RAMs
open the way to living borders
One of the pleasures of writing these
commentaries is the discovery of topics about which I have little previous
knowledge. This month I have learned about border cells, studied for several
years by Martha Hawes at the University
of Arizona, Tucson. Border cells are released from the
outer layer of the root cap by hydrolysis of the connections with the next cell
layer. The border cells remain alive in the rhizosphere where they release
signalling molecules that may alter gene expression in potential pathogens and
symbionts; they are thus free-living somatic plant cells. However, it is not a
universal phenomenon. Amongst the dicots, some families produce no (or very
few) border cells while others produce many. In plants that do not produce
border cells, the outer cells of the root cap undergo programmed cell death and
are sloughed off in sheets. So, what regulates the production of border cells?
To start to answer the question, Hawes
has collaborated with colleagues at the University
of California, Davis (Hamamoto et al., pp. 917–923).
They have found an unexpected correlation between border cells and root apical
meristem (RAM) organization. Species that produce few or no border cells have
closed RAMs in which it is possible to trace specific cell files back to
specific initials in the RAM. Species that produce significant numbers of
border cells possess open RAMs in which differentiated cell types cannot be
traced back to specific initials. Why should this correlation exist? Is there
something about closed meristems that also directs the programmed cell death of
the outer cell layer of the root cap or is there something about open meristems
that activates the expression of the genes regulating release of these cells as
border cells? Do the differences reflect evolutionary history relating to how
different types of plant interact with micro-organisms? This story will surely
run and run.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
