Fungal elicitor stimulates a multicomponent defense response in cultured parsley cells (Petroselinum crispum). Early elements of this receptor-mediated response are ion f luxes across the plasma membrane and the production of reactive oxygen species (ROS), sequentially followed by defense gene activation and phytoalexin accumulation. Omission of Ca 2؉ from the culture medium or inhibition of elicitor-stimulated ion f luxes by ion channel blockers prevented the latter three reactions, all of which were triggered in the absence of elicitor by amphotericin B-induced ion f luxes. Inhibition of elicitor-stimulated ROS production using diphenylene iodonium blocked defense gene activation and phytoalexin accumulation. O 2 ؊ but not H 2 O 2 stimulated phytoalexin accumulation, without inducing proton f luxes. These results demonstrate a causal relationship between early and late reactions of parsley cells to the elicitor and indicate a sequence of signaling events from receptor-mediated activation of ion channels via ROS production and defense gene activation to phytoalexin synthesis. Within this sequence, O 2 ؊ rather than H 2 O 2 appears to trigger the subsequent reactions.

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A series of systematically modified cyclic AMP (cAMP) analogues, including newly synthesized benzimidazole ribofuranosyl 3',5'-monophosphates was used to map the essential molecular interactions between cAMP and the monoclonal antibody 4/2C2 (mab 4/2C2) directed against 2'-O-succinoyl cAMP [Colling, Gilles, Nass, Moka & Jaenicke (1988) Second Messengers Phosphoproteins 12, 123-133]. Its paratope binds the purine base in syn conformation by dipole-dipole interactions and hydrophobic forces and/or stacking interactions. The ribose phosphate moiety is recognized by a combination of charge interactions and H-bonds to the exocyclic and the 5'-oxygen atoms and a hydrophobic interaction at the 2'-position. There is no regioselectivity for the exocyclic oxygen atoms. Compared with the known types of binding, mab 4/2C2 thus shows a new combination of molecular interactions which may be the basis of its strikingly specific recognition and binding of the cyclic adenylates. On this account mab 4/2C2 may become an important tool in studies on cAMP metabolism.

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