Cyclic adenosine 3’:5’-monophosphate and cyclic guanosine 3’:5’-monophosphate, common- ly know as cAMP and cGMP, are key second messengers in living organisms ranging from bacteria to Homo sapiens. Cyclic AMP and cyclic GMP are produced from ATP and GTP by the action of adenylyl cyclase and guanylyl cylase and are hydrolyzed to AMP and GMP by the cyclic nucleotide phosphodie- sterase. The natural occurrence of cyclic nucleotides in higher plants is now established, as is the presence of enzymes involved in their metabolism.

Nitric oxide (NO), synthesized from L-arginine by NO synthases, is a small, diffusible, highly reactive molecule with dichotomous regulatory roles under physiological and pathological con- ditions. Apoptosis plays an important role in the development of the organism but also under various pathological conditions. NO can exert both pro- and anti-apoptotic effects, depending on the conditions and cell type.

Stentor and Blepharisma possess photoreceptor systems that render the cells capable of avoiding light. The observed photobehaviour is largely the result of a step-up photophobic response displayed by both ciliates. The photophobic response caused by a sudden increase in light intensity starts with a cessation of ciliary beating, then a period of ciliary reversal followed by renewed forward movement. Ciliary reversal correlates with the generation of an action potential, which is elicited by a photoreceptor potential as a result of light absorption by cellular pigments.