ABA- and cADPR-mediated effects on respiration and filtration downstream of the temperature-signaling cascade in sponges.

Recently, the thermosensing pathway in sponges (Porifera) was elucidated. The thermosensor triggering this cascade is a heat-activated cation channel, with the phytohormone abscisic acid (ABA), cyclic ADP-ribose (cADPR) and calcium acting as intracellular messengers, similarly to the drought-stress signaling cascade in higher plants. Here, we investigated the functional effects downstream of the temperature-signaling pathway in Axinella polypoides (Porifera, Demonspongiae). Short-term stimulation followed by long-term depression of amino acid incorporation, oxygen consumption and water filtration were observed after exposure of the sponge to a brief heat stress or to micromolar ABA. These effects could be prevented by the targeted interruption of the signaling pathway either at the level of the cation channel thermosensor or at the level of the cADPR-induced intracellular calcium increase. Moreover, release of cyclase activity into the sea water and generation of extracellular cADPR were observed following brief heat stress. Intact sponge cells were sensitive to extracellular cADPR and addition of purified cyclase increased sponge respiration similarly to heat stress. This is the first observation of functional effects exerted on Metazoa by the phytohormone ABA: conservation of the ABA/cADPR stress-signaling cascade points to its early evolution in a common precursor of modern Metazoa and Metaphyta. The functional effects induced by extracellular cyclase/cADPR suggest an evolutionary origin of cADPR as an ancient stress hormone in Porifera.