity plays during development. In spite of the obvious presence of polarity in plants and the attention that has been given to this topic for centuries, we are only beginning to unravel the complex mechanisms that underlie cellular polarity and its connection to the polarity of tissues,organs and the whole plant. The plant polarity field is still in its infancy;delimitation of the field is unclear, many concepts are intriguing but largely speculative (such as the analogies drawn between auxin and neurotransmitters),and there is a lack of consensus about basic nomenclature (for example, the issue of whether the tip of the root should be referred to as an apical or a basal structure). Above all, polarity exists at many different structural levels, and these issues make plant polarity a very difficult topic to discuss in a comprehensive synopsis, as it does not break down easily into small,separate entities that can be presented in an intuitive sequence.

Keith Lindsey has arranged the eleven reviews about various facets of polarity in plants that make up this book in a logical way, along the axes of both space and time. The first few chapters, which have been written by various contributors, cover polarity at the single-cell level, with special consideration given to the cytoskeleton and cell walls. The rest of the book follows the course of plant development. It starts with polarity in the zygote and during embryogenesis, and then considers the different organs and tissue structures as they develop in time, which eventually leads us to the`abominable mystery' of polarity in flowers. This structure provides for a book that is relatively well rounded, in spite of the variable scope and style of the single reviews.

By and large, the chapters are `stand alone' reviews, and this sometimes leads to a somewhat redundant treatment of certain subjects. However, this arrangement facilitates the selective reading of single topics, which is probably the way this book will be used most often. Much of the data reviewed here, of course, come from the major model plant Arabidopsis, but where appropriate, work from other plants, such as Fucus or Antirrhinum, is also discussed.

The first chapter, about cell growth and the plant cytoskeleton, comes closest to an introduction to the whole book, as it is very comprehensive and introduces some of the key molecules and mechanisms that are involved in polarity at the cellular level. To some extent, this chapter even serves as a glossary. Comparisons with polarity in animal cells make this a good overview,and also a starting point for those interested in more than one kingdom. Here,the reader encounters root hairs and their growth for the first time, and since root hairs are a great model system for growing cells, they also appear in several other chapters, although with varying success. Chapter 2 attempts to connect the role of ROPs (RhoGTPases of Plants) with polar cell-to-cell auxin transport, and contains broad information about both topics. Chapter 3 is the last of the general chapters, and thoroughly covers plant cell walls as major determinants of the cell's growth and shape, and their role in polarity. In particular, the roles of targeted vesicle trafficking and auxin transport are highlighted; these and the connection to the cytoskeleton seem to be the recurring themes of the first part of the book. Cell walls are a discriminating feature of plant cells and, with the inherent philosophical tendencies of Chapter 3, they can indeed be seen at the `transition zone between uni- and multicellularity'. With the little knowledge that we currently have about the molecular mechanisms of polar auxin transport, you can also follow the speculative notion that the smaller, non-elongating transversal walls of root cells resemble animal synapses. This otherwise stimulating analogy is brought to its extreme in Chapter 7, which is on root polarity, where these cell walls are almost exclusively referred to as `plant synapses'.

FIG1 

All subsequent chapters focus on special structures or processes involving polarity. The reader will learn about trichomes (the little hairs on leaves that are actually single, branched cells), and early events in the development of fucoid algae, and will find an excellent chapter on polarity in Arabidopsis embryogenesis, which does not only review the literature thoroughly but expands on it in a truly inspiring way. As mentioned above, the chapter on roots is maybe a bit too inspired by the idea of auxin behaving like a neurotransmitter, but otherwise you will find a thorough review about root and lateral root development, which also considers comparisons between different plant species.

There are no surprises about polarity in the chapter on the shoot apical meristem, but it does give comprehensive information on the meristem's amazing self-regulatory capacities. The next chapter on vascular development is maybe a bit more substantial with regards to polarity, and is also easier to read. The last two chapters cover lateral organ and flower development. It becomes especially clear in the latter that polarity is a phenomenon closely linked to development. During flower development, a multitude of polar growth axes emerge, along which floral development takes place. Actually, most forms of development do require or lead to polarity, and so it is not surprising that this book also serves as quite a comprehensive review of plant development. As such, this book might also prove useful in higher-level graduate classes,although it is aimed at the professional in the field and is certainly not a textbook. Moreover, the price of about £100 will definitely limit the accessibility of this book to students.

Now, what does the reader learn from this book? Overall, it provides a mixed bag of information (a big bag of quality information, that is) that unfortunately lacks synergy. A useful addition might have been a more general introduction or synopsis of the known mechanisms of polarity in a special chapter. This would have helped the reader to see the context more clearly and would have improved some redundant parts; for example, those on the role of the phytohormone auxin. But let us not be greedy but content with what we get:a comprehensive and, for the most part, well-written collection of reviews that deal with polarity in plants, which, to our best knowledge, can also claim the prize of being the first book on this topic.

From the concluding remarks of most of the chapters, one can infer that still little is known about the molecular mechanisms underlying polarity. However, there seems to have been a recent boom in identifying molecular players involved in cell polarity, and, consequently, any attempt to review plant polarity is inevitably condemned to be slightly out of date, even before appearing. A recent major topic in the field is the polar flow of auxin, which on the one hand depends on polar, actin-dependent vesicle trafficking of PIN proteins and on the other determines polarity at higher levels. This model was lately reinforced when PIN-dependent auxin flow was identified as a common player in both embryonic and de novo organ axes formation. However, despite the recent fast pace of findings in this field, there is still a long way to go before we have a clear idea about how polarity is established in plants,and we look forward to a second edition of this book in, maybe, five years from now.