Expression and promoter methylation of succinate dehydrogenase and fumarase genes in maize under anoxic conditions.

Succinate dehydrogenase (SDH) and fumarase enzyme activity and expression of genes encoding the SDH subunits A (Sdh1-2), B (Sdh2-3), C (Sdh3), D (Sdh4) and the mitochondrial (Fum1) and cytosolic (Fum2) isoforms of fumarase were quantified in maize (Zea mays L.) seedlings exposed to atmospheres of air (control), N and CO. The catalytic activity of SDH gradually declined in plants exposed to N atmospheres, with ∼40% activity remaining after 24h. In seedlings incubated in CO the suppression was even more pronounced. Fumarase activity was more stable, decreasing by one third after 24h of anoxia. The level of Sdh1-2 transcripts in seedlings declined significantly under N and even more rapidly upon exposure to CO, with a concomitant increase in methylation of the corresponding promoters. The level of Sdh2-3 and Sdh3 transcripts also decreased under N and CO but the changes in promoter methylation were less pronounced, whereas the changes in the level of Sdh4 expression and promoter methylation were minor. Expression of Fum1 and Fum2 was affected by N and CO atmospheres, however without changes in corresponding promoter methylation. It is concluded that, under conditions of oxygen deficiency, succinate accumulates mainly due to downregulation of SDH gene expression and reduction of enzyme activity, and to a lesser extent due to the decrease of fumarase gene expression.