Triose phosphate utilization and beyond: from photosynthesis to end product synthesis

Abstract

During photosynthesis, plants fix CO2 from the atmosphere onto ribulose-bisphosphate, producing 3-phosphoglycerate, which is reduced to triose phosphates (TPs). The TPs are then converted into the end products of photosynthesis. When a plant is photosynthesizing very quickly, it may not be possible to commit photosynthate to end products as fast as it is produced, causing a decrease in available phosphate and limiting the rate of photosynthesis to the rate of triose phosphate utilization (TPU). The occurrence of an observable TPU limitation is highly variable based on species and especially growth conditions, with TPU capacity seemingly regulated to be in slight excess of typical photosynthetic rates the plant might experience. The physiological effects of TPU limitation are discussed with an emphasis on interactions between the Calvin–Benson cycle and the light reactions. Methods for detecting TPU-limited data from gas exchange data are detailed and the impact on modeling of some physiological effects are shown. Special consideration is given to common misconceptions about TPU.

This Zenodo record is a permanently preserved version of a PREreview. You can view the complete PREreview at https://prereview.org/reviews/7621978.

UIUC Plant Physiology Journal Club: TPU and beyond. 2018/10/30

The UIUC Plant physiology journal club chose to review the preprint "Triose phosphate utilization and beyond: from photosynthesis to end-product synthesis" (http://dx.doi.org/10.1101/434928), by McClain and Sharkey, about the role of phosphate levels in relation to CO2 assimilation during photosynthesis. The paper explains that when the rate of photophosphorylation which produces ATP, exceeds that of starch or sucrose synthesis which consumes ATP, phosphate levels drop causing photosynthetic electron transport rates to slow. This process is defined as Triose phosphate limitation (TPU).

We found the paper provided a very thorough definition of TPU and thought it was a really good introduction to the topic. In particular, the explanation about the way TPU limitation can be identified by no change in photosynthesis and decline in PhiPSII at high CO2 was very useful. The paper raises a number of points we were interested to discuss further including why TPU limitation is only slightly more than photosynthetic rate? Are some plants are more subjected to TPU limitation than others? Why is TPU limitation is slightly more than photosynthetic rate (one might predict that natural selection would select for utilization of all their photosynthate)? Why C4 plants do not experience any TPU limitation? Is it more common to see TPU limitation in Li6800s?

The question of the potential impact of a rising atmospheric CO2 concentrations on TPU is a fascinating topic. We thought it would be beneficial for readers to include reference to Experimental free air CO2 enrichment (FACE) experiments which have sought to address this issue, and some degree of guidance about what types of increases in atmospheric CO2 would be required to potentially see an effect.

In addition it would be useful to mention that many of the TPU studies have been performed in pot bound plants, which experience an altered shoot/root ratio creating an artificial sink limitation, and a bit more discussion about how often TPU limitation might occur under field conditions (Arp 1991). Finally it might be good to include a bit more discussion about how the xanthophyll cycle kicks in to protect the plant under stress and that decreased PhiPSII could be an example of non-photochemical quenching.

In summary, we thoroughly enjoyed the paper, we learnt a lot and look forward to seeing it published.

Minor points

In equation 5 it is not entirely clear what the phi symbol is?
The symbol R is used to represent three different variables, could maybe consider changing this to prevent confusion.
Many color combinations used in the figures would be unsuitable for those with color blindness, and suggest using color oracle (https://colororacle.org/) to help.
Fig 2: Clarification on what 'a little' and 'a lot' means would be helpful
Fig 2: We found it a bit confusing, many members were unclear about what it was demonstrating and suggest considering reformulating the description as we believe it could be useful as a teaching aid.
Fig 3: Many members really liked this figure and thought it provided a clear image to explain the various limitations a plant experiences during an A/Ci curve. There was some discussion about where the boundary lines were drawn, this might be addressed by having zones of colour fade into each other rather than represented as sharp boundaries. It was also suggested that some kind of upper bound to J limitation of ETR might be included (possibly fading out to white?)

References

Arp (1991) https://doi.org/10.1111/j.1365-3040.1991.tb01450.x

Rodgers et al. (2004) https://doi.org/10.1111/j.1365-3040.2004.01163.x

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Triose phosphate utilization and beyond: from photosynthesis to end product synthesis

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