Engineering photorespiration: current state and future possibilities.

Reduction of flux through photorespiration has been viewed as a major way to improve crop carbon fixation and yield since the energy-consuming reactions associated with this pathway were discovered. This view has been supported by the biomasses increases observed in model species that expressed artificial bypass reactions to photorespiration. Here, we present an overview about the major current attempts to reduce photorespiratory losses in crop species and provide suggestions for future research priorities.

Perspectives on plant photorespiratory metabolism.

Being intimately intertwined with (C3) photosynthesis, photorespiration is an incredibly high flux-bearing pathway. Traditionally, the photorespiratory cycle was viewed as closed pathway to refill the Calvin-Benson cycle with organic carbon. However, given the network nature of metabolism, it hence follows that photorespiration will interact with many other pathways. In this article, we review current understanding of these interactions and attempt to define key priorities for future research, which will allow us greater fundamental comprehension of general metabolic and developmental consequences of perturbation of this crucial metabolic process.

Promoter strength and tissue specificity effects on growth of tomato plants transformed with maize sucrose-phosphate synthase.

When sucrose-phosphate synthase (SPS; EC 2.4.1.14) is expressed in tomato (Lycopersicon esculentum Mill.) from a ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) small subunit (rbcS) promoter, yields are often unchanged but when SPS is expressed from a Cauliflower Mosaic Virus 35S promoter, yield is enhanced up to 80%. Two explanations for this phenomenon are (i) that expression of SPS in tissues other than leaves accounts for the increased yield or (ii) that the lower level of expression directed by the 35S promoter is more beneficial than the high level of expression directed by the rbcS promoter. To test the first hypothesis, we conducted a reciprocal graft experiment, which showed that root SPS activity did not substantially affect growth. To test the second hypothesis, we conducted a field trial using a backcrossed, segregating, population of SPS-transformed plants derived from 35S and rbcS lines. The optimal dose of SPS activity for growth was approximately twice that of the wild type regardless of which promoter was used. The effect of SPS on growth was the result of a shift in partitioning of carbon among starch, sucrose, and ionic compounds (primarily amino acids), rather than of an increase in net photosynthesis. Excessive SPS activity resulted in a decreased rate of amino acid synthesis, which could explain the non-linear response of plant growth to the level of SPS expression.

Evolutionary significance of isopreneemission from mosses.

Isoprene emission has been documented and characterized from species in all major groups of vascular plants. We report in our survey that isoprene emission is much more common in mosses and ferns than later divergent land plants but is absent in liverworts and hornworts. The light and temperature responses of isoprene emission from Sphagnum capillifolium (Ehrh.) Hedw. are similar to those of other land plants. Isoprene increases thermotolerance of S. capillifolium to the same extent seen in higher plants as measured by chlorophyll fluorescence. Sphagnum species in a northern Wisconsin bog experienced large temperature fluctuations similar to those reported in tree canopies. Since isoprene has been shown to help plants cope with large, rapid temperature fluctuations, we hypothesize the thermal and correlated dessication stress experienced by early land plants provided the selective pressure for the evolution of light-dependent isoprene emission in the ancestors of modern mosses. As plants radiated into different habitats, this capacity was lost multiple times in favor of other thermal protective mechanisms.