A Response to Stress Does Not Necessarily Constitute Adaptation to Stress.

Abiotic stress causes a host of molecular modifications in plants. Hundreds of genes respond to any specific stress and their products may or may not have a role in adaptation to stress. The physiological role of most of these molecular modifications is yet unknown. Better understanding of the role of physiological and developmental modifications caused by stress exists at the whole plant level.

Since the beginning of the use of molecular genetics in dissecting plant stress response and stress adaptation there has been a trend to assume that any molecular stress response has a role in stress adaptation or that a stress responsive gene must also be stress adaptive. In recent years more care is being exercised in assuming such an obligatory and automatic link, especially since some modifications caused by stress were recognized as steps in programmed cell death rather than adaptation. However, once in a while the assumption that a specific molecular stress response must be adaptive is still being put forward.

An educational case is the paper by Parida et al. (2005).* This study revealed that a certain low molecular weight protein (designated as SSP-23) disappears from the cytosol of cells of leaves of a mangrove (salt tolerant) plant subjected to salinity stress. The authors recognized that protein breakdown under salinity stress is a common observation also in other plant species. They speculated that the breakdown of this protein (into amino acids) serve to provide for osmotic adjustment which helps adaptation to salinity. There is no qualitative data on the amino acids resulting from this protein breakdown and whether these amino acids have any osmotic value. There is no quantitative estimate or even a theoretical calculation of the expected reduction in osmotic potential due to these putative amino acids. Still the main result of this paper as stated by the authors is that "this protein is salt-sensitive and has a possible role in salt adaptation" (Abstract).

(*) Asish Kumar Parida, Bhabatosh Mittra, Anath Bandhu Das, Taposh Kumar Das and Prasanna Mohanty (2005). High salinity reduces the content of a highly abundant 23-kDa protein of the mangrove Bruguiera parviflora. Planta 221:135-140.

Abstract A significant decrease in the amount of a protein, whose migration in two-dimensional gel electrophoresis corresponds to an apparent molecular mass of 23 kDa and pI=6.5, was observed in leaves of NaCl-treated Bruguiera parviflora (Roxb.) Wt. & Arn. ex Griff. seedlings. This particular salt-sensitive protein, designated as SSP-23, almost disappeared after 45 days of treatment in 400 mM NaCl as compared to untreated seedlings (0 mM NaCl) where the presence of the protein was significant. A polyclonal antibody raised against the 23-kDa protein was used to determine the subcellular localization of this protein in leaves by cross-reaction with proteins from isolated chloroplasts, mitochondria, peroxisomes and cytosol fractions on Western blots. SSP-23 was confirmed to be localized in the cytosol by immunoblotting. The disappearance of SSP-23 as a result of high NaCl treatment suggests that this protein is salt-sensitive and has a possible role in salt adaptation.