It is becoming increasingly apparent that the long-distance
signalling associated with many developmental processes
is complex and that novel hormone-like signals may play
substantial roles. The past decades have seen several
substances (e.g. brassinosteroids, systemin and other
polypeptides, mevalonic and jasmonic acids, polyamines,
oligosaccharides, flavonoids, and quinones) vie for a place
among the classical plant hormones (e.g. Spaink, 1996).
Recent microinjection and grafting studies have also shown
that RNA may act as a long-distance signal (Jorgensen et
al., 1998; Xoconostle-Cázares et al., 1999). In this issue,
Hannah et al. describe long-distance signalling and the
regulation of root–shoot partitioning in dwarf lethal or
dosage-dependent lethal (DL) mutants of common bean
(Shii et al., 1980, 1981), and present evidence indicating
that substances in addition to classical plant hormones
(e.g. cytokinins) may be involved.
As in the report by Hannah et al., much of the evidence
for roles of unidentified long-distance signals in the control
of plant development is indirect. The possibility that a
small number of long-distance signals might control a
multitude of developmental processes arises through the
potential for differences in tissue sensitivity, fluctuations
in hormone levels and differences in the nature of responses
of different tissues to the same hormone. Consequently,
particular hormones may influence numerous processes
seemingly simultaneously, yet independently. Even so,
long-distance signalling is involved in processes as diverse
as root–shoot balance, senescence, branching, flowering,
nodulation, stress responses and nutrient uptake. Through
comparison of even a few different developmental processes, progress can be made to reveal the true complexity
of plant development. Using this approach it is also clear
that many unknown signals may be involved.