Sujata Bhargava

Variation in the Antioxidant Metabolism of Drought Tolerantand Drought Susceptible Varieties of Sorghum Bicolor (L.) Moench. Exposed to High Light, Low Water and High Temperature Stress

Summary Seedlings of four varieties of Sorghum bicolor differing in drought tolerance under field conditions wereexposed to three components of drought stress in the laboratory. They were exposed to high light (10,000 lux), high temperature (36 ± 2 °C, and low water stress (−0.5 MPa) in different experiments. After 72 hours of exposure to each of these stresses, changes in lipid peroxidation, and the activities of catalase, superoxide dismutase and guaicol dependent peroxidase were assayed. Out of the four varieties, CSV-5, which is a drought susceptible variety, showed maximum increases in the level of malondialdehyde, which is a product of lipid peroxidation, under all three stress conditions. IS-22380, PA-59 and E-36 showed lesser lipid peroxidation. Superoxide dis mutase and catalase activities increased in the drought tolerant varieties. While IS-22380 seemed to adapt to stress conditions by manipulating the catalase levels, E-36 showed higher adaptability by manipulating the superoxide dismutase levels under water and temperature stress. Adaptation to light stress was manifested by increases in the levels of superoxide dismutase in the drought tolerant varieties. Thus, a drought tolerant variety can adapt to drought stress by diverse manipulations in the antioxidant system.

Salt and drought tolerance of sugarcane under iso-osmotic salt and water stress: growth, osmolytes accumulation, and antioxidant defense

In order to discriminate between the ionic and osmotic components of salt stress, sugarcane (Saccharum officinarum L. cv. Co 86032) plants were treated with salt-NaCl or polyethylene glycol-PEG 8000 solutions (−0.7 MPa) for 15 days. Both the salt and PEG treatments significantly reduced leaf width, number of green leaves, and chlorophyll stability index. Osmotic adjustment (OA) indicated that both the stresses led to significant accumulation of osmolytes and sugars. Salt stressed plants appeared to use salt as an osmoticum while the PEG stressed plants showed an accumulation of sugars. Oxidative damage to membranes was not severe in plants subjected to salt or PEG stress. The salt stressed plants showed an increase in the activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX), while PEG stress led to an increase in SOD but not APX activity as compared to the control. Thus, results indicate that the iso-osmotic salt or PEG stress led to differential responses in plants especially with respect to growth, OA, and antioxidant enzyme activities.

Stem sugar accumulation in sweet sorghum – Activity and expression of sucrose metabolizing enzymes and sucrose transporters

Sugar metabolism was studied in sweet sorghum (SSV74) that is known to store sugars in the mature internodes and which is reported to give grain yields twice that of a grain sorghum variety (SPV1616). Comparison of sugar accumulation in these two varieties was carried out at three stages of growth and in the upper and lower internodes. In spite of large differences in the level of sugar accumulation, osmolarity of the sap did not vary as significantly in the two varieties. Significant contribution of variety, stage and internode position was seen for the variation observed in sugar content. Though the activities of sugar metabolizing enzymes namely sucrose synthase (in the synthesis and cleavage directions), sucrose phosphate synthase and invertase (cytoplasmic and vacuolar) also varied in a stage- and internode-specific manner in the two varieties, these enzymes did not contribute significantly to the variation observed in sugar content. Transcriptional expression of one sucrose synthase (SUC1), two sucrose phosphate synthase (SPS2 and SPS3) and a vacuolar invertase (INV3) gene were lower in sweet sorghum as compared to grain sorghum. Sweet sorghum also showed lower expression of two sucrose transporters (SUT1 and SUT4), which correlated to higher sugar accumulation in this variety. Differential expression of the sugar metabolizing enzymes and sucrose transporters in sweet and grain sorghum suggest a role for signaling molecules and transcription factors in regulating sugar accumulation observed in the mature internodes of sweet sorghum, which needs to be investigated.

Bark and Fruit Extracts of Gmelina arborea. Protect Liver Cells from Oxidative Stress

Abstract The effect of Gmelina arborea Roxb. (Verbenaceae) bark and fruit aqueous extracts on paraquat- and hydrogen peroxide–induced oxidative stress was investigated using liver slice culture. Both paraquat and hydrogen peroxide were found to be cytotoxic as measured by release of lactate dehydrogenase from liver slice culture. Addition of bark and fruit extracts along with these cytotoxic agents led to a decrease in lactate dehydrogenase release. Activities of three antioxidant enzymes, namely super-oxide dismutase, catalase, and glutathione reductase, were found to increase on treatment with these pro-oxidants. Addition of the plant extracts along with the pro-oxidants suppressed the enzyme activities. The extracts also displayed antioxidant activity in in vitro radical scavenging assays. Results indicate that Gmelina bark and fruit extracts protected liver slice culture cells by alleviating oxidative stress–induced damage to liver cells.

Seasonal variation in antioxidant enzymes and the sprouting response of Gmelina arborea Roxb. nodal sectors cultured in vitro

Nodal sector explants of Gmelina arborea Roxb. showed seasonal variation in the sprouting of axillary buds in vitro. Explants from mature trees showed only 20% sprouting in summer, while those from seedlings and young trees showed over 85% sprouting in this season. In winter, there was a significant decrease in the sprouting response of explants from young and mature trees but not in that of seedling explants. We have attempted to correlate the sprouting response of the explants with their antioxidant status. Activities of three enzymes, namely ascorbate peroxidase, superoxide dismutase and guaiacol-dependent peroxidase which form a part of the antioxidant defense system of plants, were studied in the excised nodal sectors before and after placing them on culture medium. Prior to culture, higher activities of ascorbate peroxidase, superoxide dismutase and guaiacol-dependent peroxidase were observed during winter than during summer. During the culture period, ascorbate peroxidase, superoxide dismutase and guaiacol-dependent peroxidase showed variation depending on the season in which the explants were isolated, and the age of the donor. Axillary bud sprouting in vitro appeared to depend more on the physiological state of the donor, than on the oxidative stress generated during culture.

Genotypic variation in the photosynthetic competence of Sorghum bicolor seedlings subjected to polyethylene glycol-mediated drought stress

Eleven varieties of Sorghum bicolor, subjected to PEG-mediated drought stress were compared for their photosynthetic performance. The varieties differed in their relative water content over a range of PEG concentrations (0-25%). CO2 assimilation, stomatal conductance and the quantum yield of PSII electron transport decreased with increasing PEG concentrations in all varieties. However the intercellular CO2 concentration showed a nonlinear PEG concentration-dependent change. At lower PEG concentrations there was a decrease in the levels of intercellular CO2 concentration in all varieties that could be attributed to stomatal closure. At higher PEG concentrations, some varieties showed an increase in the intercellular CO2 concentration, indicating an inhibition of photosynthetic activity due to non-stomatal effects, while others did not. It was seen that the varieties differed in the stress thresholds at which stomatal and metabolic limitations to photosynthesis occur. These differences in the photosynthetic adaptation of Sorghum varieties could be useful in identifying genotypes showing large differences in photosynthetic adaptation, which could be useful in mapping photosynthetic traits for drought stress tolerance.

Provenance- and subculture-dependent variation during micropropagation of Gmelina arborea

Axillary shoot elongation, formation of multiple shoots and rooting of shoots were compared in nodal segment cultures of Gmelina arborea Roxb. from seedlings obtained from six provenances, over several subcultures. Provenance-dependent variation was observed with respect to these parameters. In addition, a subculture-dependent decrease was observed in multiple shoot formation and root induction. Seventy percent of the rooted plantlets were successfully hardened and transferred to soil. A transient decrease in photochemical efficiency was observed during the early stages of hardening, whereas ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) levels increased gradually as the plants acclimatized to photoautotrophic growth.

Over-represented promoter motifs in abiotic stress-induced DREB genes of rice and sorghum and their probable role in regulation of gene expression

Genes coding for drought response element binding (DREB) proteins regulate transcription of a large number of downstream genes involved in the plant response to abiotic stresses. However the regulation of DREB genes themselves is not well understood. Using a bioinformatics approach, we identified the over-represented motifs in promoters of DREB genes of sorghum and rice as compared to all the other promoters in their genomes. Aligned orthologous promoter pairs of sorghum and rice DREBs were then used to identify co-localized motifs from among the over-represented ones, assuming that such motifs were likely to play a regulatory role. Finally the motifs over-represented in sorghum DREBs in comparison to their rice orthologs were identified. Results indicated over-representation of motifs pertaining to calcium, light, sugar, and hormone signaling in the DREB promoters. The co-localized motifs in DREB promoters were mainly those involved in abscisic acid-, light- and calcium-mediated regulation. These motifs along with others pertaining to ethylene signaling were over-represented in sorghum DREB promoters as compared to their orthologs from rice and could possibly contribute to its drought tolerance. Besides calcium, an integration of abscisic acid, ethylene, auxin and methyl jasmonate signaling was probably involved in regulating expression of the drought response through DREB transcription factors.

Alterations in stem sugar content and metabolism in sorghum genotypes subjected to drought stress

Changes in stem sugar concentrations due to drought stress at the early reproductive stage were studied in seven sorghum (Sorghum bicolor (L.) Moench) genotypes that differ in their stem sugar storage ability. Total sap sugar concentration increased in most genotypes. ANOVA showed a significant contribution of genotype and treatment to the variation in sugar levels. Two genotypes showed little variation in total sugar levels at the fifth internode from the peduncle and five genotypes showed significant increases in total sugar levels under drought; these groups were used to compare sugar metabolism. Drought led to a decrease in catabolic sucrose synthase activity in both groups. Invertase activities increased significantly in two genotypes and correlated with the increase in reducing sugar concentrations under drought. Stem sugar hydrolysis probably had a role in osmotic adjustment under drought and correlated with retention of sap volume. However, the activities of sugar-metabolising enzymes did not correlate with their gene expression levels. After resuming irrigation, grain yields, stalk yields and juice volume at physiological maturity were lower in plants recovering from drought stress compared with the controls. In some genotypes, there were similar losses in grain yields and stem sugars due to drought, indicating photoassimilate source limitation; in other genotypes, grain yield losses were less than stem sugar losses in drought-exposed plants compared with the controls, suggesting mobilisation of sugars from the storage internodes to the developing panicle. Accumulation of stem sugars appears to be an adaptive strategy against drought stress in some sorghum genotypes.

Reduced mycorrhizal colonization (rmc) tomato mutant lacks expression of SymRK signaling pathway genes.

Comparison of the expression of 13 genes involved in arbuscular mycorrhizal (AM) symbiosis was performed in a wild type tomato (Solanum lycopersicum cv 76R) and its reduced mycorrhizal colonization mutant rmc in response to colonization with Glomus fasiculatum. Four defense-related genes were induced to a similar extent in the mutant and wild type AM colonized plants, indicating a systemic response to AM colonization. Genes related to nutrient exchange between the symbiont partners showed higher expression in the AM roots of wild type plants than the mutant plants, which correlated with their arbuscular frequency. A symbiosis receptor kinase that is involved in both nodulation and AM symbiosis was not expressed in the rmc mutant. The fact that some colonization was observed in rmc was suggestive of the existence of an alternate colonization signaling pathway for AM symbiosis in this mutant.