Peter W. Barlow

Actin: A Dynamic Framework For Multiple Plant Cell Functions

Recent progress in understanding the plant actin gene family is reviewed, focusing on the Arab idopsis actins. Taking an evolutionary perspective, we have focused on the functional significance of the conserved but ancient vegetative and reproductive actin classes, which date back to the origin of vascular plants. We propose that the conservation of ancient family members is due to differential gene regulation and/or to functional differences among isovariants. The eight functional actin genes are widely dispersed on four of the five Arabidopsis chromosomes. Each of the five actin gene subclasses are strongly expressed at some time and place during plant development, and they are highly differentially regulated. A handful of surface epitope differences among plant and vertebrate actins enabled the isolation of general and subclass-specific anti-plant actin monoclonal antisera. These reagents give an excellent resolution to the switch from vegetative to reproductive actin protein expression during floral development. Combined with refined fixation protocols, these reagents resolve the intimate relationship between the chloroplasts and the actin cytoskeleton in leaf cells. Sequence-based screening procedures were developed for the isolation of the first mutant alleles of plant actins. These mutants have strong deleterious effects on the survival of plants and are effectively lethal mutations over several generations. Sequence differences among the co-expressed plant actin isovariants should produce complex dynamics within actin filaments and with actin-binding proteins. Future work on the significance of this ancient family will focus on the cell biology, genetics, and biochemistry of the isovariants.

Root apex transition zone: a signalling-response nexus in the root.

Longitudinal zonation, as well as a simple and regular anatomy, are hallmarks of the root apex. Here we focus on one particular root-apex zone, the transition zone, which is located between the apical meristem and basal elongation region. This zone has a unique role as the determiner of cell fate and root growth; this is accomplished by means of the complex system of a polar auxin transport circuit. The transition zone also integrates diverse inputs from endogenous (hormonal) and exogenous (sensorial) stimuli and translates them into signalling and motoric outputs as adaptive differential growth responses. These underlie the root-apex tropisms and other aspects of adaptive root behaviour.

The Root Cap: Cell Dynamics, Cell Differentiation and Cap Function

The root cap is a universal feature of angiosperm, gymnosperm, and pteridophyte roots. Besides providing protection against abrasive damage to the root tip, the root cap is also involved in the simultaneous perception of a number of signals – pressure, moisture, gravity, and perhaps others – that modulate growth in the main body of the root. These signals, which originate in the external environment, are transduced by the cap and are then transported from the cap to the root. Root gravitropism is one much studied response to an external signal. In the present paper, consideration is given to the structure of the root cap and, in particular, to how the meristematic initial cells of both the central cap columella and the lateral portion of the cap which surrounds the columella are organized in relation to the production of new cells. The subsequent differentiation and development of these cells is associated with their displacement through the cap and their eventual release, as “border cells”, from the cap periphery. Mutations, particularly in Arabidopsis, are increasingly playing a part in defining not only the pattern of genetic activity within different cells of the cap but also in revealing how the corresponding wild-type proteins relate to the range of functions of the cap. Notable in this respect have been analyses of the early events of root gravitropism. The ability to image auxin and auxin permeases within the cap and elsewhere in the root has also extended our understanding of this growth response. Images of auxin distribution may, in addition, help extend ideas concerning the positional controls of cell division and cell differentiation within the cap. However, firm information relating to these controls is scarce, though there are intriguing suggestions of some kind of physiological link between the border cells surrounding the cap and mitotic activity in the cap meristem. Open questions concern the structure and functional interrelationships between the root and the cap which surmounts it, and also the means by which the cap transduces the environmental signals that are of critical importance for the growth of the individual roots, and collectively for the shaping of the root system.

A Polarity Crossroad in the Transition Growth Zone of Maize Root Apices: Cytoskeletal and Developmental Implications

Due to their simple and regular anatomy, root apices represent a unique model object for studying growth, polarity, and morphogenesis. This advantageous anatomy has been exploited to characterize the developmental changes that occur as root cells progress from their origin in the meristem up to their final nongrowing state at the proximal limit of the elongation region. A new growth region located between the apical meristem and the distal portion of the region of rapid cell elongation was discovered and designated as the ‘transition zone.’ Cells of this zone accomplish a developmental transition recently from cytoplasmically driven expansion to vacuome-driven elongation. Cells traversing the transition zone use cytoskeletal elements to regulate both growth polarity and the maintenance of cellular growth per se. Transition zone cells are also sensitive to diverse endogenous clues and exogenous factors such as auxin, ethylene, extracellular calcium, mechanical pressure, aluminum, and microorganisms. This high sensitivity of transition zone cells, which are not engaged in mitotic divisions, seems to be related to their specific cytoarchitecture whereby postmitotic nuclei occupy a central position within the cell, with their radial perinuclear microtubules extending to the cell periphery. Future studies are challenged to identify genes and proteins that determine the various sensory behaviors of cells in this transitional phase of development, and which, in turn, drive directed growth responses (tropisms) of root apices in response to diverse external stimuli.

A role for gibberellic acid in orienting microtubules and regulating cell growth polarity in the maize root cortex

The role of gibberellins and cortical microtubules in determining the polarity of cell growth in the root cortex of maize (Zea mays L.) was examined. Inhibition of gibberellin biosynthesis, either naturally through mutation (d5 mutant) or by means of chemicals such as 2S,3S paclobutrazol, caused thickening of root apices and increased their starch content. Immunofluorescence microscopy of cortical microtubules, coupled with a comparison of cell widhts, lengths and shapes, indicated that the meristem and immediate post-mitotic zone were the targets of gibberellin deficiency. Cortical cells in these regions were impaired in their ability to develop highly ordered transversal arrays of cortical microtubules. Consequently, the cells became wider and shorter. Application of gibberellic acid re-established the arrangements of cortical microtubules and the polarity of cell growth characteristic for roots having normal levels of gibberellins, it also decreased the starch content. These results indicate that gibberellins are morphogenetically active substances, not only in shoots but also in roots of maize.

The 'root-brain' hypothesis of Charles and Francis Darwin: Revival after more than 125 years

This year celebrates the 200th aniversary of the birth of Charles Darwin, best known for his theory of evolution summarized in On the Origin of Species. Less well known is that, in the second half of his life, Darwin’s major scientific focus turned towards plants. He wrote several books on plants, the next-to-last of which, The Power of Movement of Plants, published together with his son Francis, opened plants to a new view. Here we amplify the final sentence of this book in which the Darwins proposed that: “It is hardly an exaggeration to say that the tip of the radicle thus endowed [with sensitivity] and having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals; the brain being seated within the anterior end of the body, receiving impressions from the sense-organs, and directing the several movements.” This sentence conveys two important messages: first, that the root apex may be considered to be a ‘brain-like’ organ endowed with a sensitivity which controls its navigation through soil; second, that the root apex represents the anterior end of the plant body. In this article, we discuss both these statements.

Cell-Cell Channels

Cell-Cell Channels and Their Implications for Cell Theory.- Cell-Cell Channels and Their Implications for Cell Theory.- Prokaryotic Cells.- Mating Cell-Cell Channels in Conjugating Bacteria.- Ciliate Cells.- The Tetrahymena Conjugation Junction.- Algal Cells.- Cytoplasmic Bridges in Volvox and Its Relatives.- Fungal Cells.- Vegetative Hyphal Fusion in Filamentous Fungi.- Plant Cells.- Plasmodesmata: Cell-Cell Channels in Plants.- Sieve-Pore Plugging Mechanisms.- Actin and Myosin VIII in Plant Cell-Cell Channels.- Cell-Cell Communication in Wood.- TMV Movement Protein Targets Cell-Cell Channels in Plants and Prokaryotes.- Viral Movement Proteins Induce Tubule Formation in Plant and Insect Cells.- Cell-Cell Movements of Transcription Factors in Plants.- Animal Cells.- Gap Junctions.- Tunneling Nanotubes.- Cytoplasmic Bridges as Cell-Cell Channels of Germ Cells.- Fusome as a Cell-Cell Communication Channel of Drosophila Ovarian Cyst.- Cytonemes as Cell-Cell Channels in Human Blood Cells.- Paracellular Pores in Endothelial Barriers.- Channels across Endothelial Cells.- Molecular Transfers through Transient Lymphoid Cell-Cell Channels.- Cell-Cell Transport of Homeoproteins.- Virological Synapse for Cell-Cell Spread of Viruses.- Cell-Cell Fusion.

Gravitropism of the primary root of maize: a complex pattern of differential cellular growth in the cortex independent of the microtubular cytoskeleton

The spatio-temporal sequence of cellular growth within the post-mitotic inner and outer cortical tissue of the apex of the primary root of maize (Zea mays L.) was investigated during its orthogravitropic response. In the early phase (0–30 min) of the graviresponse there was a strong inhibition of cell lengthening in the outer cortex at the lower side of the root, whereas lengthening was only slightly impaired in the outer cortex at the upper side. Initially, inhibition of differential cell lengthening was less pronounced in the inner cortex indicating that tissue tensions which, in these circumstances, inevitably develop at the outer-inner cortex interface, might help to drive the onset of the root bending. At later stages of the graviresponse (60 min), when a root curvature had already developed, cells of the inner cortex then exhibited a prominent cell length differential between upper and lower sides, whereas the outer cortex cells had re-established similar lengths. Again, tissue tensions associated with the different patterns of cellular behaviour in the inner and outer cortex tissues, could be of relevance in terminating the root bending. The perception of gravity and the complex tissue-specific growth responses both proceeded normally in roots which were rendered devoid of microtubules by colchicine and oryzalin treatments. The lack of involvement of microtubules in the graviresponse was supported by several other lines of evidence. For instance, although taxol stabilized the cortical microtubules and prevented their re-orientation in post-mitotic cortical cells located at the lower side of gravistimulated roots, root bending developed normally. In contrast, when gravistimulated roots were physically prevented from bending, re-oriented arrays of cortical microtubules were seen in all post-mitotic cortical cells, irrespective of their position within the root.

A Strong Nucleotypic Effect on the Cell Cycle Regardless of Ploidy Level

BACKGROUND AND AIMS In published studies, positive relationships between nucleotype and the duration of the mitotic cell cycle in angiosperms have been reported but the highest number of species analyzed was approx. 60. Here an analysis is presented of DNA C-values and cell cycle times in root apical meristems of angiosperms comprising 110 measurements, including monocots and eudicots within a set temperature range, and encompassing an approx. 290-fold variation in DNA C-values. METHODS Data for 110 published cell cycle times of seedlings grown at temperatures between 20-25 degrees C were compared with DNA C-values (58 values for monocots and 52 for eudicots). Regression analyses were undertaken for all species, and separately for monocots and eudicots, diploids and polyploids, and annuals and perennials. Cell cycle times were plotted against the nuclear DNA C-values. KEY RESULTS A positive relationship was observed between DNA C-value and cell cycle time for all species and for eudicots and monocots separately, regardless of the presence or absence of polyploid values. In this sample, among 52 eudicots the maximum cell cycle length was 18 h, whereas the 58 monocot values ranged from 8-120 h. There was a striking additional increase in cell cycle duration in perennial monocots with C-values greater than 25 pg. Indeed, the most powerful relationship between DNA C-value and cell cycle time and the widest range of cell cycle times was in perennials regardless of ploidy level. CONCLUSIONS DNA replication is identified as a rate limiting step in the cell cycle, the flexibility of DNA replication is explored, and we speculate on how the licensing of initiation points of DNA replication may be a responsive component of the positive nucleotypic effect of C-value on the duration of the mitotic cell cycle.

The microtubular cytoskeleton in cells of cold-treated roots of maize (Zea mays L.) shows tissue-specific responses

SummaryMicrotubules (MTs) in cells of various tissues at different distances from the apex of the maize root exhibited different sensitivities to cold (5 °C), as judged by MT reorientation and tendency to depolymerization. Their responses seem to be related to their initial intracellular arrangements. Generally, MTs in cells which were ceasing elongation were the least sensitive during the early stages (6–24 h) of cold treatment, but during the later stages (5–7 d) MTs in most of these cells eventually depolymerized. Pericycle cells showed a unique cold response. Here the MTs were conspicuously cold-labile and quickly depolymerized near the root-tip. However, after 1 d many pericycle cells in more proximal regions had repolymerized their MTs as dense, randomly organized arrays. These persisted for the remainder of the cold treatment. A similar resistance to longterm chilling, by means of MT repolymerization, was found in cells of the root cap, quiescent centre and cells of the distal part of the former meristem. MT repolymerization in the cold may enable the apex to resume growth when more favourable (warmer) conditions return.