THE ROLE OF ACONITATE ISOMERASE IN TRANS-ACONITATE ACCUMULATION IN PLANTS

Authors: Thompson, John; Schaefer, Stephen; MADISON, JAMES - RETIRED-USDA, ARS, PSNL

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/17/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: Grass tetany is a serious problem of ruminants that causes ataxia and death when the forage is predominantly grasses. Grass tetany is caused by a deficiency of Mg and Ca in the blood. There are a number of contributing causes, such as low Mg and Ca in the forage, high nitrogen in the forage, high lipid content of the forage and high organic acid (that chelate Mg and Ca) levels in the forage. Our goal was to find out if the accumulation of the organic acid, trans-aconitate, in wheat (a tetany prone species) is caused by aconitate isomerase, the enzyme involved in its formation. Our experiments showed that aconitate isomerase is necessary for the accumulation of trans-aconitate in wheat but that it does not control the level of accumulation. Our results suggest at least two ways of reducing the trans-aconitate content of wheat and hence the its contribution to the grass tetany problem. First, cultivars of wheat could be selected that have low or no aconitate isomerase. Second, an antisense gene to aconitate isomerase could be inserted into the wheat genome to eliminate the aconitate isomerase activity. Elimination of grass tetany will benefit farmers and ranchers whose cows die as a result of this disease.

Technical Abstract: In order to obtain information on one of several possible causes of grass tetany (i.e., accumulation of trans-aconitate), we have investigated the role of mineral nutrition and aconitate isomerase (an enzyme that may be involved in aconitate synthesis) in trans-aconitate accumulation in wheat (Triticum aestivum). Wheat seedlings were grown for 5 days on buffered calcium sulfate solution and then transferred for two days to single salt solutions containing cations and anions that are known to be absorbed by plant roots at similar or different rates. The shoots were analyzed for trans-aconitate and aconitate isomerase. The trans-aconitate content at the end of the two-day experimental period and changes in trans-aconitate content during the two-day treatment varied widely and were related to the cation but not the anion of the single salt solution. Aconitate isomerase activity did not vary as much as trans-aconitate or change in trans-aconitate content, and was not closely correlated with the trans-aconitate content. Analysis of diverse species and tissues for aconitate isomerase and trans-aconitate showed that the presence of aconitate isomerase was necessary for appreciable trans-aconi- tate accumulation. We conclude that the activity of aconitate isomerase does not control trans-aconitate content but that its presence is necessary for any significant accumulation of trans-aconitate.