In addition to improving the production of ethanol by modifications to the process, modern molecular biology offers chances to alter all aspects of ethanol production. In terms of bioethanol genetic modification can be applied both to the biomass sources and the fermenting organisms.

The biomass sources for ethanol are sugars, starches and lignocellulose. Increas­ing sugar content of sugarcane and sugarbeet plants had reached a limit but recently a sucrose isomerase enzyme has been targeted to sugarcane vacuoles, which converts the glycosidic linkage in sugar to a 1,6-fructoside. This allows the accumulation of 0.5M isomaltose (palatinose) in sugarcane stems in addition to sucrose which increases the overall sugar content (Gressel, 2008). However, the one enzyme capable of degrading isomaltose only degrades it slowly but it is hoped to increase the rate greatly by gene shuffling. A change in starch composition to make its conversion into sugar easier is possible and is under investigation.

The development of some second — and third-generation biofuels depends on the ability to process lignocellulose. If lignocellulose is to be used for ethanol produc­tion, it has to be broken down into sugars. The lignin content of wood, straw and grasses reduces the rate of cellulose hydrolysis due to steric hindrance of the cellulo­lytic enzymes. A reduction in lignin or an increase in cellulose would increase the production of sugars. Plant material with more cellulose and less lignin has been reported where partial silencing of the phenylpropanoid pathway enzymes leading to lignin reduces the lignin content (Morohoshi and Kajita, 2001; Gressel and Zilberstein, 2003). In this way an increase in the digestibility of maize, sorghum, pearl millet, poplar and pine has been achieved. A possible problem with the reduc­tion in lignin is loss of structural strength and subsequent lodging (blowing down in strong winds). The dwarf and semi-dwarf wheat and rice will probably not suffer from this and there appears to be no correlation between lodging and lignin content (Gressel, 2008).

The best and most widely used ethanol-producing microorganism Saccharo- myces cerevisiae has a rapid growth rate, and a tolerance to ethanol accumulating in the medium. However, S. cerevisiae has only a limited substrate range restricted to a few sugars and it is unable to metabolize starch and lignocellulose or the pen­tose sugar from lignocellulose. There are ethanol-producing bacteria, perhaps the best known is Zymomonas mobilis, but these also have a restricted range of sugars that they can ferment. One solution to this dilemma is to use genetic manipulation to introduce the ability to use alternative substrates like starch and xylose into these organisms.