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Normal growth and development of plants is greatly dependent on the capacity to overcome environmental stresses. Environmental stress conditions like high salinity, drought, high incident light and low or high temperature cause major crop losses worldwide. A common denominator in all these adverse conditions is the production of reactive oxygen species (ROS) within different cellular compartments of the plant cell. Plants have developed robust mechanisms including enzymatic or nonenzymatic scavenging pathways to counter the deleterious effects of ROS production. There are a number of general reviews on oxidative stress in plants and few on the role of ROS scavengers during stress conditions. Here we review the regulation of antioxidant enzymes during salt stress in halophytes, especially mangroves. We conclude that (i) antioxidant enzymes protect halophytes from deleterious ROS production during salt stress, and (ii) genetic information from mangroves and other halophytes would be helpful in defining the roles of individual isoforms. This information would be critical in using the appropriate genes for oxidative stress defence for genetic engineering of enhanced stress tolerance in crop systems.

Ascophyllum nodosum extract products are used commercially in the form of liquid concentrate and soluble powder. These formulations are manufactured from seaweeds that are harvested from natural habitats with inherent environmental variability. The seaweeds by themselves are at different stages of their development life-cycle. Owing to these differences, there could be variability in chemical composition that could in turn affect product consistency and performance. Here, we have tested the applicability of using Arabidopsis thaliana as a model to study the activity of two different extracts from A. nodosum. Three different bioassays: Arabidopsis root-tip elongation bioassay, Arabidopsis liquid growth bioassay and greenhouse growth bioassay were evaluated as growth assays. Our results indicate that both extracts promoted root and shoot growth in comparison to controls. Further, using Arabidopsis plants with a DR5:GUS reporter gene construct, we provide evidence that components of the commercial A. nodosum extracts modulates the concentration and localisation of auxins which could account, at least in part, for the enhanced plant growth. The results suggest that A. thaliana could be used effectively as a rapid means to test the bioactivity of seaweed extracts and fractions.

Plant photosynthesis results in the production of molecular oxygen. An inevitable consequence of this normal process is the production of reactive oxygen species (ROS) by the transfer of electrons to molecular oxygen. Plants are adequately protected by the presence of multiple antioxidative enzymes in different organelles of the plant such as chloroplasts, cytosol, mitochondria and peroxisomes. Under high light and CO(2) limiting conditions caused by environmental stress like salinity, these antioxidative enzymes play an important role in scavenging toxic radicals. To investigate the functions of antioxidative enzymes in a mangrove plant, we isolated three cDNAs encoding cytosolic Cu-Zn SOD (Sod1), catalase (Cat1) and ferritin (Fer1) from Avicennia marina cDNA library. Sod1, Cat1 and Fer1 cDNA encoded full-length proteins with 152, 492 and 261 amino acids respectively. We studied the expression of these antioxidant genes in response to salt, iron, hydrogen peroxide, mannitol and light stress by mRNA expression analysis. Cat1, Fer1 showed short-term induction while Sod1 transcript was found to be unaltered in response to NaCl stress. A decrease in mRNA levels was observed for Sod1, Cat1 while Fer1 mRNA levels remained unaltered with osmotic stress treatment. Sod1, Cat1 and Fer1 mRNA levels were induced by iron, light stress and by direct H(2)O(2) stress treatment, thus confirming their role in oxidative stress response.

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