EXPRESSION OF CELL-CYCLE GENES DURING SESBANIA-ROSTRATA STEM NODULE DEVELOPMENT

Upon infection of Sesbania rostrata with Azorhizobium caulinodans, nod ules are formed on roots and stems. Stem nodules develop from abundant ly distributed dormant root primordia. To acquire more insight into th e meristem organization during stem nodule development, the expression patterns of a mitotic B1-type cyclin gene (Sesro; CycB1;1), a cyclin- dependent kinase gene (Cdc2-1Sr), and a histone H4 gene (H4-1Sr) of S. rostrata were followed by in situ hybridization, Cdc2-1Sr transcripts were found in all cells of uninfected and infected root primordia. In uninfected root primordia, Sesro;CycB1;1 transcripts were detected in a few cells of the apical root meristem whereas H4-1Sr transcripts we re abundant in this region. Interestingly, after inoculation with A. c aulinodans, H4-1Sr transcripts disappeared in the root meristem and a patchy pattern of Sesro;CycB1;1 and H4-1Sr expression appeared in the cortex of the root primordium, reflecting the formation of globular no dule primordia. When bacterial invasion started, a distal nodule meris tem was delimited wherein Sesro;CycB1;1 and H4-1Sr expression was conc entrated. Approximately 1 week after inoculation, meristem activity ce ased, indicated by the loss of Sesro;CycB1;1 and H4-1Sr expression.