Agriculture Reference
In-Depth Information
4
The plant extracellular matrix and signalling
Andrew J. Fleming
4.1
Introduction
Plant cells are fixed relative to another via a cellulose-based cell wall. As a conse-
quence, cell migration during the ontogeny of a plant is extremely limited, yet cells
in spatially separated parts of an organ must differentiate in a coordinated fashion
if a fully functional plant is to be produced. This coordination of differentiation
could be based on cell lineage (i.e. if all cells underwent a precise pattern of growth
and division, then it would be possible to reliably generate a functioning organism).
However, the vast majority of data indicate that cell-lineage-based mechanisms of
differentiation are rare in plants (Kessler
et al.
, 2002) and the general consensus is
that the patterns of differentiation observed in plants are based on extensive net-
works of intercellular communication. Significant progress has been made in this
area over the last few years. Indeed, there has been a veritable boom in the inter-
est and advances in our understanding of how plant cells communicate with each
other. As will be seen from even a brief perusal of the other chapters in this topic,
plants employ a plethora of different signalling mechanisms, and a major challenge
for the future is not simply to decipher these signals, but also to understand how
the different signals are integrated. Added to the complexity of the basic signalling
network is the further complication that plants are sessile organisms that must sense
and respond to their environment. Thus, the precise timing and position of events
of differentiation are frequently influenced by environmental factors, ranging from
temperature and light to a massive spectrum of potential pathogens and herbivores.
These responses may occur both locally at the point of stimulation and at a dis-
tance. Thus, the signalling schemes employed by plants must somehow combine a
consistency that maintains species-specific forms of growth and differentiation with
an adaptability that can encompass the myriad external factors that can potentially
influence the precise form and timing of growth and differentiation.
Much classical work on intercellular communication in plants has concentrated
on traditional hormones (e.g. auxin, cytokinin, gibberellin, abscisic acid and ethy-
lene). However, the last decade has seen intense interest (and progress) in novel
signalling components. Thus, recent research has led to exciting discoveries on the
function of peptides and RNA in intercellular communication in plants, and many
of the following chapters of this topic reflect this progress. By necessity, this means
that less emphasis is given to some of the classical hormones implicated in cell-
to-cell signalling. Readers are directed towards a number of excellent reviews that
cover this area (e.g. Finkelstein
et al.
, 2002; Hutchison & Kieber, 2002; Wang
et al.
,
