STRESS RESPONSES OF TONOPLAST PROTEINS - AN EXAMPLE FOR MOLECULAR ECOPHYSIOLOGY AND THE SEARCH FOR ECO-ENZYMES

Molecular ecophysiology aims at understanding ecological adaptations a t the level of molecules, and vice versa, the role of molecules in the ecological comportment of whole organisms. Hence, it continuously mov es up and down a ladder of systems characterized by different levels o f scaling like ecosystems and habitats, whole organisms, organs, tissu es and cells, membranes and molecules. Membranes with controlled trans port mechanisms are essential for the separation from and contact with the environment. The vacuole of plant cells is an intermediary or per manent sink for solutes which are resources in metabolism or waste com pounds. Therefore the transport molecules of the tonoplast-the membran e separating the vacuole from the cytoplasm-play a key role in stress responses. Among the membrane-transport enzymes of the tonoplast the H +-pumping V0V1-ATPase has been characterized structurally and function ally, and it has recently been recognized as an enzyme both serving st ress responses and undergoing stress-related modifications. Therefore, we call it an 'eco-enzyme'. We define an eco-enzyme as an enzyme whic h shows ecophysiological reactions by (i) mediating adaptations (i.e. in contrast to a house-keeping enzyme), and (ii) undergoing modificati on itself (i.e. in contrast to a stress enzyme). The H+-pumping tonopl ast pyrophosphatase is known structurally, but its function in the who le plant remains enigmatic and therefore also its role in ecophysiolog y. Secondary-active transporters are known to occur and to be essentia l in stress responses, but their molecular identity and therefore thei r precise role in molecular ecophysiology is as yet unknown.