Summary

The scientific challenges encountered during the last decade by attempts at improving photosynthetic productivity, even when successful, generated further questions, but even the lack of success has taught us many things. As the conclusion for this chapter, we would like to explore the approaches necessary for future achievements in improvement of crop productivity.

One most important requisite for manipulating physiology of an organism is to accumulate information about the precise mechanisms of function of the key protein(s) or enzyme(s) in question. This includes detailed knowledge on gene structure and the regulation of gene and protein expression of enzymatic properties and subcellular location. ADPglucose pyrophosphorylase, for example, had been studied extensively over a long period, from its biochemistry in vitro through to regulation of activity in vivo (Preiss et al., 1991). However, only the introduction of a gene, modified to be insensitive to feedback regulation, into potato tuber amyloplasts resulted in increased starch synthesis (Preiss, 1996). FBP/SBPase from a cyanobacterium has been shown to improve productivity in tobacco (Miyagawa et al., 2001). Since the functional sites of these enzymes are the chloroplast stroma, the selection of the promoter and the transit sequences for expression of these proteins could easily be accomplished based on previous knowledge. Another strategy, antisense suppression of resident genes has revealed the significance of particular enzymes in a postulated metabolic pathway.

Similar considerations are also valid for RuBisCO research. We are still ignorant, for example, about either the residues that determine the S_rel value, or how carbon and oxygen atoms are enabled to overcome spin prohibition on the RuBisCO protein for the oxygenation of RuBP, and about which residues limit the reaction rate in overall catalysis (Cleland et al., 1998; Roy and Andrews, 2000). Translation of rbcL mRNA and association of RuBisCO peptides are important topics about which not enough is known (Houtz and Portis, 2003; Roy and Andrews, 2000). In general, the steps of posttranslational folding in plants and other organisms, whether E. coli, yeast, or human, must become known (Frydman, 2001). RuBisCO should provide an excellent model protein for study, considering that plants are able to synthesize up to 200 mg/ml of RuBisCO protein within days during the greening of leaves.

Engineering of the chloroplast genome has become the transformation strategy that promises to overcome problems encountered in the genetic manipulation of nuclear chromosomes for functions that must reside in plastids (Daniell, 1999). The technology will be indispensable for the metabolic engineering of pathways such as the PCR cycle, and starch and lipid biosyntheses. In this context, establishing methods for chloroplast genome engineering in the major crop species is an important priority.

Introduction of the cyanobacterial CO₂-pumping system into the plasma membrane of mesophyll cells or the chloroplast envelope may be one future direction. Some improvement in the photosynthetic performance of transgenic plants has already been reported with Arabidopsis (Lieman-Hurwitz et al., 2003).

Interspecies crosses that might lead to the transfer of beneficial genes are not possible in plants or any higher organism. Attempts at improving physiological performance in diverse environments can be realized by varying the expression of genes inherited from the parents. This requires that we understand in more detail the networks of reactions that constitute the evolutionarily established reaction bandwidth and allelic plasticity of a species. Science is now beginning to elucidate the potential of natural intraspecies variation and to probe the upper limits of plants physiologically, biochemically, and at the molecular genetic levels. Furthermore, we are learning, as we have pointed out, that it is possible to raise the potential of organisms and to exceed the intrinsic limits of plant productivity by introducing genes across species barriers that of a species that cannot be crossed by traditional breeding.