The role of proline in plant cells and tissues has been the subject of intensive research (Dashek and Erickson, Bot. Rev. 47:349-385, 1981). It has been repeatedly observed that plants accumulate a considerable amount of free proline in response to different biological and environmental stresses. Though the physiological significance of this accumulation is not clear, it has been suggested that it is either a consequence of the stress or alternatively an adaptive response of the plant in terms of survival. It seemed therefore interesting to us to analyze the effect of water stress on pro1 mutants whose lethality is overcome by growth on media supplied with proline.
PEG (MW 6000) solutions at different concentrations (20%, 30% and 40%) were used to simulate water stress conditions. Intact seedlings (1-2 leaf stage) and leaf discs (5mm) were treated for 24 hours with PEG solutions and then analyzed for their proline and soluble proteins content. For whole seedlings, shoots and roots were analyzed separately. As far as mutants are concerned, care was taken to initiate PEG treatment before any sign of lethality due to the proline requirement. The free proline content, in response to PEG treatments, shows a significant increase in shoots and leaf discs but not in roots where the content is unaffected. The threshold of PEG induction (concentration of PEG at which an accumulation of proline begins to take place) is lower in mutants than in normals.
The polypeptide pattern of soluble proteins present in roots seems in agreement with the hypothesis that the mutant is more sensitive to PEG than the control. Both mutant and normal show changes in some polypeptide classes as a consequence of PEG treatments; however, they differ in the PEG concentration required to induce such changes. So mutant roots begin to show modifications in the polypeptide pattern at 20% PEG while normal roots require 30% or 40%.
Chiara Tonelli and Alcide Bertani
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