THE PHYSIOLOGICAL FUNCTIONS OF NITROGENOUS SOLUTES ACCUMULATED BY HIGHER PLANTS SUBJECTED TO ENVIRONMENTAL STRESS
The contribution of the so-called osmotic solutes to osmotolerance of higher plants still remains a strong matter of debate. We have assessed some of the possible functions of two of them i.e. proline (PRO) and glycine betaine (GB) using a simple in vitro rape leaf discs (RLDs) assay. Some of the processes associated with PRO accumulation were investigated with RLDs induced to accumulate PRO through application of osmotic upshift using the non-permeant osmoticum PEG 6000 (-3 MPa). Turgid RLDs devoid of endogenous GB were also treated with exogenous GB to investigate the actual compatibility of this solute. When subjected to osmotic upshift RLDs were found to accumulate PRO up to 600 µmol.g-1 DW (i.e. 6% DW) within 24 h. This response depended not only on stimulation of PRO synthesis and inhibition of PRO degradation but also on negative effects on protein synthesis and positive ones on protein degradation. The 6% DW of PRO accumulated corresponded to the 6% DW of proteins lost by treated RLDs. Amino acids released from degradation of RubisCO-LSU were assumed to be recycled to produce PRO. Soluble non structural carbohydrates (NSCs) were also strongly accumulated prior to PRO and this according to current model of osmotic adjustment in glycophytic plants. We have demonstrated that the PRO response results from uncontrolled metabolic disorders that could be curable by suitable antidotes such as phytooxylipines, aliphatic polyamines, and interestingly by the two other putative osmolytes citrulline and GB. The function of GB, especially its compatibility, has been further studied using turgid RLDs that were first shown to absorb and to accumulate this compound up to high level. The GB induced effects on leaf discs were then compared to those exerted on spinach leaf discs (SLDs) that already contained significant amount of endogenous GB. In Rape plants, the non producers of GB, this compound provoked detrimental effects especially upon nitrogen metabolism. Thus glutamine and glycine accumulated when GB was provided under light conditions. This was associated with inhibition of protein synthesis and stimulation of proteolysis. One target for the GB detrimental effect could be the conversion of the photo respiratory produced glycine into serine that takes place in mitochondria. In contrast in SLDs, exogenously supplied GB was also found to be absorbed and accumulated rapidly but it did not induce any of the damaging effects observed in RLDs. Our results clearly demonstrate that the compatibility of GB is not a general feature in higher plants.
Larher, F.R., Gagneul, D., Deleu, C. and Bouchereau, A. (2007). THE PHYSIOLOGICAL FUNCTIONS OF NITROGENOUS SOLUTES ACCUMULATED BY HIGHER PLANTS SUBJECTED TO ENVIRONMENTAL STRESS. Acta Hortic. 729, 33-41
osmotic stress, compatible solutes, proline, glycine, betaine, stress tolerance