Abstract
Transgenic tobacco plants (M51-1) constitutively over-expressing a modified gene for the proline biosynthetic enzyme △2-pyrroline-5-carboxylate synthetase (P5CSF129A) and the corresponding wild-type plants (WT) were compared during drought or heat stress and under combination of both stresses. The proline content in M51-1 was several times higher than in WT plants. Under optimal conditions, the transpiration rate and stomatal conductance of M51-1 plants were lower than those in WT plants. The differences in net photosynthetic rate were not significant and water use efficiency and contents of chlorophyll and xanthophyll cycle pigments were higher in M51-1 than in WT plants. Drought induced by cessation of watering for 7 d resulted in decrease of all gas exchange parameters and chlorophyll content, but in an increase of the content of xanthophyll cycle pigments and degree of their de-epoxidation. After application of heat stress (40°C/60 min) to control or water-stressed plants the gas exchange parameters decreased considerably. Short-term heat stress alone, however, did not affect pigment contents. The responses of M51-1 and WT plants to the tested stresses did not differ significantly. Therefore, a decisive contribution of elevated proline content to drought or heat stress tolerance of tobacco was not proved.
Highlights
IntroductionOsmotic adjustment (lowering of the osmotic potential in order to maintain the pressure potential) occurs in response to water stress induced by drought, salinity or low temperature
In many plant species, osmotic adjustment occurs in response to water stress induced by drought, salinity or low temperature
The differences in net photosynthetic rate were not significant and water use efficiency and contents of chlorophyll and xanthophyll cycle pigments were higher in M51-1 than in wild-type plants (WT) plants
Summary
Osmotic adjustment (lowering of the osmotic potential in order to maintain the pressure potential) occurs in response to water stress induced by drought, salinity or low temperature. The step involves the reduction of P5C to proline by ∆1-pyrroline5-carboxylate reductase (P5CR). Proline biosynthesis steadily occurs in the cytosol, while it is augmented to the chloroplasts during stress conditions 4. Elevation of proline content coincided with modulation of the enzyme activities (increase in case of P5CR and decrease in PDH, respectively), as well as with changes in expression of the corresponding genes 6,9,10. Two closely related genes, P5CS1 and P5CS2, were identified in Arabidopsis 11 The former one is involved in regulation of development, while the latter one is stress-responsible 4. Two genes for proline degradation, MsPHD1 and MsPHD2 were distinguished in Medicago sativa 9
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