Role of Jasmonate in Modulation of Mycorrhizae-Induced Resistance Against Fungal Pathogens.

Symbiotic association of plants with arbuscular mycorrhizal (AM) fungi brings about changes in levels of the phytohormone jasmonate (JA) in root and shoot tissues of a plant. The enhanced JA levels not only play a role in controlling the extent of AM colonization but are also involved in the expression of mycorrhizal-induced resistance (MIR) against pathogens. We describe a method used to study the levels of a volatile jasmonate derivative, methyl jasmonate (MeJA), in tomato plants colonized by AM fungi and in response to subsequent attack by the foliar pathogen Alternaria alternata.

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.

Polyketide synthesis in tobacco plants transformed with a Plumbago zeylanica type III hexaketide synthase.

A type III polyketide synthase from Plumbago zeylanica (PzPKS) was cloned and expressed in tobacco plants to study whether the transgenic tobacco plants expressing PzPKS synthesize the pharmacologically important polyketide, plumbagin. High resolution mass spectrometry based metabolite profiling of two transgenic events and wild type tobacco plants was carried out to investigate changes in polyketides, including plumbagin. Ten polyketides, which included six pyrones and four naphthalene derivatives, were identified in PzPKS transgenic plants. While one pyrone, styryl-2-pyranone, was detected in both, wild type and transgenic tobacco plants, three pyrones were expressed only in the leaves of transgenic tobacco plants. The transgenic tobacco plants did not accumulate plumbagin, but showed accumulation of isoshinanolone in the roots, which is postulated to be the reduction product of plumbagin. In addition, leaves of transgenic tobacco plants accumulated 3-methyl-1,8-naphthalenediol, a postulated precursor of plumbagin. The results indicated the requirement of additional Plumbago-specific components in the biosynthetic pathway of this polyketide.

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.

Halopriming mediated salt and iso-osmotic PEG stress tolerance and, gene expression profiling in sugarcane (Saccharum officinarum L.).

Seed priming is a well known pre-germination strategy that improves seed performance. However, biochemical and molecular mechanisms underlying priming mediated stress tolerance are little understood. Here, we report results of the study on growth, physiological characteristics and expression of stress responsive genes in salt primed sugarcane cv. Co 86032 plants in response to salt (NaCl, 150 mM) or iso-osmotic (-0.7 MPa) polyethylene glycol-PEG 8000 (20 % w/v) stress exposure for 15 days. Variable growth, osmolyte accumulation and antioxidant capacity was revealed among the primed and non-primed plants. The primed plants showed better tolerance to the salt or PEG stress, as revealed by better growth and lower membrane damage, through better antioxidant capacity as compared to the respective non-primed controls. Further, steady state transcript expression analysis revealed up regulation of sodium proton antiporter (NHX) while, down regulation of sucrose transporter (SUT1), delta ( 1 )-pyrolline-5-carboxylate synthetase (P5CS) and proline dehydrogenase (PDH) in primed plants on exposure to the stress as compared to the non-primed plants. Transcript abundance of catalase (CAT2) decreased by about 25 % in leaves of non-primed stressed plants, however, the expression was maintained in leaves of the stressed primed plants to that of non-stressed controls. Thus, the results indicated priming mediated salt and PEG stress tolerance through altered gene expression leading to improved antioxidant capacity in sugarcane.

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.

Transcript expression profiling of stress responsive genes in response to short-term salt or PEG stress in sugarcane leaves.

Semi-quantitative RT-PCR based transcript expression of stress responsive genes was studied in leaves of sugarcane plants exposed to short-term (up to 24 h) salt (NaCl, 200 mM) or polyethylene glycol-PEG 8000 (20% w/v) stress. Transient increase in expression of NHX (sodium proton antiporter), SUT1 (sucrose transporter1), PDH (proline dhydrogenase) and CAT2 (catalase2) was observed in response to 2-4 h PEG stress. However, salt stress imposed repression of NHX, PDH and CAT2 at these time points. The transcript level of the delta (1)-pyrolline-5-carboxylate synthetase (P5CS) increased slightly in salt treatment while in response to the PEG stress, the gene expression increased at 4 h treatment but then decreased considerably by 80% at 24 h. The results thus indicated differential regulation of these stress responsive genes in response to salt or PEG stress in sugarcane. Further, the transcript expression data was compared with that available for the Arabidopsis homologs at Arabidopsis eFP Browser and Genevestigator V3 tools. Understanding transcript gene expression patterns of the stress responsive genes may provide insights into complex regulatory network of stress tolerance.

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.

Salt-induced respiration in Bruguiera cylindrica - role in salt transport and protection against oxidative damage.

Leaves of Bruguiera cylindrica plants grown in the greenhouse and irrigated with fresh water (FW plants) and those from salt-adapted plants from the seacoast (SW plants) showed about 5-fold and 3-fold increase in respiration, respectively, when immersed in 4M NaCl solution. The increase in respiration was not due to dehydration effect of high salt concentration, since PEG-imposed dehydration stress to the leaves led to an inhibition of respiration rates in both FW and SW plants. The salt-induced increase in respiration rate was specific to monovalent cations, especially Na(+) and K(+), but not divalent or trivalent cations, and to Cl(-), but not other anions. Pretreatment of leaves of FW plants with 1mM amiloride, an inhibitor of the Na(+) / H(+) antiporter, reduced the NaCl-induced respiration surge. At least some part of the observed respiratory increase could therefore be for providing energy for ion transport, since the Na(+) / H(+) antiport activity is driven by activities of the tonoplast and plasma membrane H(+)-ATPases and H(+)-PPases. Respiration of the leaves from both FW and SW plants was accounted for by the COX pathway and was inhibited by KCN. But 4M NaCl-induced increase in FW, but not SW plants, was inhibited by the AOX inhibitor, SHAM. Also, generation of ROS was reduced by treatment with KCN, but increased with SHAM. This pointed to a protective role of AOX in reducing ROS generation during salt-induced respiration. Our results indicated that NaCl-induced increase in leaf respiration of B. cylindrica plants irrigated with fresh water was required for (a) salt transport and (b) reducing the harmful effects of ROS that are known to accompany increased respiratory activity.

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.