Photosynthetic CO2 assimilation in crop plants is not optimal for the current or predicted future environments (Long et al., 2006b). The kinetic properties of Rubisco are widely recognized as a major limitation to crop productivity. This is not only because of Rubisco's weak affinity for CO2 and catalysis of a competing reaction with O2 but also because the enzyme has a very low kcat (catalytic rate). High photosynthetic rates therefore demand large amounts of Rubisco which is often more than 50% of soluble leaf protein and 50% of leaf N. Thus, both crop productivity and the demand for N fertilizer could be addressed (Fig. 8.1) by overcoming Rubisco's manifest inadequacies. This could increase photosynthetic rates by as much as 100% in C3 crops (Long et al., 2006b; Parry et al., 2007; Reynolds et al., 2009) and increase N-use efficiency (NUE) in both C3 and C4 crops (Ghannoum et al., 2005). While the advantages of addressing individual parameters are briefly discussed below it is essential to take into account the overall impact, if any, of the other kinetic parameters and the overall costs in terms of both energy and N.
The balance between Rubisco and other photosynthetic components does not appear to be correct in C3 crops even under current conditions (Mitchell et al., 2000). In tobacco, increasing a single component of ribulose-
1,5-bisphosphate (RuBP; a substrate of Rubisco) regeneration, sedoheptulose-1,7-bisphosphatase, increased photosynthesis, leaf area and plant productivity (Harrison et al, 1998; Lefebvre et al, 2005; Tamoi et al, 2006). So it should be relatively easy to rebalance the investment in photosynthetic machinery in crop plants with existing technology.
Rubisco's weak affinity for CO2 and the competing reaction with O2 could be partially overcome by selecting for either natural variants with higher affinity for CO2 or a specificity factor (Zhu et al., 2004). However, Rubiscos with higher affinity for CO2 and a specificity factor do exist in some species (Galmes et al., 2005) and these could be introduced into crop plants; currently chloroplast transformation has only been developed for a few crop plants and not in monocots. Even where chloroplast transformation is available, foreign Rubiscos do not always fold and assemble correctly and further research is needed to address these technological challenges (Whitney and Andrews, 2001; Whitney et al., 2001; Whitney and Sharwood, 2007).
Considerable benefits could be achieved by increasing the CO2 concentration at the active site of Rubisco. The simplest approach would be to decrease stomatal and mesophyll conductance, for example by altering phenology of crops to avoid stomatal closure during drought stress. A more complex alternative would be the introduction of C4 metabolism into C3 crops (Hibberd et al., 2008). However, such a strategy is extremely complex, requiring the simultaneous introduction of both multiple structural and metabolic traits into the C3 plant. Negative impacts of this will be the need to divert light energy away from the Calvin cycle to the operation of the C-concentrating mechanism and the increase in N needed for the proteins associated with the C4 plant, although this would be in part offset by a lower Rubisco requirement pathway. At present this strategy remains at the initial stages of development in rice and is therefore a very long-term option requiring substantial investment.
An alternative approach to decrease the negative impact of photorespiration is to decrease the energy required and to increase the probability of released CO2 being recaptured. This can be achieved using metabolic engineering to introduce genes encoding proteins that short circuit the photorespir-atory cycle (Parry et al., 2003b; Kebeish et al., 2007). One possible negative impact is the possible accumulation of toxic intermediates.
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