Possible role of inositol phospholipid signal transduction system in Dunaliella salina under hypoosmotic shock

摘要

IN the course of plant life, many physiological and biochemical processes are controlled by extracellular signals (light, temperature, gravity, water, hormone, etc.). How the cells perceive these environmental signals, transduce them inwards and leadto alterations of intracellu-lar physiology has become a focus in life science these years. Research in this field was started earlier and further developed in animal cells. When specific receptors on the external surface of animal cells perceive an extracellular signal, specific phospholipase C (PLC) isozymes are activated, signaled by the cleavage of inositol phospholipids to release second messengers, i.e. inositol 1, 4,5-trisphosphate (IP_3) diacylglycerol (DAG). IP_3 could release Ca~(2+) from intra-cellular Ca~(2+) stores, and thus stimulate the activity of Ca~(2+)-CaM system or Ca~(2+)-dependent protein kinases. As DAG may activate protein kinase C (PKC), the activities of certain functional proteins are promoted. Since the discovery of inositolphospholipids in carrot plasma membrane by Boss and Massel (1985), people have characterized the existence of all the above signal transduction components (inositol phospholipids, protein kinases, PLC, IP_3, DAG, etc.) within photosynthetic organisms, and studied their biochemical characteristics, metabolism and responses to outside stimuli. The accumulative evidence shows that inositol phospholipid signal transduction system exists in plants in a form similar to that of the animal model. Dunaliella salina is a naked single-celled green alga which can grow under saline conditions of 0.5--5 mol/L NaCl and produce glycerol as its main osmotically active compound. It is halo-tolerant and very responsive to osmotic stimuli, so it is used as a model systemto study coupling mechanisms between stimulus and cell responses. Although the experiments done have indicated that inositol phospholipid signaling system in D. salina may participate in extracellular signal transduction, evidence on the role of the second messengers ( such as IP_3) in signal transduction and its coupling to physiological alterations is still lacking. Here, we show the relationship between the degradation of polyphosphoinositides and the concomitant elevation of IP_3 levels in D. salina cells during hypoosmotic shock. By using the G protein activator and PLC inhibitor, we preliminarily prove the stimulus-response coupling mechanism in D. salina, i.e. hypoosmotic shock --> G protein --> PLC-->IP_3 --> glycerol conversion.