Cell cycle: the key to plant growth control?

Experimental evidence on the role of the cell cycle in plant growth regulation does not exclusively fit the cellular (division drives growth) or the organismal perspective (division merely accompanies growth). Here we present a broader, integrated concept of plant growth regulatory interactions, which accommodates experimental results gathered to date. This model can serve as a basis for future research, and prompts experimental approaches to encompass both measurements of cell growth and division parameters.     

Nest predation risk and growth strategies of passerine species: grow fast or develop traits to escape risk?

Abstract Different body components are thought to trade off in their growth and development rates, but the causes for relative prioritization of any trait remains a critical question. Offspring of species at higher risk of predation might prioritize development of locomotor traits that facilitate escaping risky environments over growth of mass. We tested this possibility in 12 altricial passerine species that differed in their risk of nest predation. We found that rates of growth and development of mass, wings, and endothermy increased with nest predation risk across species. In particular, species with higher nest predation risk exhibited relatively faster growth of wings than of mass, fledged with relatively larger wing sizes and smaller mass, and developed endothermy earlier at relatively smaller mass. This differential development can facilitate both escape from predators and survival outside of the nest environment. Tarsus growth was not differentially prioritized with respect to nest predation risk, and instead all species achieved adult tarsus size by age of fledging. We also tested whether different foraging modes (aerial, arboreal, and ground foragers) might explain the variation of differential growth of locomotor modules, but we found that little residual variation was explained. Our results suggest that differences in nest predation risk among species are associated with relative prioritization of body components to facilitate escape from the risky nest environment.     

Development of foraging skills in two orangutan populations: needing to learn or needing to grow?

Background

Orangutans have one of the slowest-paced life histories of all mammals. Whereas life-history theory suggests that the time to reach adulthood is constrained by the time needed to reach adult body size, the needing-to-learn hypothesis instead suggests that it is limited by the time needed to acquire adult-level skills.To test between these two hypotheses, we compared the development of foraging skills and growth trajectories of immature wild orangutans in two populations: at Tuanan (Pongo pygmaeus wurmbii), Borneo, and Suaq Balimbing (Pongo abelii), Sumatra. We collected behavioral data on diet repertoire, feeding rates and ranging competence during focal follows, and estimated growth through non-invasive laser photogrammetry.

Results

We found that adult-like diet repertoires are attained around the age of weaning and that female immatures increase their repertoire size faster than their male peers. Adult-level feeding rates of easy techniques are reached just after weaning, but several years later for more difficult techniques, albeit always before adulthood (i.e. age at first reproduction). Independent immatures had faster feeding rates for easy to process items than their mothers, with male immatures achieving faster feeding rates earlier in development relative to females. Sumatran immatures reach adult-level feeding rates 2–3 years later than their Bornean peers, in line with their higher dietary complexity and later weaning. The range-use competence of independently ranging and weaned immatures is similar to that of adult females. Body size measurements showed, immatures grow until female age of first reproduction.

Conclusions

In conclusion, unlike in humans, orangutan foraging skills are in place prior to reproduction. Growth trajectories suggest that energetic constraints, rather than skills, best explain the length of immaturity. However, skill competence for dietary independence is reached later where the adult niche is more complex, which is consistent with the relatively later weaning age with increasing brain size found generally in primates, and apes in particular.

     

Cell cycle regulation in the postmitotic neuron: oxymoron or new biology?

Adult CNS neurons are typically described as permanently postmitotic but there is probably nothing permanent about the neuronal cell cycle arrest. Rather, it appears that these highly differentiated cells must constantly keep their cell cycle in check. Relaxation of this vigilance leads to the initiation of a cell cycle and entrance into an altered and vulnerable state, often leading to death. There is evidence that neurons which are at risk of neurodegeneration are also at risk of re-initiating a cell cycle process that involves the expression of cell cycle proteins and DNA replication. Failure of cell cycle regulation might be a root cause of several neurodegenerative disorders and a final common pathway for others.     

Cell cycle: who turns the crank?

The oscillating activity of a single CDK-cyclin fusion protein can drive the orderly progression of yeast cells through DNA replication, mitosis and cell division.     

Cell size: a matter of life or death?

Proper growth and development of multicellular organisms require the tight regulation of cell growth, cell division and cell death. A recent study has identified a novel regulatory link between two of these processes: cell growth and cell death.     

Prolonged clonal growth: escape route or route to extinction?

Many plant species have the capability to reproduce sexually as well as clonally. The balance between clonal reproduction and sexual reproduction varies between different species. It was estimated that 66.5% of all central European flora may form independent but genetically identical daughter plants. Also within species there is great variation in the ratio clonal/sexual reproduction. Clonal reproduction can be considered as an alternative life cycle loop that allows persistence of a species in the absence of the ability to complete the normal life cycle (i.e. seed production, germination and recruitment). Plant populations exhibiting prolonged clonal growth have been referred to as 'remnant populations'. A remnant population in general is defined as "a population capable of persistence during extended time periods despite a negative population growth rate (λ<1) due to longlived life stages and life cycles, including loops, that allow population persistence without completion of the whole life cycle". Here we argue that prolonged and nearly exclusive clonal growth through environmental suppression of sexual reproduction can ultimately lead to local sexual extinction and to monoclonal populations of a species, and that this may imply significant consequences for population viability. Especially obligate or mainly outcrossing clonal plant species may be vulnerable for sexual extinction. We argue that the consequences of reduced sexual recruitment in clonally propagating plants may be understudied and underestimated and that a re-evaluation of current ideas on clonality may be necessary.     

Inside or outside? Localization of the receptor relevant to auxin-induced growth

The localization of the auxin receptor relevant to the control of elongation growth is still a matter of controversy. Auxin-induced elongation of maize coleoptile segments was measured by means of a high resolution auxanometer. When indole-3-acetic acid (IAA) was removed from the bathing solution, a rapid cessation of auxin-induced elongation was detected. This decline was delayed when the auxin efflux carrier was blocked by the phytotropins naphthylphthalamic acid (NPA) and pyrenoylbenzoic acid (PBA) or by triiodobenzoic acid (TIBA). The IAA concentration in NPA-pretreated segments was 2–3 times higher than in NPA-free controls 35 min after the removal of IAA in the bathing medium.
A similar rapid drop of growth after removal of auxin was observed for the rapidly-transported synthetic auxin, naphthaleneacetic acid (NAA). When the auxin efflux was blocked, growth induced by NAA was sustained much longer than IAA-stimulated elongation.
In comparison with NAA, the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) is known to be excreted very slowly by the efflux carrier. 2,4-D-induced growth remained at a stimulated level when the auxin was washed off, even in the absence of any auxin efflux inhibitor. We conclude from these results that the presence of intracellular auxin is a necessary and sufficient condition for sustained auxin-induced elongation growth, at least for the phases during the 2 h after its application. Consequently, we postulate the existence of an intracellular auxin receptor relevant to the control of growth.     

Cell differentiation: midbody remnants - junk or fate factors?

The midbody is an electron-dense structure that forms between two dividing daughter cells, and a midbody remnant is left after completion of cell separation. This structure has been regarded as a piece of cellular debris, but two recent papers suggest an unexpected function for the midbody remnant in promoting an undifferentiated cellular phenotype.     

Cell wall growth during elongation and division: one ring to bind them?

The role of the cell division protein FtsZ in bacterial cell wall (CW) synthesis is believed to be restricted to localizing proteins involved in the synthesis of the septal wall. In this issue of Molecular Microbiology, the groups of Christine Jacobs-Wagner and Waldemar Vollmer provide compelling evidence that in Caulobacter crescentus, FtsZ plays an additional role in CW synthesis in non-dividing cells. During elongation (cell growth) FtsZ is responsible for the incorporation of CW material in a zone at the midcell by recruiting MurG, a protein involved in peptidoglycan (PG) precursor synthesis. This resembles earlier findings of FtsZ mediated PG synthesis activity in Escherichia coli. A role of FtsZ in PG synthesis during elongation forces a rethink of the current model of CW synthesis in rod-shaped bacteria.     

How does a bacterium grow during its cell cycle?

Rod-shaped bacteria such as Escherichia coli and Bacillus subtilis appear to extend continuously in length between divisions. However, the kinetics of growth of the individual cell in the steady state is still unknown. A brief, critical account of the main approaches used to determine the pattern of surface extension is given. In general, these approaches are of three types. Firstly, attempts have been made to relate average cell size to growth rate of the culture and to determine possible stages in the cell cycle at which the rate of length extension might change. Secondly, comparisons have been made between the measured length distribution of cells and theoretical distributions, based on three primary hypotheses (linear, bilinear and exponential growth). Thirdly, the principle of Collins and Richmond, involving the calculation of growth rate from the length distributions of extant, separating and new-born cells, is described. It is emphasized that there is a strong element of variation in size at different stages of the cell cycle. This variation imposes severe limitations on models which utilize only average cellular dimensions. We conclude that the Collins-Richmond principle affords the most powerful approach to the analysis of bacterial growth kinetics. However, we propose that the method be modified to permit calculation of separate rates of growth of cells between discernible events in the cell cycle, as well as simply between birth and division.     

Cell polarity: intrinsic or externally imposed?

A basic question in studies of the genesis of cell polarity is whether the polarity is an intrinsic and permanent property of cells or whether it is externally imposed by signals at the cell periphery. Current models favor the possibility that an external signal selectively imposes a polarized cell morphology. However, recent data from different experimental systems are discussed here that support the idea that an intrinsic polarity in animal cells is maintained through a dynamic process involving specific activities of the cortical microfilament system and the centrosome-microtubule complex. In this view, external signals capable of modulating cell polarity, for example, during chemotaxis or histogenesis, do so by acting on mechanisms that maintain cells permanently polarized. The contribution of the cytoskeleton to the genesis of cell polarity is discussed, with particular reference to experimental evidence for global cytoskeletal dynamics, and it is suggested that critical advances in our understanding of the maintenance of cell polarity will depend on our obtaining further knowledge of the molecular mechanisms controlling interactions between microtubules and microfilaments. Microtubules appear to exert an inhibitory control on the recruitment of cytoplasmic myosin into the cortex, and there are data indicating that the centrosome and centrioles could actively contribute to the establishment of cell polarity.     

Intercellular junctions: downstream and upstream of Ras?

Most human tumors are of epithelial origin, and these tumors gradually lose their epithelial character in a process termed the epithelial-mesenchymal transition. Approximately 40% of human tumors have activating mutations in one of the three RAS genes. Given these statistics, it is critically important to understand the role of Ras signaling in the epithelial-mesenchymal transition. This review considers the mechanisms and effectors through which Ras may regulate intercellular junction formation in epithelial cells. Conversely, intercellular junction proteins themselves may play a role in regulating Ras activation and signaling.     

How does a hypha grow? The biophysics of pressurized growth in fungi

The mechanisms underlying the growth of fungal hyphae are rooted in the physical property of cell pressure. Internal hydrostatic pressure (turgor) is one of the major forces driving the localized expansion at the hyphal tip which causes the characteristic filamentous shape of the hypha. Calcium gradients regulate tip growth, and secretory vesicles that contribute to this process are actively transported to the growing tip by molecular motors that move along cytoskeletal structures. Turgor is controlled by an osmotic mitogen-activated protein kinase cascade that causes de novo synthesis of osmolytes and uptake of ions from the external medium. However, as discussed in this Review, turgor and pressure have additional roles in hyphal growth, such as causing the mass flow of cytoplasm from the basal mycelial network towards the expanding hyphal tips at the colony edge.     

Regulation of Myc by miR-34c: A mechanism to prevent genomic instability?

Over the past 8 years several lines of compelling evidence have indicated that microRNAs are critical downstream effectors of classic oncogene/tumour suppressor networks. The archetypal examples of oncogene and tumour suppressor microRNAs are the miR-17-92 (oncomir 1) polycistron and miR-34 respectively. Whilst the involvement of these two opposing families of microRNAs in oncogenesis has been known for some time, the mRNA targets through which they exert their phenotypes are only just beginning to be uncovered. Moreover, several recent reports have demonstrated that the relevant physiological targets of certain individual microRNAs are actually fairly limited, with repression of just one or two major targets sufficient to explain the observed phenotype. In this review we will discuss the emerging role of microRNAs in tumourigenesis with a specific focus on miR-34c-dependent regulation of Myc.