Annals of Botany 89: 129-131, 2002
© 2002 Annals of Botany Company
Book Reviews
The plant cell cycle and its interfaces.
Francis D. (ed.) 2001.Sheffield: Sheffield Academic Press.
£69 (hardback). 220 pp.
What are the ‘interfaces’ of the cell cycle mentioned in the title of this book? One interface is with the stimuli received from the immediate environment of the nucleus, where many cell cycle-related events are executed, and from neighbouring cells and super-symplasmic conducting channels. Another interface is with the outputs of the cycle—that is, those developmental events which require new cells for their successful accomplishment. However, in addition to the biotic inputs from the living plant system itself, cycling cells certainly also have inputs from the external abiotic environment. With the exception of the chapter by M. Sauter (‘Gibberellic acid-mediated regulation of the cell cycle during stem growth’), which basically comes down to a discussion of how submergence of deep-water rice plants, with the consequential change in internal gas atmosphere, leads to cell division, abiotic factors are not discussed.
Concerning the output of new cells to meristems and their associated tissues, the question is broached in a number of chapters as to whether cells actually do contribute to development. Are cells only a means to an end in much the same way that stairs of a stairway are the means of moving between storeys of a house? Movement would be more difficult if the stairway were without the stairs. So, the reader is invited to consider a number of related questions: what is the cell cycle for; does cell enlargement drive the cell cycle and, if so, how; and what is the significance of endoreduplication, a modification of the mitotic cycle where mitosis is absent but DNA synthesis continues and generates either somatic polyploidy or chromosomal polyteny?
For rapidly growing organs, such as roots, shoots and leaves, which rely on vacuole-driven growth for the major part of their enlargement, the presence of an extensible cell wall around the protoplasm is clearly a prerequisite for the regulation of growth. However, in meristems, where cell growth is relatively slow, the process is not dependent upon vacuolar development but upon the increase of ground cytoplasm. It might be argued, therefore, that cells, or more precisely the walls of cells, are not essential either for the cytoplasmic mode of growth or for morphogenesis, except in so far as this latter process depends upon differential extension of some boundary, such as the external wall of certain critical tissues.
A cell comes into existence at the end of mitosis when a new wall partitions the cytoplasm. But there are plenty of instances where no wall is interpolated between divided nuclei, as in, say, the coenocytic stages of the embryos of Paeonia spp. and many gymnosperms. Thus, we could, if we wished, redefine what most books, including the present one under review, refer to as the ‘cell’ cycle, as actually having to do with a ‘cytoplast’ or ‘cell body’ cycle. How is it, therefore, that cytokinesis, i.e. the insertion of a cell plate and thence a cell wall, can be dispensed with (as in coenocytes), but more often than not has become an obligatory adjunct to cell body reproduction? Although not phrased in these terms, this interesting and central question of how cell body reproduction interfaces with new cell formation, via cell wall formation, is partially resolved by S. Wick and H. Rogers in their chapter entitled ‘The cytoskeletal interface with cell cycle control’. The issue addressed is whether products of cell cycle genes directly affect the properties of the preprophase band of microtubules (MTs) and thereby influence the site of insertion of a new cell wall at cytokinesis. But what cytoskeletal and metabolic rearrangements enable a callosic (not cellulosic) cell plate to form in the phragmoplast during cytokinesis? Moreover, given that interfacing is the theme, it is a pity that no interface was created between the cytoskeleton and the induced growth of deep-water rice discussed in the above-mentioned chapter by Sauter! Throughout that chapter, the suspicion lingers that the gibberellins which provide the stimulus to stem growth might do so partly by effecting a realignment of cortical MTs, thereby conferring upon extension growth a strong anisotropy.
The remarkable properties of cultured Bright Yellow (BY) tobacco cells, described by T. Nagata, F. Kumagai and T. Sano, appear to answer many questions about the physiological control of cell division in a much more tractable manner than do complex meristematic cell groups. What the BY cells have not yet shown, however, is the link between cell division and the generation of significant groups of cells that are capable of producing recognizable plant forms. Although D. Francis and D. Inzé, in their introductory chapter, marvel at the proliferative ability of these BY cells, equally marvellous are those many other systems which, although they may divide more slowly and more erratically than BY cells do at least show a capacity for organogenesis. One challenge for the BY system, therefore, might be to find cell lines that express different degrees of morphogenetic potential.
The cell cycle is the impulse for meristem productivity. Not only does it regulate the rate at which cells are produced within a meristem, but the number of cycling units determines the rate of output of cells from the meristem, these cells then being invested in axis extension and the formation of new organs. In their chapter ‘Cell cycle regulation in the shoot apical meristem’, M. Lenhard et al. discuss the cytological structure of shoot meristems and, in particular, the genetic control, via the activity of WUSCHEL and CLAVATA genes, over the passage of cells from the initial zone at the summit of the shoot to the pool of proliferative cells in the ground meristem. But there is another side to the cell proliferation problem, that of how mitotic cycling activity is turned off and, hence, how the proximal limit of the meristem is defined. Which check-point within cycle processes is used to implement the decision of whether to divide or not? At one time, the concept of a G0 phase was useful in this respect. Later, the G0 idea was recast into a hypothesis that cells could exist in either a probablistic A-state or in deterministic B-phase. This concept fitted proliferation/non-proliferation patterns in plant and animals cells rather well, and was even espoused by the editor himself in the course of his cell cycle research. It is rather odd, therefore, that G0 receives no more than a passing mention. The intriguing result from arabidopsis showing that increased root elongation results from an increased expression of cyc1At, a B-type cyclin gene, may indicate that one more cycle can be added into the meristematic cell files, thus delaying the onset of stringent control at the G0/G1 interface. (The description of experiments with over-expression of cyc1At never unequivocally demonstrated, as some of the authors in this book imply, that the cell cycle was faster in the transformed roots, only that the roots grew faster.)
In their chapter entitled ‘Cell division patterns and root apical construction’, E. P. Groot and T. L Rost take note of the constancy of cell-cycle-counting in the cell lineages of cap/protoderm tissue, particularly in arabidopsis root meristems. This study begins to make the link between the cell cycle and a deterministic concept of rhizogenesis. Incidentally, the authors give the impression that there is some mystery about 16 being the number of cells within packets in protoderm derivatives as well as the number of protoderm initial cells at the base of the cap. The latter group of 16 cells arises because this is the number of cells (assuming them to be hexagonal) that will fit around the group of central initials (i.e. there are four central founder cells and then there are, respectively, ten and 16 cells in the first and second concentric rings of cells surrounding these four central cells). In the cell packets, 16 cells are evident in longitudinal section because some precursor cell (a daughter issuing from one of the protoderm initials) has accomplished four transverse divisions (24 = 16). Groot and Rost also provide modern terms that are equivalent to Hanstein and Janczewski’s histogens. While these are satisfactory, less convincing is the terminology of cellular groups. ‘Cell packet’ is simply a convenient item of histological observation wherever there are cells that are the product of a few cycles of division within a common cell boundary. However, the term ‘module’, which the authors favour, seems redundant given that it is equivalent in some cases to a cell packet and in other cases to a ‘merophyte’. It is also worth observing that the relative size of cells in symplastically growing cell packets adequately and directly throws light on the existence, or non-existence, of non-cycling (G0) cells within meristems (see Fig. 8 in Barlow, 1987; see also p. 7 in the present book). No amount of juggling with data gained from experimental studies of cell cycle kinetics can satisfactorily determine whether such non-cycling cells exist. Direct observation of relative cell sizes can do so.
Despite the pioneering work of J. van’t Hof on sucrose-regulated cell cycle check-points and of H. Street on sugar controls of root growth in vitro, it is strange that so little can be said of the link between sugar availability and the mitotic cell cycle. Indeed, any insights into the problem as it relates to plants have been reduced here to a mere couple of paragraphs in the chapter by N. Halford and J. Dickenson (‘Sugar sensing and cell cycle control’). The other 14 pages of this chapter review the interaction of sugar with various metabolic pathways in yeast. Only after 11 pages does the chapter get anywhere near to touching upon the cell cycle, and then it is yeast, not plants, that provides the examples.
Two chapters are devoted to the elusive link between cytokinin and the cell cycle (J. Murray et al. ‘G1 cyclins, cytokinins and the regulation of the G1/S transition’ and P. John and K. Zhang ‘Cytokinin control of cell proliferation’). Overlap between these chapters is minimal, with no more than 18 % of all references being common to both chapters.
Small details of style caused some puzzlement during reading. Sometimes it is not clear from the text whether genes or proteins are meant, particularly when, on occasion, both are said ‘to be differentially expressed’. For example, on p. 68 we read ‘Differential expression of R2 was observed in partially synchronised cell suspension cultures . . .’, while on p. 69 we read ‘The expression studies of R2 and cycH in rice suggest that activating phosphorylation of CDKs . . .’ (note the italics). The context of both these sentences appears to deal with proteins but, since genes are also being mentioned, there is quite a lot of scope for confusion should italic and Roman scripts have been incorrectly used to denote proteins and genes. For those readers unfamiliar with Okazaki fragments mentioned on pp. 47–49 and in Fig. 3.1, these units of DNA replication were first described by R. Okazaki et al. in 1968. Another confusing point about Fig. 3.1 is that its panels are numbered 1–4, whereas the legend to the figure refers to panels A–E (i.e. one panel more than the number of numbered panels).
Lastly, there is a mystery contributor to the book—a ‘ghost’ writer, perhaps! In the list of contributors on p. vii a Mr Ir G. van den Berg appears. His name does not appear anywhere else.
In all, and taking into account the reservations mentioned above, this stimulating book should be sought out by undergraduates, postgraduates, tutors and supervisors alike.
LITERATURE CITED
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Barlow PW. 1987. Cellular packets, cell division and morphogenesis in the primary root meristem of Zea mays L. New Phytologist 105: 27–56.
Okazaki R, Okazaki T, Sakabe K, Sugimoto K, Kainuma R, Sugino A, Iwatsuki N. 1968. In vivo mechanism of DNA chain growth. Cold Spring Harbor Symposia on Quantitative Biology 33: 129–143.
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