Regulation of secondary antioxidants and carbohydrates by gamma-aminobutyric acid under salinity-alkalinity stress in rice (Oryza sativa L.).

Gamma-aminobutyric acid (GABA) is known to improve tolerance against abiotic stresses but less studied for salinity-alkalinity stress (SAS). In the present study, GABA regulation of secondary antioxidants and carbohydrates was studied in rice in the presence and absence of SAS. About 1.5 mM GABA, 200 mM SAS, GABA + SAS were applied to 5-day old seedlings, and thereafter measurements were done in shoots and roots at 24, 48, 72 h in rice cultivars CSR 43 (tolerant) and Pusa 44 (susceptible). SAS was applied in molar ratio of 1:9:9:1 of NaCl:Na2SO4:NaHCO3:Na2CO3. Peroxidases (POX), polyphenol oxidases (PPO), lignin, flavonoids and phenolics from secondary metabolism and invertases, hexoses, sucrose and starch from carbohydrate metabolism were studied. Pusa 44 increased soluble POX, lignin, flavonoids in shoots but deficient in roots during stress period but improved under GABA + SAS. CSR 43 increased soluble POX, lignin, flavonoids in roots consistently throughout the stress period and also improved under GABA + SAS. Early increase in cell wall POX/PPO under SAS was seen in CSR 43 only, while Pusa 44 improved this under GABA + SAS. During stress period, CSR 43 showed an increasing trend of cell wall invertase activity, sucrose, sucrose-to-hexose ratio and starch in roots but Pusa 44 showed poor such response but Pusa 44 improved starch, sucrose, sucrose-to-hexose ratio by significant amount in both shoots and roots under GABA + SAS. The overall study indicated GABA as an important regulator of secondary and carbohydrate metabolisms. Besides improving secondary antioxidants, GABA under stress may improve cellular reserves like starch and protective sugars like sucrose.

Hydrogen peroxide and nitric oxide regulation of phenolic metabolism under water stress and ABA in wheat.

Wheat cultivar PBW644 (drought tolerant) and PBW343 (drought sensitive) were found as ABA-higher sensitive and ABA-lesser sensitive, respectively, in the screen of six wheat cultivars. Both cultivars were studied for H2O2 (ROS)/nitric oxide (NO)-regulation of growth and phenolic metabolism under ABA and water stress (WS) by supplying ROS/NO producers as well as scavengers. Endogenous ROS/NO under ABA/WS increased growth, such effect was higher in PBW644. In PBW343, reduced growth under WS was improved by exogenous ROS/NO. Exogenous ROS/NO under ABA/WS decreased lignin and increased phenolics in PBW343 but such relation was not found in PBW644. Endogenous NO under WS increased flavonoids in both cultivars. Both ROS/NO under ABA/WS increased flavonoids in PBW644, however, in PBW343, only ROS increased these in roots. Under WS, PBW644 showed higher levels of cell wall peroxidase (CW-POX) and lower levels of soluble peroxidase (S-POX) than PBW343. However, under ABA, it showed higher levels of both peroxidases. ROS/NO signals under ABA increased both types of POX in both cultivars while under WS, these signals increased both types in PBW343 but CW-POX only in PBW644. Polyphenol oxidases were ABA-upregulated in PBW644 only. Under WS, these enzymes were maintained higher in PBW343. This study indicated that tolerant cultivar under WS contained sufficient endogenous ROS/NO signalling to which susceptible cultivar lacked but showed improvement on exogenous applications. Secondly, tolerant cultivar was using less phenolic activity under WS which could be due to the presence of sufficient levels of primary antioxidants.