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Salt-tolerant Pokkali rice plants accumulate higher polyamines (PAs) such as spermidine (Spd) and spermine (Spm) in response to salinity stress, while the sensitive cultivarM-1-48 is unable to maintain high titres of these PAs under similar conditions. The effects of the triamine Spd and the tetramine Spm on physiological and biochemical changes in 12-day-old rice seedlings were investigated during salinity stress to determine whether they could protect the sensitive plants from stress effects. At physiological concentrations Spd and Spm significantly prevented the leakage of electrolytes and amino acids from roots and shoots induced by salinity stress. To different degrees they also prevented chlorophyll loss, inhibition of photochemical reactions of photosynthesis as well as downregulation of chloroplast-encoded genes like psbA, psbB, psbE and rbcL, indicating a positive correlation between salt tolerance and accumulation of higher PAs in rice. The inhibitory effect of salinity stress and its reversal by exogenous PAs were more pronounced in the salt-sensitiveM-1-48 plants than in the tolerant Pokkali plants.

The present study investigated the linkages between drought stress, oxidative damages and variations in antioxidants in the three rice varieties IR-29 (salt-sensitive)

The coding region of a human beta-polymerase cDNA, predicting a 335 amino acid protein, was subcloned in the Escherichia coli expression plasmid pRC23. After induction of transformed cells, the crude soluble extract was found to contain a new protein immunoreactive with beta-polymerase antibody and corresponding in size to the protein deduced from the cDNA. This protein was purified in a yield of 1-2 mg/50 g of cells. The recombinant protein had about the same DNA polymerase specific activity as beta-polymerase purified from mammalian tissues, and template-primer specificity and immunological properties of the recombinant polymerase were similar to those of natural beta-polymerases. The purified enzyme was free of nuclease activity. We studied detailed catalytic properties of the recombinant beta-polymerase using defined template-primer systems. The results indicate that this beta-polymerase is essentially identical with natural beta-polymerases. The recombinant enzyme is distributive in mode of synthesis and is capable of detecting changes in the integrity of the single-stranded template, such as methylated bases and double-stranded region. The enzyme recognizes a template region four to seven bases downstream of the primer 3' end and utilizes alternative primers if this downstream template region is double stranded. The enzyme is unable to synthesize past methylated bases N3-methyl-dT or O6-methyl-dG.

The full length Rab16A, from the indica rice Pokkali, was introduced into tobacco by Agrobacterium-mediated transformation. The transgene was stably integrated into the genome and they originated from different lines of integration. Expression of Rab16A transcript driven by its own promoter (stress inducible) in T2 progenies, only when triggered by salinity/ABA/PEG (Polyethylene glycol)-mediated dehydration, but not at the constitutive level, led to the stress-induced accumulation of RAB16A protein in the leaves of transgenic plants. The selected independent transgenic lines showed normal growth, morphology and seed production as the WT plants without any yield penalty under stress conditions. They exhibited significantly increased tolerance to salinity, sustained growth rates under stress conditions; with concomitant increased osmolyte production like reducing sugars, proline and higher polyamines. They also showed delayed development of damage symptoms with better antioxidative machinery and more favorable mineral balance, as reflected by reduced H2O2 levels and lipid peroxidation, lesser chlorophyll loss as well as lesser accumulation of Na+ and greater accumulation of K+ in 200 mM NaCl. These findings establish the potential role of Rab16A gene in conferring salt tolerance without affecting growth and yield, as well as pointing to the fact that the upstream region of Rab16A behaves as an efficient stress-inducible promoter. Our result also suggests the considerable potential of Group 2 lea genes as molecular tools for genetic engineering of plants towards stress tolerance.

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In an attempt to understand the molecular basis of salt-stress response in the aromatic rice Gobindobhog, a comprehensive analysis encompassing physiological or biochemical assays and gene expression studies under high salt (200 mM NaCl) supply regimes were initiated and compared with a salt-sensitive (M-1-48) and salt-tolerant (Nonabokra) rice. The detrimental effects of salinity stress were the most pronounced in Gobindobhog, as reflected by the maximally increased root to shoot ratio, the highest chlorophyll degeneration, the highest foliar concentration of Na(+) ions and peroxide content, with their maximum increment after salt treatment. The amplification of oxidative damages was further stimulated by the accumulation of putrescine and lipid peroxidation-derived toxic degradation products (increased malondialdehyde and lipoxygenase activity), which were comparable in M-1-48 and Gobindobhog. Antioxidants like anthocyanin and particularly cysteine and the osmolytes like reducing sugar, proline and polyamines (spermidine and spermine) showed the highest level in Nonabokra. While the inhibition of catalase activity occurred in all the varieties following salt-stress, the maximum induction in guaiacol peroxidase activity, elevated cysteine and proline levels in Gobindobhog probably constituted the detoxification mechanism obligatory for its survival. Intensification of the aroma content with salt treatment was markedly noted in Gobindobhog. A very low abundance of Rab16A/SamDC transcript and the corresponding proteins were observed both in M-1-48 and Gobindobhog, induced only after salt-stress, whereas they were constitutively expressed in Nonabokra. Thus, our data reflect Gobindobhog as a salt-sensitive cultivar, susceptible to high-stress-induced growth-inhibition, ion imbalances, membrane/oxidative damages with lower expression of stress-tolerant genes.

Translational effects of the RNA leader and