7 Single-Cell Gravitropism and Gravitaxis
Markus Braun* and Ruth Hemmersbach1417.1 Introduction
Over the last few decades, single-cell systems have increasingly attracted attention as
model organisms for research on gravity-related biological processes. Gravity is the only
constant environmental stimulus and provides the most reliable cue that free-swimming
and sessile organisms learned to use for orientation. This capacity is found in organisms
ranging from single cells up to multicellular animals and plants. In all these systems grav-
itropism and gravitaxis involve the pure physical step of susception followed by physio-
logical steps comprising gravity perception, signal transduction, and signal transmission,
eventually resulting in a gravity vector-related response in the form of a reorientation of
growth or movement direction.
In this review, free-swimming protozoa like flagellates (e.g., Euglena) and ciliates
(e.g., Parameciumand Loxodes) are referred to as single-cell systems, as are tip-growing
cell types such as protonemata of mosses and ferns and the rhizoids and protonemata of
algae. Although such cells are part of a multicellular organism, they extend away from it.
They have to cope directly with the environment and they have to adapt to it in a most
beneficial way in order to survive. Communication of single-cell systems with other cells
of the organ is limited and they are not integrated into complex signal transduction net-
works and response pathways required for multicellular response. Thus, for single-celled
systems, the stimulus response is dependent only on the cell’s own orientation. All single-
cell systems share a number of advantageous features for study. The unobstructed access
to the cell body permits a great variety of experimental approaches and allows easy iso-
lation for biochemical and molecular analyses. The signaling pathways are relatively
short and all phases occur in a single cell.
This chapter accentuates the substantial contribution single-cell systems have added to
our understanding of the intracellular mechanisms underlying gravity sensing and the
molecular basis of the gravity-dependent signaling pathways.
7.2 Definitions of Responses to Environmental Stimuli that Optimize the
Ecological Fitness of Single-Cell Organisms
Microorganisms (bacteria and protists), animals, and plants respond to environmental stim-
uli in a multitude of ways. The capacity of free-living organisms to orient the direction of
their movement with respect to the source of an external stimulus is called taxis: positive
taxis if the direction of movement is toward the source of the stimulus, negative taxis if it
is away from it, and diataxis or transverse taxis if it is at an angle to the stimulus direction.
*Corresponding author