Chloroplasts are the major site for the production of various sugars and sugar alcohols
By Ashwani Kumar
| September 16th 2009 06:33 PM | Print
Chloroplasts are the major site for the production of various sugars and sugar alcohols.
However their number in cells has been found to be regulated by the sugars themselves in presence of appropriate growth regulators.
Sugars and sugar alcohols have a protective role in osmoregulation and also in protection of enzymes and pigment molecules against adverse effects of abiotic stresses. Inositol, mannitol, trehalose, sucrose and glycerol did not bring any significant alteration in the carboxylase activity of RuBISCO in Sebania sesban ( Sivakumar, 2002). However sorbitol caused a notable reduction in carboxylase activity. Interestingly with the exception of sorbitol, all the compatible sugars and sugar alcohols tested significantly lowered oxygenase activity of RuBISCO in Sesbania sesban. NaCl also had suppressive effect on carboxylase activity while it promoted oxygenase. ( Sivakumar, 2000). Maize is C4 plant which may not have recordable Oxygenase activity. It remains to be seen what effect it will have on the RuBISCO oxygenase activity. Most markedly NaCl induced enhancement in oxygen activity of RuBISCO was totally nullified in the presence of any of the compatible sugars or sugar alcohols tested including sorbitol, even at concentrations as low as 50 mM. Unlike proline whose presence brought about an additive inhibitory effect on the NaCl suppressed carboxylase activity of RuBISCO, no change/ recovery with NaCl included suppression in the carboxylase activity was observed in presence of any of the compatible sugars/sugar alcohols. The initial and total activities of RuBISCO in fully expanded tomato leaflets increase ( Ca 40 % ) due to stress treatments. GB application increased slightly in initial and total activity of RuBISCO in well watered and salt stressed tomato which is in accordance with earlier studies with A. halophytica ( Incharoensakdi, et al 1986).
Stitt ( 1991) proposed that accumulating carbohydrate could lead to a direct inhibition of photosynthesis through mechanical damage by large starch grains or inorganic phosphorus ( Pi) limitation due to initiation of sucrose synthesis but indirect or adaptive regulation is probably more important, involving decreases in the amount of key photosynthetic enzymes, including RuBISCO. Whether down regulation of RuBISCO
and /or upregulation of RuBP regeneration are enough to avoid limitation of RuBP regeneration to photosynthesis during photosynthesis acclimation to elevated CO2 in rice is not clear now ( Mitchell, 200).
It has generally accepted that PEPC is one of the rate limiting steps of the C4 cycle, especially under low CO2 condition ( Von Caemmerer and Furbank, 1999). To evaluate the physiological importance of PEPC phosphorylation its regulation due to salinity and ultimate influence on PEPC activity determinations shall be made on contents and activity of PEPC and if possible on its phosphorylated state.
1. Single gene controls glycine betaine accumulation in Zea mays under saline conditions ( Saneoka et al 1995, Quan, 2004)
2. The compatible osmolytes - Low molecular weight sugars, organic acids, polyols and N containing compounds such as amino acids, amides, imino acids, ecotoine ( 1, 4, 5, 6, tetrahydro-2-methyl-4-carboxylpyrimidine) proteins and quaternary ammonium compounds develop under osmotic stress. They are reported to have protective or suppressive role on the enzyme activity of RuBISCO and / or PEP case.
3. In vascular plants PEPC are regulated by reversible phosphorylation at the conserved Ser located near the N terminus ( Vidal and Chollet, 1997).
4. Most PEPC are subject to allosteric regulation. Effectors of monocot plants are Gly or Ala ( Tovar-Mendez, 2000, 2001).
5. Enhanced phosphorylation of C4 PEPC under salt stress ( Gracia-Maurino et al 2003) also suggest multiple cues for regulatory phosphorylation. Protein kinase involved in this phosphorylation is Calcium independent and highly specific to PEPC with estimated molecular weights of 30 and 37 kDa.
6. Nimmo et al. (2001) detected 55 kDa inhibitor protein in maize leaf cell extracts and proposed that its function is to mark the basal level of the kinase activity. PEPCase could be readily inactivated under mild oxidative conditions and reactivated efficiently by thioredoxin mediated reduction.