Stress responses of tonoplast proteins: an example for molecular ecophysiology and the search for eco-enzymes

Molecular ecophysiology aims at understanding ecological adaptations at the level of molecules, and vice versa, the role of molecules in the ecological comportment of whole organisms. Hence, it continuously moves up and down a ladder of systems characterized by different levels of scaling like ecosystems and habitats, whole organisms, organs, tissues and cells, membranes and molecules. Membranes with controlled transport mechanisms are essential for the separation from and contact with the environment. The vacuole of plant cells is an intermediary or permanent sink for solutes which are resources in metabolism or waste compounds.Therefore, the transport molecules of the tonoplast—the membrane separating the vacuole from the cytoplasm—play a key role in stress responses. Among the membrane-transport enzymes of the tonoplast the H+-pumping V0V,-ATPase has been characterized structurally and functionally, and it has recently been recognized as an enzyme both serving stress responses and undergoing stress-related modifications. Therefore, we call it an ‘eco-enzyme’. We define an eco-enzyme as an enzyme which shows ecophysiological reactions by (i) mediating adaptations (i.e. in contrast to a house-keeping enzyme), and (ii) undergoing modification itself (i.e. in contrast to a stress enzyme). The H+-pumping tonoplast pyrophosphatase is known structurally, but its function in the whole plant remains enigmatic and therefore also its role in ecophysiology. Secondary-active transporters are known to occur and to be essential in stress responses, but their molecular identity and therefore their precise role in molecular ecophysiology is as yet unknown.