General Biology Seminar

We study the symbiosis of rhizobia with plants, which results in formation of complex nodule structures on the plant roots.  The bacteria invade the plant in a cooperative process involving major changes in gene expression of both microbe and host.  The bacteria are released into host cells and differentiate to bacteroids that carry out nitrogen fixation.  Bacterial transcription during symbiosis is regulated by multiple genetic and environmental inputs.  Early stages are characterized by specific exchange of chemical signals such as plant-derived flavonoids and bacterially synthesized Nod Factor.  Nitrogen fixing bacteroids are regulated by signal transduction triggered by low oxygen, and by nodule specific plant peptides.  The intermediate stages of symbiosis are poorly understood.  We used laser-capture microdissection (LCMD) to analyze bacteria at different stages of symbiosis.  This revealed a set of genes that are selectively up-regulated after nodulation begins, then shut off again before the final differentiated state.  These intermediate-stage genes include some that are involved in outer membrane and peptide maintenance. Based on transcription analysis and mutant phenotype, we hypothesize that the stringent response is a key element in the tranition from bacteria to bacteroids.