Chapter 1 Investigation of Possible Relationships Between the Chlorophyll Biosynthetic Pathway, the Assembly of Chlorophyll–Protein Complexes and Photosynthetic Efficiency

Summary

The world population of about six billion is expected to increase to nine billion by the year 2030. It may reach 18 billion by the end of the century. Worldwide, there has been a progressive decline in cereal yield and at present the annual rate of yield increase is below the rate of population increase. Since it will be difficult to increase the land area under cultivation without serious environmental consequences, the increased demand for food and fiber will have to be met by higher agricultural plant productivity. Plant productivity depends in turn upon photosynthetic efficiency. We have reason to believe that agricultural productivity can be significantly increased by alteration of the photosynthetic unit size. On the basis of recent advances in the understanding of the chemistry and biochemistry of the greening process and significant advances in molecular biology, we believe that alteration of the PSU size has become a realistic possibility. The thorough understanding of photosynthetic membrane assembly requires a deeper knowledge of the coordination of chlorophyll (Chl) and thylakoid apoprotein biosynthesis. As a working model for future investigations we have proposed three Chl–thylakoid apoprotein biosynthesis models namely a single-branched Chl biosynthetic pathway (SBP)-single location model, a SBP-multilocation model, and a multi-branched Chl biosynthetic pathway (MBP)-sublocation model. Rejection or validation of these models was probed by determination of resonance excitation energy transfer between various tetrapyrrole intermediates of the Chl biosynthetic pathway and various thylakoid Chl–protein complexes. The occurrence of resonance excitation energy transfer between several Chl precursors namely protoporphyrin IX (Proto) Mg-Proto + Mg-Proto monomethyl ester [Mp(e)] and divinyl (DV) and monovinyl (MV) Pchlide a and several Chl–protein complexes was demonstrated in situ. It was possible to calculate the distances separating Proto Mp(e) and Pchl(ide) a donors from Chl a acceptors. The calculated distances were incompatible with the operation of the SBP-single location Chl–protein biosynthesis model but were compatible with the operation of the MBP-sublocation model. The compatibility of the MBP-sublocation model of Chl–thylakoid protein complexes assembly opened the way for testing the hypothesis of whether certain Chl biosynthetic routes are indeed involved in the formation of specific Chl–protein complexes. The experimental strategy to tackle this issue was described. Based on the results so far acquired it was suggested that the greening process may now be manipulated to bioengineer genetically modified plants with a smaller PSU i.e. with more PSU units having fewer antenna Chl per unit thylakoid area, and ensuing higher photosynthetic efficiencies.

Epilogue

As suggested above, future research dealing with the bioengineering of smaller PSU sizes will have to focus on the MBP-sublocation Chl a-thylakoid protein biosynthesis model. The first order of business will have to deal with determining which Chl biosynthetic routes gives rise to PS I, PS II, and LHCII Chl–protein complexes. Appropriate genes of the greening process may then be manipulated by molecular biological techniques to bioengineer genetically modified plants with a smaller PSU, i.e., PSU units having more RC complexes and fewer antenna Chl per unit thylakoid area. Nevertheless this type of agriculture using genetically modified plants with smaller PSU sizes and higher photosynthetic conversion efficiencies will still be at the mercy of extrinsic factors and weather uncertainties.

In our opinion the ultimate agriculture of the future should consist of bioreactors populated with bioengineered, highly efficient photosynthetic membranes, with a small PSU size, and operating at efficiencies that approach the 12% maximal theoretical efficiency of the PETS that may be observed under white light, or the 27% maximal theoretical efficiency that may be achieved under red light. Such conditions may be set up during space travel or in large space stations (Rebeiz et al., 1982). The photosynthetic product may well be a short chain carbohydrate such as glycerol that can be converted into food fiber and energy. In the meanwhile let us not forget that a journey of 10,000 miles starts with the first step.

Note: Unless preceded by MV or DV, tetrapyrroles are used generically to designate metabolic pools that may consist of MV and/or DV ­components.