times from the same plants, and that they will grow efficiently with less need for watering
and fertilizers. Although they are currently not efficient, improved methods for this cellu-
lytic conversion of plant material to ethanol may hold some of the best promise for sustain-
able fuel production from plants. Transgenic approaches are being explored to produce
cellulose that would be more easily converted to simple sugars by microbes for alcohol pro-
duction, or in grasses and woody plants with decreased levels ofligninthat can interfere
with cellulose degradation. In addition, identification and engineering of microbes that
can degrade lignin or more readily convert cellulose and sugars to ethanol are also being
explored (Stephanopoulos 2007). There are a number of investigators searching for ways
to modify plant feedstocks for eventual more facile cellulosic ethanol production. One
idea is to encodecellulasesand other cell-wall-degrading enzymes by the transgenic
biomass crops directly.
Diesel fuel made from plant material, biodiesel, can also represent an alternative to fossil
fuels. Diesel currently accounts for approximately 20% of the fuel consumed for transpor-
tation in the United States; therefore, finding a renewable replacement could have a
considerable impact on the need for oil throughout the world. Biodiesel is produced
from oilseed crops such as soybean and canola, through a process calledtransesterification.
The properties of biodiesel are slightly different from those of petroleum-based diesel, but
biodiesel can be used alone as a fuel or in a blend of the two types of fuel. Although there
are currently no transgenic applications to improve biodiesel production in oilseed crops,
the two major sources for biodiesel (soybean and canola) are most often grown as trans-
genic plants.
Because of the economic, environmental, and political concerns associated with fossil
fuel consumption, the use of plants for biofuel production will almost certainly continue
to increase and develop with new strategies. Genetically engineered biofuel crops will
likely not be food or feed crop plants for several reasons—as noted above, food companies
could be opposed to altering food crops for fuel purposes if there is a viable chance of acci-
dental mixing of fuel and feed (Stewart 2007).
8.5 Conclusions
Clearly we are at the proverbial tip of the iceberg with regard to the numbers and types
of genes identified that could be useful in plant biotechnology. Genes are currently
limited by insufficient knowledge of diverse kinds of genomes and the ability to engineer
in metabolic pathways. Simple solutions to problems that can be fixed with the insertion of
one gene coding for one protein are myriad, but how much more important will be the
ability to engineer into plants entire metabolic pathways such as was done to produce
Golden Rice.
210 GENES AND TRAITS OF INTEREST FOR TRANSGENIC PLANTS