Genetic improvement of leaf photosynthesis and intrinsic water use efficiency in C3 plants: Why so much little success?

There is an urgent need for simultaneously increasing photosynthesis/yields and water use efficiency (WUE) in C3 crops. Potentially, this can be achieved by genetic manipulation of the key traits involved. However, despite significant efforts in the past two decades very limited success has been achieved. Here I argue that this is mostly due to the fact that single gene/single trait approaches have been used thus far. Photosynthesis models demonstrate that only limited improving of photosynthesis can be expected by large improvements of any of its single limiting factors, i.e. stomatal conductance, mesophyll conductance, and the biochemical capacity for photosynthesis, the latter co-limited by Rubisco and the orchestrated activity of thylakoid electron transport and the Calvin cycle enzymes. Accordingly, only limited improvements of photosynthesis have been obtained by genetic manipulation of any of these single factors. In addition, improving photosynthesis by genetic manipulation in general reduced WUE, and vice-versa, and in many cases pleiotropic effects appear that cancel out some of the expected benefits. I propose that success in genetic manipulation for simultaneous improvement of photosynthesis and WUE efficiency may take longer than suggested in previous reports, and that it can be achieved only by joint projects addressing multi-gene manipulation for simultaneous alterations of all the limiting factors of photosynthesis, including the often neglected phloem capacity for loading and transport the expected surplus of carbohydrates in plants with improved photosynthesis.