Effects of foliarly applied amino acids on sunflower tolerance to arsenic and molybdenum, and DNA methods to characterize associated rhizosphere bacterial communities

Arsenic contamination of soil is a worldwide concern due to its impact on human health and agriculture. Phytoremediation is an emerging technology that uses plants to remediate metal contaminated sites in situ. Helianthus argophyllus is a sunflower plant that is native to South Texas, and is currently under study as a promising prospect for phytoremediation. This investigation focused on the use of foliarily applied amino acids, glycine or a mixture of cysteine and glutamate, to enhance the tolerance of Helianthus argophyllus to weekly treatments of arsenic+molybdenum (2 mM each). Plants typically died over a treatment period of 2-5 weeks, but foliar applications of glycine, in particular, reduced the yellowing of leaves and delayed the onset of toxicity symptoms and death. Other data suggests that the foliar amino acid effects are due to enhanced exclusion of the metalloids from the above-ground plant parts, possibly implicating amino acid-altered plant-rhizosphere interactions The second component of this study developed methods for the comparison of the rhizosphere communities of sunflower plants among the various treatment groups by using PCR amplification of 16S rRNA followed by denaturing gradient gel electrophoresis. Two DNA extraction kits were compared for their effectiveness at extracting amplifiable DNA from sandy and organic soils. Two sets of primers were also compared. Universal rRNA primers failed to produce any bands by DGGE. However, DGGE banding patterns form a second eubacteria-specific primer set revealed a more complex profile, including a number of unique bands, for a growth chamber soil containing a sunflower plant compared to a growth chamber control soil (without a plant) and a wild sunflower sample.