Alcoholic fermentation is often used by plants in anaerobic conditions to produce ATP and regenerate NAD to allow for glycolysis to continue.  For most plant tissues, fermentation only occurs in anaerobic conditions, but there are a few exceptions.  In the pollen of maize (Zea mays) and tobacco (Nicotiana tabacum & Nicotiana plumbaginifolia), the fermentation enzyme ADH is abundant, regardless of the oxygen level.  In tobacco pollen, PDC is also highly expressed in this tissue and transcript levels are not influenced by oxygen concentration.  Tobacco pollen, similar to Crabtree-positive yeast, perform high levels of fermentation dependent on the sugar supply, and not oxygen availability.  In these tissues, respiration and alcoholic fermentation occur simultaneously with high sugar availability.  Fermentation produces the toxic acetaldehyde and ethanol, that can build up in large quantities during pollen development.  It has been hypothesized that acetaldehyde is a pollen factor that causes cytoplasmic male sterility.  Cytoplasmic male sterility is a trait observed in maize, tobacco and other plants in which there is an inability to produce viable pollen.  It is believed that this trait might be due to the expression of the fermentation genes, ADH and PDC, a lot earlier on in pollen development than normal and the accumulation of toxic aldehyde.