Does nitric oxide activate antioxidant enzymes in plants exposed to arsenic?

Nitric oxide (NO) is an important cell signaling agent in responses to various kinds of abiotic stress conditions. Under oxidative stress plants produce toxic levels of oxygen reactive intermediates (ROIs), causing damage to cell metabolism. Experimental data is suggestive of an antioxidant action of the NO in plants subjected to abiotic stresses, such as exposed to toxic levels of arsenic (As). The objective of this research was to evaluate the effect of NO, exogenously supplied, on the activity of antioxidant enzymes: superoxide dismutase (SOD), catalase (CAT) and peroxidase (POX) in water hyacinth (Eichhornia crassipes) exposed to toxic levels of As. Plants grown in nutrient solution were exposed to the following treatments: T0 (control), T1 (20 mu M As), T2 (20 mu M As + 100 mu M SNP) and T3 (100 mu M SNP). The SNP (sodium nitroprusside) was the exogenous supplement of NO. After 0, 4, 12 and 24 hr root samples were taken and the antioxidant enzymes activities were determined. The activities of the antioxidant enzymes increased in the plant exposed to toxic levels of As (T1) and with time. Addition of SNP (T2) to the nutrient solution, however, did not increase the activities of the enzymes as expected, probably as a result of a reduction in ROIs content, substrate of the antioxidant enzymes. Probably, NO may be directly used as antioxidant agent reducing the availability of substrates to the enzymes, and therefore reducing cell damage.