SYMBIOTIC AND NONSYMBIOTIC HEMOGLOBIN GENES OF CASUARINA-GLAUCA

Casuarina glauca has a gene encoding hemoglobin (cashb-nonsym). This g ene is expressed in a number of plant tissues. Casuarina also has a se cond family of hemoglobin genes (cashb-sym) expressed at a high level in the nodules that Casuarina forms in a nitrogen-fixing symbiosis wit h the actinomycete Frankia. Both the nonsymbiotic and symbiotic genes retained their specific patterns of expression when introduced into th e legume Lotus corniculatus. We interpret this finding to mean that th e controls of expression of the symbiotic gene in Casuarina must be si milar to the controls of expression of the leghemoglobin genes that op erate in nodules formed during the interaction between rhizobia and le gumes. Deletion analyses of the promoters of the Casuarina symbiotic g enes delineated a region that contains nodulin motifs identified in le gumes; this region is critical for the controlled expression of the Ca suarina gene. The finding that the nonsymbiotic Casuarina gene is also correctly expressed in L. corniculatus suggests to us that a comparab le nonsymbiotic hemoglobin gene will be found in legume species.