An Examination of Leaf Morphogenesis In The Moss, Physcomitrella Patens
Abstract
Physcomitrella patens is a simple model plant belonging to the bryophytes, which
diverged from the tracheophytes approximately 500 million years ago. The leaves of the
moss are similar in form to vascular plant leaves although leaves evolved independently
in the bryophyte and tracheophyte lineages. Close examination of the morphology of
Physcomitrella leaves and investigation of the morphogenetic processes that result in the
leaf form and of the hormonal and genetic regulation of those processes will elucidate the
evolutionary trajectory of moss leaves.
Photomicroscopy and measurement of moss leaves were performed to provide
detailed descriptions of leaves in strains of Physcomitrella that exhibited normal and
aberrant morphology. Low concentrations of two major phytohormones, auxins and
cytokinins, were applied to growing moss cultures to investigate their effects on leaf
morphogenesis. A bioinformatic analysis of homologues of vascular plant genes that are
involved in leaf development was performed to identify candidate genes that may have
been co-opted for regulation of leaf morphogenesis during the early course of plant
evolution.
The transition from the basal leaf form to the adult form is gradual for all of the
heteroblastic features of Physcomitrella leaves with the possible exception of the midrib.
Low concentrations of exogenous auxin and cytokinin stimulate cell expansion and cell
divisions respectively and auxin may be a key regulator of leaf heteroblasty.
Homologues of almost all tracheophyte genes that are known to be involved in leaf
morphogenesis are present in the Physcomitrella genome. However, few of the
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Physcomitrella homologues exhibit both a high degree of sequence similarity to the
vascular plant genes and high levels of expression in leaves. Of these, class I and class II
homeodomain-leucine zipper genes are predicted to play key roles in leaf development in
the moss. Other genes may have been co-opted to regulate the processes of cell division,
cell expansion and adaxialization/abaxialization of the midrib in bryophyte leaves.
Preliminary models of auxin-cytokinin activity and of genetic and hormonal regulation of leaf morphogenetic processes in Physcomitrella provide testable hypotheses for further
investigation.