Chapter 16 - Molecular symbiotic interactions of cyanobacterial association in nonvascular seedless plants

Abstract

Cyanobacteria form a variety of associations with nonvascular seedless plants ranging from the almost accidental to very close symbiosis. Nostoc is a widespread cyanobacterial genus that is a common symbiont and the association between Nostoc and hornworts is an excellent example of a close symbiosis. Cyanobacteria form symbiotic associations with a wide range of eukaryotic hosts including plants, fungi, sponges, and protists. The cyanobacterial symbionts are often filamentous and fix N2 in specialized cells known as heterocysts, enabling them to provide the host with fixed nitrogen and, in the case of nonphotosynthetic hosts, with fixed carbon. Nonvascular seedless plants associations with nitrogen-fixing cyanobacteria are of specific interest because the cyanobacteria function in supplying fixed nitrogen to the plant tissue and are, in many ways, analogous to the symbiotic relationship between agronomically important legumes\\and rhizobia. The cyanobionts undergo morphological and physiological changes, including reduced growth rate and CO2 fixation, and enhanced N2 fixation, and release to the plant much of the dinitrogen fixed. Although current understanding of the molecular basis for many of the interactions involved in cyanobacteria–nonvascular seedless plants symbioses is poor, continuing advances in molecular genetic techniques for symbiotically competent cyanobacteria should lead to more rapid advances in this area.