Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/14/2004
Publication Date: 6/1/2004
Citation: Chen, Z.-Y., Brown, R.L., Damann, K.E., Cleveland, T.E. 2004. Identification of a maize kernel stress-related protein and its effect on aflatoxin accumulation. Phytopathology. 94:938-945. Interpretive Summary: Aflatoxins are poisons produced by the fungus Aspergillus flavus after it infects agricultural commodities, such as corn. Contamination of corn with aflatoxins causes financial loss to growers and is a potential health hazard to animals and humans. Corn kernel proteins have been shown to play a role in resistance against aflatoxin contamination. Using state-of-the-art protein separation techniques to compare resistant corn lines to susceptible ones, one protein associated with resistance has been identified as an enzyme called glyoxalase I. It is involved in the detoxification of methylglyoxal produced by cells under normal and stress conditions. The gene encoding this protein has been cloned and its activity in resistant and susceptible corn genotypes has also been measured. The glyoxalase activity is high in resistant lines and remained the same after fungal infection, whereas it is low in susceptible genotypes and decreased after infection. This decrease in activity could be due to the accumulation of aflatoxin, which was found to inhibit glyoxalase I activity in vitro. This study also found methylglyoxal at low concentrations could induce aflatoxin production in vitro. It is believed that this protein plays an active role in host resistance by actively removing its substrate, an aflatoxin inducer, and therefore, the high levels of glyoxalase activity may contribute to lower levels of aflatoxins seen in resistant lines. Enhancing the expression of this protein in commercial susceptible corn lines could result in development of resistant corn lines and the control of aflatoxin contamination, which would, in turn, result in increased financial savings to growers, and enhanced protection to the public.
Technical Abstract: Aflatoxins are carcinogens produced mainly by Aspergillus flavus during infection of susceptible crops such as maize. Though resistant maize genotypes have been identified, the incorporation of resistance into commercial lines has been slow due to the lack of selectable markers. Through comparisons of kernel embryo proteins of resistant and susceptible genotypes separated using 2-D gels, several protein spots were previously found to be unique or up-regulated in resistant embryos. In the present study, one of such protein spots was sequenced and identified as glyoxalase I (GLX-I). This enzyme detoxifies the methylglyoxal produced under stress conditions. To assess the relationship between glyoxalase I and host resistance, the full-length cDNA of glyoxalase I gene (glx-I) has been cloned and its expression during kernel development was examined. Its transcript in immature kernels was up-regulated in both resistant and once susceptible genotypes upon fungal inoculation. GLX-I activity was also found to be significantly higher in the resistant maize lines compared to susceptible ones, and was inhibited in vitro in the presence of aflatoxins. Most interestingly, methylglyoxal, the substrate of GLX-I, was found to induce aflatoxin production in A. flavus culture at a concentration as low as 5.0 uM. These data suggest that the glyoxalase I plays an active role in host resistance by actively removing its substrate, an aflatoxin inducer, and therefore, it may contribute to lower levels of aflatoxins in resistant maize genotypes.