Gravitaxis in unicellular microorganisms

Modelling the key drivers of an aerial Phytophthora foliar disease epidemic, from the needles to the whole plant

Understanding the epidemiology of infectious diseases in a host population is a major challenge in forestry. Radiata pine plantations in New Zealand are impacted by a foliar disease, red needle cast (RNC), caused by Phytophthora pluvialis. This pathogen is dispersed by water splash with polycyclic infection affecting the lower part of the tree canopy. In this study, we extended an SI (Susceptible-Infectious) model presented for RNC to analyse the key epidemiological drivers. We conducted two experiments to empirically fit the extended model: a detached-needle assay and an in vivo inoculation. We used the detached-needle assay data to compare resistant and susceptible genotypes, and the in vivo inoculation data was used to inform sustained infection of the whole plant. We also compared isolations and real-time quantitative PCR (qPCR) to assess P. pluvialis infection. The primary infection rate and the incubation time were similar for susceptible and resistant genotypes. The pathogen death rate was 2.5 times higher for resistant than susceptible genotypes. Further, external proliferation of mycelium and sporangia were only observed on 28% of the resistant ramets compared to 90% of the susceptible ones. Detection methods were the single most important factor influencing parameter estimates of the model, giving qualitatively different epidemic outputs. In the early stages of infection, qPCR proved to be more efficient than isolations but the reverse was true at later points in time. Isolations were not influenced by the presence of lesions in the needles, while 19% of lesioned needle maximized qPCR detection. A primary infection peak identified via qPCR occurred at 4 days after inoculation (dai) with a secondary peak observed 22 dai. Our results have important implications to the management of RNC, by highlighting the main differences in the response of susceptible and resistant genotypes, and comparing the most common assessment methods to detect RNC epidemics.

Decoding the chemotactic signal

From an individual bacterium to the cells that compose the human immune system, cellular chemotaxis plays a fundamental role in allowing cells to navigate, interpret, and respond to their environments. While many features of cellular chemotaxis are shared among systems as diverse as bacteria and human immune cells, the machinery that guides the migration of these model organisms varies widely. In this article, we review current literature on the diversity of chemoattractant ligands, the cell surface receptors that detect and process chemotactic gradients, and the link between signal recognition and the regulation of cellular machinery that allow for efficient directed cellular movement. These facets of cellular chemotaxis are compared among E. coli, Dictyostelium discoideum, and mammalian neutrophils to derive organizational principles by which diverse cell systems sense and respond to chemotactic gradients to initiate cellular migration.

Circular motion of asymmetric self-propelling particles

Micron-sized self-propelled (active) particles can be considered as model systems for characterizing more complex biological organisms like swimming bacteria or motile cells. We produce asymmetric microswimmers by soft lithography and study their circular motion on a substrate and near channel boundaries. Our experimental observations are in full agreement with a theory of Brownian dynamics for asymmetric self-propelled particles, which couples their translational and orientational motion.

Origin and early evolution of neural circuits for the control of ciliary locomotion

Behaviour evolved before nervous systems. Various single-celled eukaryotes (protists) and the ciliated larvae of sponges devoid of neurons can display sophisticated behaviours, including phototaxis, gravitaxis or chemotaxis. In single-celled eukaryotes, sensory inputs directly influence the motor behaviour of the cell. In swimming sponge larvae, sensory cells influence the activity of cilia on the same cell, thereby steering the multicellular larva. In these organisms, the efficiency of sensory-to-motor transformation (defined as the ratio of sensory cells to total cell number) is low. With the advent of neurons, signal amplification and fast, long-range communication between sensory and motor cells became possible. This may have first occurred in a ciliated swimming stage of the first eumetazoans. The first axons may have had en passant synaptic contacts to several ciliated cells to improve the efficiency of sensory-to-motor transformation, thereby allowing a reduction in the number of sensory cells tuned for the same input. This could have allowed the diversification of sensory modalities and of the behavioural repertoire. I propose that the first nervous systems consisted of combined sensory-motor neurons, directly translating sensory input into motor output on locomotor ciliated cells and steering muscle cells. Neuronal circuitry with low levels of integration has been retained in cnidarians and in the ciliated larvae of some marine invertebrates. This parallel processing stage could have been the starting point for the evolution of more integrated circuits performing the first complex computations such as persistence or coincidence detection. The sensory-motor nervous systems of cnidarians and ciliated larvae of diverse phyla show that brains, like all biological structures, are not irreducibly complex.

The beating pattern of the flagellum of Euglena gracilis under altered gravity during parabolic flights

The unicellular freshwater flagellate Euglena gracilis shows negative gravitactic behavior. Previous experiments have revealed that the orientation is most likely an active physiological process in which the beating pattern of the flagellum is controlled by gravity and mediated by a change in the calcium concentration inside the cell. In a signal transduction chain, the calcium signal activates a calmodulin, which in turn raises the concentration of cAMP. This alters the beating pattern of the flagellum; reorientation is therefore not a passive process driven by buoyancy. In a recent parabolic flight experiment (ESA 45th parabolic flight campaign), we observed the beating of the flagellum with a high-resolution light microscope. Transition from hyper g to microg as well as from microg to hyper g caused a change of the beating pattern of the flagellum, which confirmed the physiological nature of the process. In microg cells stopped moving the flagellum or tried to reorient, while in hyper g, the cells realigned consecutively. The reaction times for the flagellar responses in previous experiments are confirmed.

Bioconvection and front formation of Paramecium tetraurelia

We have investigated the bioconvection of Paramecium tetraurelia in high-density suspensions made by centrifugal concentration. When a suspension is kept at rest in a Hele-Shaw cell, a crowded front of paramecia is formed in the vicinity of the bottom and it propagates gradually toward the water-air interface. Fluid convection occurs under this front, and it is driven persistently by the upward swimming of paramecia. The roll structures of the bioconvection become turbulent with an increase in the depth of the suspension; they also change rapidly as the density of paramecia increases. Our experimental results suggest that lack of oxygen in the suspension causes the active individual motions of paramecia to induce the formation of this front.

Physiological characterization of gravitaxis in Euglena gracilis and Astasia longa studied on sounding rocket flights

Euglena gracilis is a photosynthetic, unicellular flagellate found in eutrophic freshwater habitats. The organisms control their vertical position in the water column using gravi- and phototaxis. Recent experiments demonstrated that negative gravitaxis cannot be explained by passive buoyancy but by an active physiological mechanism. During space experiments, the threshold of gravitaxis was determined to be between 0.08 and 0.12 x g. A strong correlation between the applied acceleration and the intracellular cAMP and Ca2+ was observed. The results support the hypothesis, that the cell body of Euglena, which is denser than the surrounding medium exerts a pressure onto the lower membrane and activates mechanosensitive Ca2+ channels. Changes in the membrane potential and the cAMP concentration are most likely subsequent elements in a signal transduction chain, which results in reorientation strokes of the flagellum.