versity of Rubisco molecules may be an important part of future strategies to solve the molecular basis of
CO 2 /O 2 specificity [112]. There has been no report so far that engineered Rubisco leads to higher effi-
ciency with respect to CO 2 fixation [113], although it has been a prime focus for genetically engineering
an increase in photosynthetic productivity [114].
In genetic engineering aimed at improving photosynthesis, another important target is the enzyme
phosphoenolpyruvate carboxylase (PEPC), which catalyzes the first reaction of the C 4 cycle of photosyn-
thetic carbon fixation. High-level expression of maize PEPC in transgenic rice plants has been successful.
Some transgenic rice leaves have an enhanced PEPC activity two- to threefold higher than that in maize,
and they showed a reduced O 2 inhibition of photosynthesis but no increased photosynthetic rates compared
with those of untransformed plants [115]. Higher photosynthetic efficiency in C 4 plants depends not only
on PEPC and the other enzymes of C 4 pathway but also on the special anatomic structure of their leaves.
Thus, overexpression of PEPC alone is not likely to result in an improvement of crop yield [116].
Genetic engineering targeted at other enzymes of photosynthetic carbon metabolism such as sucrose
phosphate synthase (SPS) and ADP-glucose pyrophosphorylase (AGPase), which participate in sucrose
and starch biosynthesis, respectively, has been performed. These efforts are undoubtedly useful in un-
derstanding the regulatory role of these enzymes but not necessarily in increasing the yield of crop plants.
SPS has been considered to be a key enzyme in the regulation of carbon assimilation and export from the
leaf [117]. Although increased photosynthetic rates were observed in transformed tomato plants express-
ing a maize SPS gene in addition to the native enzyme, total dry matter production and fruit yield were
not significantly increased [118,119]. Up- and down-regulation of SPS activity may lead to expected
changes with respect to carbon partitioning. However, it remains questionable whether the actual rate of
photosynthetic sucrose formation does determine final crop yield [120].
In addition, there has been a report that genetic engineering–transformed potato plants with reduced
accumulation of a protein located in chloroplasts showed stunted growth, decreased tuber yield, and re-
duced values of nonphotochemical quenching of chlorophyll afluorescence. These results indicate a pref-
erential association of the protein with the light-harvesting complex of PSII (LHCII) and its functional
role of modulating photosynthetic efficiency and dissipating excessive absorbed light energy within the
antenna complex [121].
Regardless of success or failure in increasing crop yield, such efforts themselves have implied that
the central object will be the improvement of photosynthetic efficiency of crops, and the sharpest tool will
be genetic engineering for a new green revolution. Transforming crops through genetic engineering to get
good varieties with high photosynthetic efficiency seems to be the major hope for the new revolution.
However, although the study of photosynthesis has benefited from the techniques of molecular biology,
these techniques alone rarely permit a mechanistic understanding of the process. Cooperative efforts for
integrated experimental approaches that combine the strategies used in physiology, biochemistry, and
other relevant fields will be imperative to evaluating the process fully [122]. In the process many basic
details still remain to be understood, especially about the regulation of photosynthetic efficiency, includ-
ing the step most seriously limiting photosynthetic efficiency, which the physiologists and biochemists of
photosynthesis are exploring. A comprehensive understanding of this regulatory mechanism will be the
basis of success of the molecular biologists in engineering crops with increased photosynthetic efficiency.
It is impossible to find a successful target(s) in engineering crops without a clear understanding of the
mechanism. Moreover, even allowing for the enormous advances in molecular biology that are being
made, results are not expected to be obtained soon in genetic improvement of crop productivity [123].
Recently, it has been reported that it is now possible to insert a single, genetically dominant, poten-
tially yield-enhancing, dwarfing gene into the genome of any transformable crop without the need for
long-term conventional breeding programs and with minimal disruption of genetic background [124].
This study indicates that plant height reduction associated with yield increase is still an important aim for
high-yield breeding, especially in high-stalk crops such as maize and sorghum. It appears that the two
green revolutions cannot be totally separated. The second revolution will be a continuation of the first one,
but it will have new characteristics. If it may be said that the first revolution was characterized by im-
proving plant type, then the second one will be characterized mainly by improving photosynthetic effi-
ciency.
Besides the central object of improving photosynthetic efficiency, of course, the new revolution may
involve more aims such as improving grain quality, manipulating plant nutrients for human health,
achieving herbicide resistance, etc. by engineering crops genetically. The initial phase of a new revolu-
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