An Examination of Leaf Morphogenesis In The Moss, Physcomitrella Patens
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 ii 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.