The remarkable plasticity of plant ontogeny is shaped by
hormone pathways, which not only orchestrate intrinsic
developmental programs, but also convey environmental
inputs. Several classes of plant hormones exist, and
among them auxin, brassinosteroid and gibberellin are
central for the regulation of growth in general and of cell
elongation in particular. Various growth phenomena can
be modulated by each of the three hormones, in a sometimes synergistic fashion, suggesting physiological redundancy and/or crosstalk between the different pathways.
Whether this means that they target a common and unique
transcriptome module, or rather separate growth-promoting transcriptome modules, remains unclear, however.
Nevertheless, while surprisingly few molecular mediators
of direct crosstalk in the proper sense have been isolated,
evidence is accumulating for complex cross-regulatory
relations between hormone pathways at the level of transcription, as exemplified in root meristem growth. The
growing number of available genome sequences from
the green lineage offers first glimpses at the evolution of
hormone pathways, which can aid in understanding the
multiple relationships observed between these pathways
in angiosperms. The available analyses suggest that
auxin, gibberellin and brassinosteroid signalling arose
during land plant evolution in this order, correlating with
increased morphological complexity and possibly conferring increased developmental flexibility.