Cyclic AMP in rhizobia and symbiotic nodules

Cyclic adenosine 3',5'-monophosphate (cAMP) content of various cultured rhizobia strains and tissues of legumes and non-leguminous plants was measured by enzyme immunoassays. Most rhizobia, cultured for 44 to 165 h, contained cAMP ranging from 0.6 to 5 pmol mg(-1) protein except for Azorhizobium caulinodans ORS571. The culture media also contained varying amounts of cAMP depending on the strain of rhizobia. Azorhizobium cells and their media contained no detectable cAMP. Nodules from most legumes and non-legumes had cAMP contents ranging from 2-70 pmol g(-1) f.wt. However, nodules from Sesbania rostrata, Crotalaria spectabilis and Parasponia andersonii showed undetectable cAMP levels, and those from Glycine mas and Vigna angularis occasionally showed levels below the detection limit. The leaves of non-legumes mostly had cAMP levels below detection limit (approx. 1.0 pmol g(-1) f.wt), while the leaves of a few legumes occasionally had detectable cAMP. The possible role of cAMP as a symbiotic signal is discussed. (C) 1997 Annals of Botany Company.