Aerenchyma Formation

The formation of aerenchyma or air spaces inside the root cortex to allow for oxygen flow is another key adaptation. A rise in ethylene concentration is the triggering factor for the formation of aerenchyma in maize. (Drew & Lynch, 1980; Drew, 1992; Drew at al, 1994; Drew et al, 1985). There is at least one chemical mechanism attempted to explain the rise in ethylene concetration. It begins with the conversion of the amino acid methionine to S-adenosylmethionine which is the precursor to aminoclcloprapane carboxylic acid (ACC). As ACC levels rise, so does ethylene concentrations which is the trigger for programmed cell death and aerenchyma formation. (Drew, et al, 1994). I have hypothesized a chemical mechanism that is less correlation related. It begins with normal fermentation with ethanol as the final by product. The last step is the action of ADH on acetyl aldehyde to form ethanol. All of Drew's works also report a correlation to the rise in ADH activity and ethylene concentration. As I have stated earlier, there is a build up of acids formed as the reduction cascade is carried out by soil microoganisms. All of this data came from working on the root tips of Zea mays, the part of the root closest to the acidic substrate. Both the cascade reactions and the fermentation reactions would be occurring in the same area at the same time and the ethanol is being released by the cells a by product. Therefore, ethanol in the presence of acid produces ethylene (Morrison & Boyd, 1973).

This is a photo of aerenchmya tissue in the roots of Zea mays, corn.  Notice the air spaces in the cortex (center) where root cells have died.  Also note the cells along the edge are healthy.  There seems to be a pattern for programed cell death to allow for air passages so that oxygen can reach the root zone during flooded conditions.  The function and formation of aerenchyma is still not fully understood.  A lot of research is being done in this area on various spices.  Our research concerns the bald cypress.  Image from Kramer, 1983.